Thermal insulation is commonly used to reduce energy consumption of HVAC systems and equipment. Minimum insulation levels for ductwork and piping are often dictated by energy codes, many of which are based on ASHRAE Standards 90.1 and 90.2. In many cases, it may be cost-effective to go beyond the minimum levels dictated by energy codes. Thicknesses greater than the optimum economic thickness may be required for other technical reasons such as condensation control, personnel protection, or noise control.

Tables 1 to 3 contain minimum insulation levels for ducts and pipes, excerpted from ANSI/ASHRAE Standard 90.1-2010.

Interest in green buildings (i.e., those that are environmentally responsible and energy efficient, as well as healthier places to work) is increasing. The LEED^® (Leadership in Energy and Environmental Design) Green Building Rating System™, created by the U.S. Green Building Council, is a voluntary rating system that sets out sustainable design and performance criteria for buildings. It evaluates environmental performance from a whole-building perspective and awards points based on satisfying performance criteria in several different categories. Different levels of green building certification are awarded based on the total points earned. The role of mechanical insulation in reducing energy usage, along with the associated greenhouse gas emissions, can help to contribute to LEED certification and should be considered when designing an insulation system.

Economics can be used to (1) select the optimum insulation thickness for a specific insulation, or (2) evaluate two or more insulation materials for least cost for a given level of thermal performance. In either case, economic considerations determine the most cost-effective solution for insulating over a specific period.

Life-cycle costing considers the initial cost of the insulation system plus the ongoing value of energy savings over the expected service lifetime. The economic thickness is defined as the thickness that minimizes the total life-cycle cost.

Labor and material costs of installed insulation increase with thickness. Insulation is often applied in multiple layers (1) because materials are not manufactured in single layers of sufficient thickness and (2) in many cases, to accommodate expansion and contraction of insulation and system components. Figure 1 shows installed costs for a multilayer application. The slope of the curves is discontinuous and increases with the number of layers because labor and material costs increase more rapidly as thickness increases. Figure 1 shows curves of total cost of operation, insulation costs, and lost energy costs. Point A on the total cost curve corresponds to the economic insulation thickness, which, in this example, is in the double-layer range. Viewing the calculated economic thickness as a minimum thickness provides a hedge against unforeseen fuel price increases and conserves energy.

Initially, as insulation is applied, the total life-cycle cost decreases because the value of incremental energy savings is greater than the incremental cost of insulation. Additional insulation reduces total cost up to a thickness where the change in total cost is equal to zero. At this point, no further reduction can be obtained; beyond it, incremental insulation costs exceed the additional energy savings derived by adding another increment of insulation.

Economic analysis should also consider the time value of money, which can be based on a desired rate of return for the insulation investment. Energy costs are volatile, and a fuel cost inflation factor is sometimes included to account for the possibility that fuel costs may increase more quickly than general inflation. Insulation system maintenance costs should also be included, along with cost savings associated with the ability to specify lower capacity equipment, resulting in lower first costs.

Chapter 37 of the 2019 ASHRAE Handbook—HVAC Applications has more information on economic analysis.

In many applications, insulation is provided to protect personnel from burns. The potential for burns to human skin is a complex function of surface temperature, surface material, and time of contact. ASTM Standard C1055 has a good discussion of these factors. Standard industry practice is to specify a maximum temperature of 140°F for surfaces that may be contacted by personnel. For indoor applications, maximum air temperatures depend on the facility and location, and are typically lower than design outdoor conditions. For outdoor installations, base calculations on summer design ambient temperatures with no wind (i.e., the worst case). Surface temperatures increase because of solar loading, but are usually neglected because of variability in orientation, solar intensity, and many other complicating factors. Engineering judgment must be used in selecting ambient and operating temperatures and wind conditions for these calculations.

Note that the choice of jacketing strongly affects a surface’s relative safety. Higher-emittance jacketing materials (e.g., plastic, painted metals) can be selected to minimize the surface temperature. Jacketing material also affects the relative safety at a given surface temperature. For example, at 175°F, a stainless steel jacket blisters skin more severely than a nonmetallic jacket at equal contact time.

For below-ambient systems, condensation control is often the overriding design objective. The design problem is best addressed as two separate issues: (1) avoiding surface condensation on the outer surface of the insulation system and (2) minimizing or managing water vapor intrusion.

Avoiding surface condensation is desirable because it (1) prevents dripping, which can wet surfaces below; (2) minimizes mold growth by eliminating the liquid water many molds require; and (3) avoids staining and possible damage to exterior jacketing.

The design goal is to keep the surface temperature above the dew-point temperature of surrounding air. Calculating surface temperature is relatively simple, but selecting the appropriate design conditions is often confusing. The appropriate design condition is normally the worst-case condition expected for the application. For condensation control, however, a design that satisfies the worst case is sometimes impossible.

To illustrate, Table 4 shows insulation thicknesses required to prevent condensation on the exterior surface of a hypothetical insulated tank containing a liquid held at 40°F in a mechanical room with a temperature of 80°F. Note that, at high relative humidities, the thickness required to prevent surface condensation increases dramatically, and becomes impractical above 90% rh.

For outdoor applications (or for unconditioned spaces vented to outdoor air), there are always some hours per year where the ambient air is saturated or nearly saturated. For these times, no amount of insulation will prevent surface condensation. Figure 2 shows the frequency distribution of outdoor relative humidity based on typical meteorological year weather data for Charlotte, North Carolina (Marion and Urban 1995). Note that there are over 1200 h per year when the relative humidity is equal to or greater than 90%, and nearly 600 h per year when the relative humidity is equal to or greater than 95%.

For outdoor applications and mechanical rooms vented to outdoor conditions, it is suggested to design for a relative humidity of 90%. Appropriate water-resistant vapor-retarder jacketing or mastics must then be specified to protect the system from the inevitable surface condensation.

Table 5 summarizes design weather data for a select number of cities. The design dew-point temperature and the corresponding dry-bulb temperatures at 90% rh are given, along with the number of hours per year that the relative humidity would exceed 90%. Additional design dew-point data can be found in Chapter 14.

This section is based on WDBG (2012).

For indoor designs in conditioned spaces, care is needed when selecting design conditions. Often, the HVAC system is sized to provide indoor conditions of 75°F/50% rh on a design summer day. However, those indoor conditions do not represent the worst-case indoor conditions for insulation design. Part-load conditions could result in higher humidity levels, or night and/or weekend shutdown could result in more severe conditions.

In addition to avoiding condensation on the exposed surface, another important design consideration is minimizing or managing water vapor intrusion, which is extremely important for piping and equipment operating at below-ambient temperatures. Water-related problems include thermal performance loss, health and safety issues, structural degradation, and aesthetic issues. Water entry into the insulation system may be through diffusion of water vapor, air leakage carrying water vapor, and leakage of surface water.

When the operating temperature is below the dew point of the surrounding ambient air, there is a difference in water vapor pressure across the insulation system. This vapor-pressure difference drives diffusion of water vapor from the ambient toward the cold surface. Piping and equipment typically create an absolute barrier to the passage of water vapor, so any vapor-pressure difference imposed across the insulation system results in the potential for condensation either in the insulation or at the cold surface. The vapor-pressure difference can range from below 0.1 in. Hg (0.05 psi) for a supply air duct operating in the return air plenum of a commercial building, to 1.2 in. Hg (0.6 psi) for a cryogenic system operating outdoors near the U.S. Gulf Coast. Although these pressure differences seem small, the effect over many operating hours can be significant.

Several fundamental design principles are used in managing water vapor intrusion. One method is to reduce the driving force by reducing the moisture content of the surrounding air. The insulation designer typically does not have control of the location of the piping, ductwork, or equipment to be insulated, but there are opportunities for the mechanical engineer to influence ambient conditions. Certainly, locating cold piping, ductwork, and equipment in unconditioned portions of buildings should be minimized. Consider conditioning mechanical rooms if feasible.

Another common method is moisture blocking, wherein passage of water vapor is eliminated or minimized to an insignificant level. The design must incorporate the following: (1) a vapor retarder with suitably low permeance; (2) a joint and seam sealing system that maintains vapor retarding system integrity; and (3) accommodation for future damage repair, joint and seam resealing, and reclosing after maintenance.

A vapor retarder is a material or system that adequately reduces transmission of water vapor through the insulation system. The vapor retarder system is seldom intended to resist entry of surface water or prevent air leakage, but can occasionally be considered the second line of defense for these moisture sources.

An effective vapor retarder material or system is essential for blocking systems to perform adequately. Mumaw (2001) showed that the design, installation, and performance of vapor retarder systems are key to the ability of an insulation system to minimize water vapor ingress. Performance of the vapor retarder material or system is characterized by the water vapor permeance: the lower the permeance, the better. The water vapor permeance can be evaluated using procedures outlined in ASTM Standard E96. In this test, a vapor pressure difference is imposed across vapor retarder material that has been sealed to a test cup, and the moisture gain or loss is measured gravimetrically.

The insulation system should be dry before applying a vapor retarder to prevent trapping water vapor in the insulation system. The insulation system also must be protected from undue weather exposure that could introduce moisture into the insulation before the system is sealed.

Faulty application techniques can impair vapor retarder performance. The effectiveness of installation and application techniques must be considered during selection. Factors such as vapor retarder structure, number of joints, mastics and adhesives that are used, as well as inspection procedures affect system performance and durability.

When selecting a vapor retarder, the vapor-pressure difference across the insulation system should be considered. Higher vapor-pressure differences typically require a vapor retarder with a lower permeance to control the overall moisture pickup of the insulated system. Service conditions affect the direction and magnitude of the vapor pressure difference: unidirectional flow exists when the water vapor pressure is constantly higher on one side of insulation system, whereas reversible flow exists when vapor pressure may be higher on either side (typically caused by diurnal or seasonal changes on one side of the insulation system). Properties of the insulation system materials should be considered. All materials reduce the flow of water vapor; the low permeance of some insulation materials can add to the overall resistance to water vapor transport of the insulation system. All vapor retarder joints should be tightly sealed with manufacturer-recommended sealants.

Another fundamental design principle is moisture storage design. In many systems, some condensation can be tolerated, the amount depending on the water-holding capacity or tolerance of a particular system. The moisture storage principle allows accumulation of water in the insulation system, but at a rate designed to prevent harmful effects. This concept is applicable when (1) unidirectional vapor flow occurs, but accumulations during severe conditions can be adequately expelled during less severe conditions; or (2) reverse flow regularly occurs on a seasonal or diurnal cycle. Design solutions using this principle include (1) periodically flushing the cold side with low-dew-point air (requires a supply of conditioned air and a means for distribution), and (2) using an insulation system supplemented by selected vapor retarders and absorbent materials such that an accumulation of condensation is of little importance. Such a design must ensure sufficient expulsion of accumulated moisture.

ASTM Standard C755 discusses various design principles. Chapters 25 to 27 of this volume thoroughly describe the physics associated with water vapor transport. Additional information is found in Chapter 10 of the 2018 ASHRAE Handbook—Refrigeration, and in ASTM (2001).

It is important to recognize that insulation retards heat flow; it does not stop it completely. If the surrounding air temperature remains low enough for an extended period, insulation cannot prevent freezing of still water or of water flowing at a rate insufficient for the available heat content to offset heat loss. Insulation can prolong the time required for freezing, or prevent freezing if flow is maintained at a sufficient rate. To calculate time θ (in hours) required for water to cool to 32°F with no flow, use the following equation:

where

θ	=	time to freezing, h
ρ	=	density of water = 62.4 lb/ft3
Cp	=	specific heat of water = 1.0 Btu/lb · °F
D1	=	inside diameter of pipe, ft (see Figure 4)
RT	=	combined thermal resistance of pipe wall, insulation, and exterior air film (for a unit length of pipe)
ti	=	initial water temperature, °F
ta	=	ambient air temperature, °F
tf	=	freezing temperature, °F

As a conservative assumption for insulated pipes, thermal resistances of pipe walls and exterior air film are usually neglected. Resistance of the insulation layer for a unit length of pipe is calculated as

where

D3	=	outer diameter of insulation, ft
D2	=	inner diameter of insulation, ft
k	=	thermal conductivity of insulation material, Btu · in/h · ft2 · °F

Table 6 shows estimated time to freezing, calculated using these equations for the specific case of still water with t_i = 42°F and t_a = –18°F.

When unusual conditions make it impractical to maintain protection with insulation or flow, a hot trace pipe or electric resistance heating cable is required along the bottom or top of the water pipe. The heating system then supplies the heat lost through the insulation.

Clean water in pipes usually supercools several degrees below freezing before any ice is formed. Then, upon nucleation, dendritic ice forms in the water and the temperature rises to freezing. Ice can be formed from water only by the release of the latent heat of fusion (144 Btu/lb) through the pipe insulation. Well-insulated pipes may greatly retard this release of latent heat. Gordon (1996) showed that water pipes burst not because of ice crystal growth in the pipe, but because of elevated fluid pressure within a confined pipe section occluded by a growing ice blockage.

Duct Insulation. Without insulation, the acoustical environment of mechanically conditioned buildings can be greatly compromised, resulting in reduced productivity and a decrease in occupant comfort. HVAC ducts act as conduits for mechanical equipment noise, and also carry office noise between occupied spaces. Additionally, some ducts can create their own noise through duct wall vibrations or expansion and contraction. Lined sheet metal ducts and fibrous glass rigid ducts can greatly reduce transmission of HVAC noise through the duct system. The insulation also reduces cross-talk from one room to another through the ducts. A good discussion of duct acoustics is provided in Chapter 48 of the 2019 ASHRAE Handbook—HVAC Applications. Duct insulation can be used to provide both attenuation loss and breakout noise reduction.

Attenuation loss is noise absorbed within the duct. In uninsulated ducts, it is a function of duct geometry and dimensions as well as noise frequency. Internal insulation liners are generally available for most duct geometries. Chapter 48 in the 2015 ASHRAE Handbook—HVAC Applications provides attenuation losses for square, rectangular, and round ducts lined with fibrous glass, and also gives guidance on use of insulation in plenums to absorb duct system noise. Internal linings can be very effective in fittings such as elbows, which can have 2 to 8 times more attenuation than an unlined elbow of the same size. For alternative lining materials, consult individual manufacturers.

It is difficult to write specifications for sound attenuation because it changes with every duct dimension and configuration. Thus, insulation materials are generally selected for attenuation based on sound absorption ratings. Sound absorption tests are run per ASTM Standard C423 in large reverberation rooms with random sound incidence. The test specimens are laid on the chamber floor per ASTM Standard E795, type A mounting. This mode of sound exposure is different from the exposure of internal linings installed in an air duct; therefore, sound absorption ratings for materials can only be used for general comparisons of effectiveness when used in air ducts of varying dimensions (Kuntz and Hoover 1987).

Breakout noise is from vibration of the duct wall caused by air pressure fluctuations in the duct. Absorptive insulation can be used in combination with mass-loaded jacketing materials or mastics on the duct exterior to reduce breakout noise. This technique is only minimally effective on rectangular ducts, which require the insulation and mass composite to be physically separated from the duct wall to be very effective. For round ducts, as with pipes, absorptive insulation and mass composite can be effective even when directly applied to the duct surface. Chapter 48 in the 2015 ASHRAE Handbook—HVAC Applications provides breakout noise guidance data.

Noise Radiating from Pipes. Noise from piping can be reduced by adding an absorptive insulation and jacketing material. By knowing the sound insertion loss of insulation and jacketing material combinations, the expected level of noise reduction in the field can be estimated. A range of jacket weights and insulation thicknesses can be used to reduce noise. Jackets used to reduce noise are typically referred to as being mass filled. Some products for outdoor applications use mass-filled vinyl (MFV) in combination with aluminum.

Pipe insertion loss is a measurement (in dB) of the reduction in sound pressure level from a pipe as a result of application of insulation and jacketing. Measured at different frequencies, the noise level from the jacketed pipe is subtracted from that of the bare pipe; the larger the insertion loss number, the larger the amount of noise reduction.

ASTM Standard E1222 describes how to determine insertion loss of pipe jacketing systems. A band-limited white noise test signal is produced inside a steel pipe located in a reverberation room, using a loudspeaker or acoustic driver at one end of the pipe to produce the noise. Average sound pressure levels are measured in the room for two conditions: with sound radiating from a bare pipe, and with the same pipe covered with a jacketing system. The insertion loss of the jacketing system is the difference in the sound pressure levels measured, adjusted for changes in room absorption caused by the jacketing system’s presence. Results may be obtained in a series of 100 Hz wide bands or in one-third octave bands from 500 to 5000 Hz.

Table 7 gives measured insertion loss values for several pipe insulation and jacket combinations. The weight of the jacket material significantly affects insertion loss of pipe insulation systems. Figure 5 represents insertion loss of typical fibrous pipe insulations with various weights of jacketing (Miller 2001).

It is very important that sound sources be well identified in industrial settings. It is possible to treat a noisy pipe very effectively and have no significant influence on ambient sound measurement after treatment. All sources of noise above desired levels must receive acoustical treatment, beginning with the largest source, or no improvement will be observed.

Materials used to insulate mechanical equipment generally must meet the requirements of local codes adopted by governmental entities having jurisdiction over the project. In the United States, most local codes incorporate or are patterned after model codes developed and maintained by organizations such as the National Fire Protection Association (NFPA) and International Code Council (International Codes). Refer to local codes to determine specific requirements.

Most codes related to insulation product fire safety refer to the surface burning characteristics as determined by the Steiner tunnel test (ASTM Standard E84, UL Standard 723, or CAN/ULC Standard S-102). These similar test methods evaluate the flame spread and smoke developed from samples mounted in a 25 ft long tunnel and subsequently exposed to a controlled flame. Results are given in terms of flame spread and smoke developed indices, which are relative to a baseline index and calibration standards of inorganic reinforced cement board (0) and select-grade red oak flooring (100). Samples are normally mounted with the exposed surface face down in the ceiling of the tunnel. Upon ignition, the progress of the flame front is timed while being tracked visually for distance down the tunnel, with the results used to calculate the flame spread index. Smoke index is determined by measuring smoke density with a light cell mounted in the exhaust stream.

Using supporting materials on the underside of the test specimen can lower the flame spread index. Materials that melt, drip, or delaminate to such a degree that the continuity of the flame front is destroyed give low flame spread indices that do not relate directly to indices obtained by testing materials that remain in place. Alternative means of testing may be necessary to fully evaluate some of these materials.

For pipe and duct insulation products, samples are prepared and mounted in the tunnel per ASTM Standard E2231, which directs that “the material, system, composite, or assembly tested shall be representative of the completed insulation system used in actual field installations, in terms of the components, including their respective thicknesses.” Samples are constructed to mimic, as closely as possible, the products as they will be used, including any facings and adhesives as appropriate.

Duct insulation generally requires a flame spread index of not more than 25 and a smoke developed index of not more than 50, when tested in accordance with ASTM Standard E84. Codes often require factory-made duct insulations (e.g., insulated flexible ducts, rigid fibrous glass ducts) to be listed and labeled per UL Standard 181. This standard specifies several other fire tests (e.g., flame penetration and low-energy ignition) as part of the listing requirements.

Some building codes require that duct insulations meet the fire hazard requirements of NFPA Standard 90A or 90B, to restrict spread of smoke, heat, and fire through duct systems, and to minimize ignition sources. Local code authorities should also be consulted for specific requirements.

For pipe insulation, the requirement is generally a maximum flame spread index of 25 and a maximum smoke developed index of 450 in nonplenum spaces (in plenums, less than or equal to 25 and 50, respectively). Consult local code authorities for specific requirements.

The term noncombustible, as defined by building codes, refers to materials that pass the requirements of ASTM Standard E136. This test method involves introducing a small specimen of the material into a furnace initially maintained at a temperature of 1380°F. The temperature rise of the furnace is monitored and the specimen is observed for any flaming. Criteria for passing include limits on temperature rise, flaming, and weight loss of the specimen. Some building codes accept as noncombustible a composite material having a structural base of noncombustible material and a surfacing not more than 1/8 in. thick that has a flame spread index not greater than 50. A related term sometimes referenced in building codes is limited combustible, which is an intermediate category that considers the potential heat content of materials determined per the testing requirements of NFPA Standard 259.

Mechanical insulation materials are often used as a component in systems or assemblies designed to protect buildings and equipment from the effects or spread of fire (i.e., fire-resistance assemblies). They can include walls, roofs, floors, columns, beams, partitions, joints, and through-penetration fire stops. Specific designs are tested and assigned hourly ratings based on performance in full-scale fire tests. Note that insulation materials alone are not assigned hourly fire resistance ratings; ratings are assigned to a system or assembly that may include specific insulation products, along with other elements such as framing members, fasteners, wallboard, etc.

Fire resistance ratings are often developed using ASTM Standard E119. This test exposes assemblies (walls, partitions, floor or roof assemblies, and through-penetration fire stops) to a standard fire exposure controlled to achieve specified temperatures throughout a specified time period. The time/temperature curve is intended to represent building fires where the primary fuel is solid, and specifies a temperature of 1000°F at 5 min, 1700°F at 1 h, and 2300°F at 8 h. In the hydrocarbon processing industry, liquid hydrocarbon-fueled pool fires are a concern; fire resistance ratings for these applications are tested per ASTM Standard E1529. This time/temperature curve rises rapidly to 2000°F within 5 min, and remains there for the duration of the test.

Fire-resistant rated designs can be found in the directories of listing agencies. Examples of such agencies include Underwriters Laboratories, Factory Mutual, UL Canada, and Intertek.

The following standard cross-references are provided for products to be tested in Canada. Although these are parallel Canadian standards, the requirements may differ from those of ASTM standards.

Corrosion of metal pipe, vessels, and equipment under insulation, though not typically caused by the insulation, is still a significant issue that must be considered during the design of any mechanical insulation system. The propensity for corrosion depends on many factors, including the ambient environment, operating temperature of the metal, proper installation, and maintenance of the insulated system.

Corrosion under insulation (CUI) is most prevalent in outdoor industrial environments such as refineries and chemical plants. Corrosion can be very costly because of forced downtime of processes and can be a health and safety hazard as well. Although insulation itself may not necessarily be the cause of corrosion, it can be a passive component because it is in direct contact with the pipe or equipment surface.

Very little information is published on corrosion in commercial environments. Although corrosion under insulation is less likely to be a major concern for most insulated surfaces located indoors, it may be a factor on indoor systems that are frequently washed down, such as in the food processing industry.

Water from condensation on cold surfaces can be present on both indoor and outdoor insulation systems if there is damage to the vapor retarder. Hot processes can also be subject to condensation during periods of system shutdown.

The following factors may lead to corrosion under insulation.

For outdoor applications, to prevent ingress of moisture and corrosive ions from precipitation, a properly designed, installed, and maintained weather-protective jacket is recommended. For more specific guidelines, consult the insulation material manufacturer. If process temperatures are lower than ambient (even for short periods of time such as during shutdowns), a vapor retarder is also required.

Even with a protective jacket and vapor retarder, it is likely that some moisture and ions will eventually enter the system because of abuse, wear, age, or improper installation. No installation is ideal in any real-world setting. Because most people only consider the chlorides arising from the insulation material, the crucial issue of water and ions infiltrating the insulation system from the environment and yielding metal corrosion remains largely unaddressed. Thus, painting the pipe is the second and most important line of defense, and is necessary if pipe temperature is in the 140 to 300°F range for significant periods of time. To minimize the potential for corrosion, metal pipe should be primed with, for example, an epoxy coating. This alternative offers superior protection against corrosion, because priming protects against ions arising from the insulation and, more importantly, from ions that enter the system from the environment.

To minimize corrosion,

Turner and Malloy (1981) categorized insulation materials into four types:

Cellular insulations are often further classified as either open-cell (i.e., cells are interconnecting) or closed-cell (i.e., cells sealed from each other). Generally, materials with greater than 90% closed-cell content are considered to be closed-cell materials.

Another material sometimes called thermal insulating paint or coating is available for use on pipes ducts and tanks. These products’ performance must be clearly understood before using them as thermal insulation. These paints and coatings have not been extensively tested and additional research is needed to verify their performance. Further discussion of these products can be found in Hart (2006).

Selecting an insulation material for a particular application requires understanding the various physical properties associated with available materials.

Operating temperature is often the primary consideration. Maximum temperature capability is normally assessed using ASTM Standard C411 by exposing samples to hot surfaces for an extended time, and assessing the materials for any changes in properties. Evidence of warping, cracking, delamination, flaming, melting, or dripping are indications that the maximum use temperature of the material has been exceeded. There is currently no industry-accepted test method for determining the minimum operating temperature of an insulation material, but minimum temperatures are normally determined by evaluating the material’s integrity and physical properties after exposure to low temperatures.

Thermal conductivity of insulation materials is a function of temperature. Many specifications call for insulation conductivity values evaluated at a mean temperature of 75°F. Most manufacturers provide conductivity data over a range of temperatures to allow evaluations closer to actual operating conditions. Conductivity of flat product is generally measured per ASTM Standards C177 or C518, whereas pipe insulation conductivity is generally determined using ASTM Standard C335. Additional information on this property is presented in the following section.

Compressive resistance is important where the insulation must support a load without crushing (e.g., insulation inserts in pipe hangers and supports). When insulation is used in an expansion or contraction joint to take up a dimensional change, lower values of compressive resistance are desirable. ASTM Standard C165 is used to measure compressive resistance for fibrous materials, and ASTM Standard D1621 is used for foam plastic materials.

Water vapor permeability is the water vapor flux through a material induced by a unit vapor pressure gradient across the material. For insulating materials, it is commonly expressed in units of perm · in. A related and often-confused term is water vapor permeance (in perms), which measures water vapor flux through a material of specific thickness and is generally used to define vapor retarder performance. In below-ambient applications, it is important to minimize the rate of water vapor flow to the cold surface. This is normally accomplished by using vapor retarders or insulation materials (e.g., cellular glass insulation) with a permeance less than or equal to 0.02 perm, or both. However, some flexible closed-cell insulation materials have been used successfully without a separate vapor retarder material. ASTM Standard E96 is used to measure water vapor transmission properties of insulation materials.

Water absorption is generally measured by immersing a sample of material under a specified head of water for a specified time period. It is a useful measure of the amount of liquid water absorbed from water leaks in weather barriers or during construction.

Typical physical properties of interest are given in Table 8. Values in this table are taken from the relevant ASTM material specification with permission from ASTM International. Within each material category, a variety in types and grades of materials exist. A representative type and grade are listed in Table 8 for each material category; refer to ASTM standards or to manufacturers for specific data.

Thermal Conductivity of Below-Ambient Pipe Insulation Systems. Mechanical pipe insulation systems are installed around cold cylindrical surfaces, such as chilled pipes, and work below ambient temperature in several industrial and commercial building applications. Thermal performance of a pipe insulation system is affected by ambient temperature and humidity and might vary gradually with time. For below-ambient temperature applications of pipe insulation, most published data are extrapolated from flat slab configurations of insulation material, and may not be accurate for cylindrical pipe insulation systems because of radial configuration and longitudinal split joints. Thus, ASHRAE research project RP-1356 (Cremaschi et al. 2012) developed an experimental apparatus to measure the thermal conductivity of mechanical pipe insulation systems below ambient temperature. Thermal conductivities of five pipe insulation systems under low-humidity, noncondensing conditions are provided in Table 9 at mean insulation temperatures of 55 and 75°F; the insulation was installed on a 3 in. nominal pipe size diameter aluminum pipe, and the test specimens were 3 ft long. Radial heat flux was inward and ranged from 7.9 to 34.6 Btu/h · ft. Nominal wall thickness of the pipe insulation systems varied from 1 to 2 in. Vapor barriers on the outer surface of the pipe insulation were not installed. The dry test were performed with the aluminum pipe surface temperature at 40.5 ± 0.5°F, ambient temperature from 73 to 110°F, and air dew-point temperature below 40°F. For these test conditions, water vapor does not condense on the aluminum pipe surface.

The relation between the pipe insulation’s thermal conductivity and its mean temperature is linear, and the thermal conductivity of pipe insulation system had a weak dependence on the nominal wall thickness. Note that, for some cases, joint sealant is recommended for the installation of the pipe insulation. Values in Table 9 represent a combined thermal conductivity of the pipe insulation with a certain type of joint sealant applied on the longitudinal joints, where two C-shells come in contact with each other. The combined thermal conductivity of the pipe insulation might be higher than the thermal conductivity of pipe insulation C-shells that are mechanically joint together without sealant on the longitudinal joints. For example, if a butyl rubber sealant with a layer thickness ranging from 1/16 to 0.1 in. is used, it is possible that the combined thermal conductivity increases up to 15% with respect to the value of the same pipe insulation system without joint sealant.

ASHRAE research project RP-1356 also measured the thermal conductivity of mechanical pipe insulation systems below ambient temperature in high humidity without vapor retarders, resulting in rapid moisture ingress. Two types of pipe insulation, installed on a 3 in. nominal pipe size diameter aluminum pipe that was 3 ft long, were exposed for less than a month to a warm, humid environment, resulting in water vapor condensation in the insulation samples and increased thermal conductivities. The nominal wall thickness of the pipe insulation systems was 2 in.

Vapor barriers were not installed on the outer surface of the pipe insulation, and the thermal conductivities increased as result of condensed water being retained into the insulation systems. For one type of pipe insulation, the thermal conductivity increased by 3.15 times when the moisture content was about 11% volume. For the other insulation, the thermal conductivity was 1.55 times of the original dry value when the moisture content reached 5% by volume. Each test was run for less than 1 month at high ambient humidity (>80% rh at 95°F. The test conditions were intentionally different from each other, and the thermal performance of the two pipe insulation systems tested in warm, high-humidity conditions should not be compared.

Caution is needed in using this data. Only two types of pipe insulation were tested, and neither had a vapor retarder. The manufacturers of these materials do not recommend these pipe insulation materials be installed in this manner for below-ambient applications. Nevertheless, these data are significant because they demonstrate both the necessity of installing an effective water vapor retarder and the negative impact of water retention in the insulation on its thermal conductivity. These results are only an example of this phenomenon, and any insulation that absorbs and retains water could be similarly affected.

Weather barriers, often referred to as jacketing, are extremely important. Premature failure can lead to insulation failure, with safety and economic consequences.

Safety consequences

Economic consequences

Many more scenarios must be considered, especially when the broad range of features required of a weather barrier are considered. Turner and Malloy (1981) define a weather barrier as “a material or materials, which, when installed on the outer surface of thermal insulation, protects the insulation from . . . rain, snow, sleet, wind, solar radiation, atmospheric contamination and mechanical damage.” With this definition in mind, several service requirements must be considered.

Materials Used as Weather Barriers for Insulation. Metal rolls or sheets of various thicknesses are available with embossing, corrugation, moisture barriers, and different banding and closure methods. Elbows and tees are also available for piping. Typical metal jacketing materials are

All metal weather barrier/jacketing should have a 3 mil thick multiple-layer moisture barrier factory heat laminated to the interior surface to help prevent galvanic and pitting/crevice corrosion on the interior surface of the jacketing (Young 2011).

Polymeric (plastic) rolls or sheets are available at various thicknesses. These materials are glued or solvent-welded, depending on the polymer. Elbows and tees are also available for piping for some type of polymers. Typical polymeric (plastic) jacketing materials include

Numerous mastics are available. Mastics are often used with fiber-glass cloth or canvas to encapsulate pipes, tanks, or other vessels; they are also used at insulation terminations and at or around protrusions such as valves or supports. It is important to choose the correct mastic for the application, considering surface temperature, insulation type, fire hazard classification, water resistance, and vapor permeability requirements. Mastics are brushed, troweled, or sprayed on the surface at a thickness recommended by the manufacturer.

Importance of Workmanship and Knowledge. Workmanship and knowledge are key to successful insulation weather barrier design. The importance of working with the installing contractor and material manufacturers regarding fitness for use of each material is paramount. The Midwest Insulation Contractors Association (MICA) publishes an excellent resource regarding materials used as weather barriers: the National Commercial and Industrial Insulation Manual (2011), in print and as a PDF file, is available from www.micainsulation.org/standards-manual.html.

Water vapor control is extremely important for piping and equipment operating below ambient temperatures. These systems are typically insulated to prevent surface condensation and control heat gain. Piping and equipment typically create an absolute barrier to passage of water vapor, so any vapor-pressure difference imposed across the insulation system results in the potential for condensation at the cold surface. A high-quality vapor retarder material or system is essential for these systems to perform adequately. Mumaw (2001) showed that the design, installation, and performance of the vapor retarder systems are key to an insulation system’s ability to minimize water vapor ingress. This research also suggests that in-place system vapor permeance may be greater than the rated performance based on standard material test methods.

Moisture-related problems include thermal performance loss, health and safety issues, structural degradation, corrosion, and aesthetic issues. Water may enter the insulation system through water vapor diffusion, air leakage carrying water vapor, and leakage of surface water. A vapor retarder is a material or system that adequately reduces the transmission of water vapor through the insulation system. The vapor retarder system is seldom intended to resist the entry of surface water or prevent air leakage, but can occasionally be considered the second line of defense for these moisture sources.

The performance of the vapor retarder material or system is characterized by its water vapor permeance. Chapter 25 has a thorough description of the physics associated with water vapor transport. Water vapor permeance can be evaluated per ASTM Standard E96, using either procedure A (desiccant method) or procedure B (water method), by imposing a vapor pressure difference across vapor retarder material that has been sealed to a test cup, and gravimetrically measuring the moisture gain or loss. It is recommended that both tests be performed and the results of both be evaluated.

Faulty application can impair vapor retarder performance. The effectiveness of installation and application techniques must be considered when selecting a vapor retarder system. Factors such as vapor retarder structure, number of joints, mastics and adhesives that are used, and inspection procedures affect performance and durability.

The insulation system should be dry before application of a vapor retarder to prevent trapping water vapor in the system. The system must be protected from undue weather exposure that could introduce moisture into the insulation before the system is sealed.

When selecting a vapor retarder, the vapor pressure difference across the insulation system should be considered. Higher vapor pressure differences typically require a lower-permeance vapor retarder to control water vapor intrusion into the system. Service conditions affect the direction and magnitude of the vapor pressure difference. Unidirectional flow exists when water vapor pressure is constantly higher on one side of insulation system.

Typically, in buildings located in humid climates, vapor pressure differences in unconditioned spaces are significantly greater than in conditioned spaces. For example, in a conditioned space at 75°F and 50% rh, the vapor pressure difference across the insulation system, on 42°F chilled-water lines, is less than 0.20 in. Hg. For below-ambient, insulated pipes running in humid unconditioned spaces, the vapor pressure difference across the insulation system can be as great as 0.80 in. Hg in the continental United States, and even greater in places with tropical climates. Hence, for below-ambient pipes running in humid unconditioned spaces, the vapor retarder may require a lower vapor permeance than for those same pipes running in conditioned spaces.

Reversible flow exists when vapor pressure may be higher on either side, typically caused by diurnal or seasonal changes on one side of the system. Properties of insulating materials used should be considered. All materials reduce water vapor flow, but low-permeance insulations can add to the overall water vapor transport resistance of the insulation system. Some low-permeability materials are considered to be vapor retarders without any additional jacket material.

Vapor Retarder Jackets. There is some inconsistency in the nomenclature used for materials used as vapor retarders for pipe, tank, and equipment insulation. Designations such as jacket, jacketing, facing, and all-service jacket (ASJ) are all applied to this component, sometimes interchangeably. On the other hand, the vapor retarder component of insulation for air-handling systems, such as duct wrap and duct board, is typically referred to only as facing. The term vapor diffusion retarder (VDR) is also used to generically describe these materials.

In this chapter, vapor retarder denotes the vapor-retarding membrane of the system, but the reader should be aware of the various terms that may be encountered. In addition, some insulation materials are considered vapor retarders in themselves without any additional retarding membrane.

Vapor Retarders for Pipe, Tank, and Equipment Insulation. Materials or combinations of materials used for vapor retarders can take many different forms. Necessarily, one component must be a material that offers significant resistance to vapor passage. A commonly used preformed material for pipe, tank, and equipment vapor retarder applications is laminated white paper, reinforcing glass-fiber scrim, and aluminum foil or metallized polyester film. These products are generally referred to as all-service jackets (ASJs), and meet the requirements of ASTM Test Method C1136 with a vapor permeance of 0.02 perm, per procedure A of ASTM Standard E96; C1136 is the accepted industry vapor retarder material standard for mechanical insulation applications. These facings are commonly used as the outer finish in low-abuse indoor areas; elsewhere, they should be covered by a protective jacket. Many types of insulation are supplied with factory-applied ASJ vapor retarders.

Note that ASJs with exposed paper may have service limitations on below-ambient systems in wet environments, particularly in unconditioned spaces in hot, humid climates. In such spaces, during periods of high humidity, expect condensation to occur on the insulation’s surface some of the time (e.g., when relative humidity exceeds 90%). Condensation on the surface can degrade the ASJ by wetting the exposed kraft paper surface, leading to mold growth on the paper, degradation of the paper itself, and/or corrosion of the aluminum vapor retarder component.

In addition to traditional ASJ vapor retarders, low-permeance monolayer plastic film and sheet, ASJ without exposed paper, laminates using aluminum foil, and other types of sheet structures are used in low- and very-low-temperature applications. They usually are water resistant; that is, when condensation occurs on their surface, they do not absorb the water. These are not always referred to as ASJ, and may be either factory applied by the insulation manufacturer or procured separately from the insulation and applied in fabrication shops or in the field. Examples of laminates include 3- to 13-ply sheet materials with thicknesses up to about 0.016 in.; these plies include at least one layer of aluminum foil and many have a permeance < 0.005 perm. There are also rubber and asphalt membranes, with an aluminum facing, with the same permeance. An example of one of the plastic films is polyvinylidene chloride (PVDC). This is typically used in more demanding applications, and often is covered by protective jacket. Many of these ASJ facings meet the requirements of ASTM Standard C1136, Type VII or VIII, or ASTM Standard C921, and generally have a permeance < 0.02 perm per ASTM Standard E96, procedure A.

The moisture-sensitive nature of paper and the relative frailty of uncoated aluminum foil can be problematic in the potentially high-humidity environment of unconditioned spaces with below-ambient applications. Exposure to water, either from condensation caused by inadequate insulation thickness or from ambient sources, can cause degradation and distortion of the paper, higher likelihood of mold growth, and foil corrosion, leading to vapor retarder failure. The presence of leachable chloride can promote corrosion of the foil or metallized film. The trend in vapor retarders for pipe insulation is toward structures without exposed paper, such as plastic films, coated metallized films, and better-protected foil laminates. Many have water vapor permeance ratings < 0.005 perm.

For most common vapor retarder jackets, matching pressure-sensitive tapes are available for making joint and puncture seals. With careful installation, these can be used to effectively seal joints in the vapor retarder system. In addition, vapor retarder mastics are available; these should be designated as such by their manufacturer and have a low vapor permeance rating, no greater than that of the vapor retarder membrane. Applying mastic thickly enough is critical to providing sufficient permeance. Vapor retarder mastics are typically used where fittings, supports, and other obstructions make a proper vapor seal difficult to achieve. Highly conformable tapes are sometimes used for this purpose, as well. Applied mastic systems are a vapor-retarding layer; they are often called vapor barriers by manufacturers, but their vapor resistance is a function of their permeability, thickness, and quality of the mastic application. Also, some mastics may not be compatible with certain insulation types. For this reason, always consult the insulation manufacturer for recommendations on the correct type of vapor retarder to use in the application. Weather barrier mastics are not vapor retarder mastics and should not be used for below-ambient applications unless they also have a low vapor permeability or are used in conjunction with a separate vapor retarder.

Below-ambient piping and equipment in general, and below-freezing applications in particular, are the most demanding applications for an insulation vapor retarder. Even though extremely low-permeance (<0.005 perm) vapor retarder materials exist, it is extremely difficult to achieve a perfect barrier in a system that is field installed and includes numerous joints and penetrations. It follows that adequate system design, proper insulation and jacketing material selection, and careful workmanship are all equally important.

For pipes operating at below-ambient temperatures, it is recommended that every 15 to 20 lineal feet, or at every fitting, a vapor stop (also called vapor dam) be installed. Should a leak occur in the vapor retarder, a vapor stop isolates vapor intrusion to that pipe insulation section and thereby prevents vapor and condensed water intrusion into the adjacent section(s) of pipe insulation or adjacent fitting insulation. A vapor stop is made by applying a vapor retarder mastic liberally to the pipe surface, for 3 in. along its length, adjacent to the end of the pipe insulation section. After installing that pipe insulation section, the mastic is then applied liberally to the end of the pipe insulation. Using a glass fiber or polyester scrim allows visual confirmation that the mastic is thick enough. For illustrations of vapor stops, see MICA’s (2011) National Commercial and Industrial Insulation Standards.

Air-Handling Systems. Vapor retarders for equipment and duct insulation take various forms. Because of the relatively less severe and demanding conditions in air-handling systems located in conditioned spaces (because of their higher operating temperatures and lower indoor ambient humidity), current vapor retarder materials have been shown to adequately meet these performance requirements. In general, moisture problems are not often encountered if insulation design is adequate for the application, and some low-permeability insulation materials are used without separate vapor retarders. For fiberglass duct wrap and duct board, a lamination of aluminum foil, scrim, and kraft paper (FSK) has long been the material of choice, although flexible vinyl and other white or black facings are occasionally used. All of these facings can be procured separately in roll form, and used on any type of insulation. ASTM Standard C1136, type II, is a typical specification for factory-applied vapor retarder on duct insulation (except flexible duct). Flexible (flex) ducting typically incorporates a plastic film or film lamination that contains a metallized substrate as a vapor-retarding component. For outdoor ducts, laminate jacketing, manufacturer-rated to have a low vapor permeance (<0.005 perm) and for outdoor use, can be installed over the previously mentioned types of duct insulation using a compatible tape for closures, to provide protection from both weather exposure and vapor intrusion to the duct insulation.

Application-specific pressure-sensitive tapes or mastics are typically used to seal joints. As in any cold system where a vapor retarder is required, design, selection of materials, and workmanship must be properly addressed. The insulation manufacturer’s recommendations should be followed.

Small pipes can be insulated with cylindrical half-sections of rigid insulation or with preformed flexible material. Larger pipes can be insulated with flexible material or with curved, flat segmented, or cylindrical half, third, or quarter sections of rigid insulation. Fittings (valves, tees, crosses, and elbows) use preformed fitting insulation, fabricated fitting insulation, individual pieces cut from sectional straight-pipe insulation, or insulating cements. Fitting insulations should always be equal in thermal performance to the pipe insulation.

Securing Methods. The method of securing varies with the type of insulation, size of pipe, form and weight of insulation, and type of jacketing (i.e., field- or factory-applied). Insulation with factory-applied jacketing can be secured on small piping by securing the overlapping jacket, which usually includes an integral sealing tape. Additional tape around the circumference may be necessary. Large piping may require supplemental wiring or banding. Insulation on large piping requiring separate jacketing is wired or banded in place, and the jacket is cemented, wired, or banded, depending on the type. Flexible closed-cell materials require no jacket for most applications and are applied using specially formulated contact adhesives.

Insulating Pipe Hangers. All piping is held in place by hangers and supports. Selection and treatment of pipe hangers and supports can significantly affect thermal performance of an insulation system. Thus, it is important that the piping engineer and insulation specifier coordinate during project design to ensure that correct hangers are used and sufficient physical space is maintained to allow for the required thickness of insulation.

A typical ring or line size hanger is illustrated in Figure 6A. This type of hanger is commonly used on above-ambient lines at moderate temperature. However, it provides a thermal short circuit through the insulation, and the penetration is difficult to seal effectively against water vapor, so it is not recommended for below-ambient applications.

Pipe shoes (Figure 6B) are used for hot piping of large diameter (heavy weight) and where significant pipe movement is expected. The design allows for pipe movement without damage to the insulation or the finish. The design is not recommended for below-ambient applications because of the thermal short circuit and difficulty in vapor sealing.

A better solution is to use clevis hangers (Figure 6C), which are sized to allow clearance for the specified thickness of insulation, and avoid the short circuit associated with ring hangers and pipe shoes. Shields (or saddles) spread the load from the pipe, its contents, and the insulation material over an area sufficient to support the system without significantly compressing the insulation material. Table 10 provides guidance on sheet metal saddle lengths for glass fiber pipe insulation. For pipe sizes above 3 in. NPS, it is recommended that high-compressive-strength inserts (e.g., foam, high-density fiberglass, calcium silicate) be used. Table 11 gives recommended saddle lengths for 2 lb/ft³ polyisocyanurate foam insulation. Preinsulated saddles are available. Note that wood blocks have poor thermal conductivity and are not recommended, especially for cold pipe systems.

When the goal is avoiding compression of low-compressive-strength insulation products, it is recommended to use high-strength insulation inserts, made of a product that offers the desired compressive strength and other necessary performance properties. Other higher-strength materials that are not thermal insulation material and interrupt the insulation envelope, or do not allow complete sealing of an insulation system against water vapor ingress, are not recommended for supporting insulated piping on pipe hangers.

Insulation Finish for Above-Ambient Temperatures. Requirements for pipe insulation finishes for above-ambient applications are usually governed by location. Appearance and durability are the primary design considerations for indoor applications. For outdoor applications, finishes are provided primarily for weather protection. The finishes may be factory-applied jackets or field-applied metal or polymeric jackets.

On indoor steam and hot-water distribution piping, it is common for flange pairs and flanged fittings such as gate valves, butterfly valves, strainers, pressure-relief valves, and other pipe fittings to be left bare (i.e., uninsulated). Whether this is done intentionally or by neglect, the end result is the same: enormous quantities of wasted thermal energy and, in unventilated spaces, very high air temperatures (Hart 2011). It is recommended that all these fittings be insulated with conventional pipe insulation or with removable/reusable insulation blankets, because a bare 350°F steam pipe indoors loses about eight times more heat than does the same pipe with only 1 in. of conventional pipe insulation. If maintenance personnel need ready access to these flanged fittings, removable/reusable insulation blankets are preferable. ASTM Standard C1695-10 includes different requirements for indoor and outdoor applications of removable/reusable insulation blankets.

Many blankets used indoors are secured in place using hook-and-loop fastening tape, which allows personnel to remove the insulation, perform maintenance, then reinstall the insulation blankets without tools. Removable/reusable insulation blankets are available either as custom-made components or as kits. If specifying custom-made blankets, time must be allowed in the schedule to have a technical support person measure for the insulation, design and fabricate the blankets, deliver them, and install them. Although ASHRAE Standard 90.1-2010 specifies 4.5 to 5.0 in. thick insulation on pipes operating above 350°F, it is recommended that permission be sought to use removable/reusable insulation blankets that are 2 in. thick or less for easier removability and reinstallation.

Insulation Finish for Below-Ambient Temperatures. Piping at temperatures below ambient is insulated to limit heat gain and prevent condensation of moisture from the ambient air. Because metallic piping is an absolute barrier to water vapor, it becomes the condensing surface. Therefore, for high-permeability insulation materials (i.e., permeability/thickness combination > 0.02 perm), the outer surface of the insulation should be covered by a low-permeance membrane. However, some flexible closed-cell insulation materials have been used successfully without a separate vapor retarder material.

Vapor retarders for straight pipe insulation are generally designed to meet permeance, operating temperature, fire safety, and appearance requirements. Sheet-type vapor retarders used on below-ambient pipe insulation should have a maximum permeance of 0.02 perm, when tested per ASTM Standard E96, procedure A (desiccant method) or B (water method). Insulation materials that meet the permeance requirements of an application can be installed without separate vapor retarders, relying on the low permeability and thickness of the insulation material to resist vapor flow, but must be carefully sealed or cemented at all joints to avoid gaps in the insulation. Jacketing used as a vapor retarder may use various materials, alone or in combination, such as paper, aluminum foil, vacuum-metallized or low-permeance plastic films, and reinforcing. The most commonly used products have been ASJ (white), foil-scrim-kraft (FSK), metallized polyester film, or plastic sheeting. An important feature of such jacketing is very low permeance in a relatively thin layer, which provides flexibility for ease of cementing and sealing laps and end-joint strips. This type of jacketing is commonly used indoors without additional treatment. In some cases of operating temperatures below 0°F, multilayer insulation and jacketing may be used. With long lines of piping, insulation should be sealed off every 15 or 20 ft with vapor stops to limit water penetration if vapor retarder damage occurs (see Figure 7).

Insulation fittings are usually vapor-sealed by applying suitable materials in the field, and may vary with the type of insulation and operating temperature. The vapor seal can be a lapped spiral wrap of plastic film adhesive tape or a relatively thin coat of vapor-seal mastic. If these do not provide the required permeance, a common practice is to double-wrap a very-low-permeance plastic film adhesive tape or apply two coats of vapor-seal mastic reinforced with open-weave glass or other fabric.

Vapor retarder mastics should be applied in a greater thickness to achieve a lower permeance. Consult the mastic manufacturer for recommendations on the thickness required to achieve a particular permeance.

Insulated cold piping should receive special attention when exposed to ambient or unconditioned air. Because cold piping frequently operates year-round, a unidirectional vapor drive may exist. Even with vapor-retarding insulation, jackets, and vapor sealing of joints and fittings, moisture inevitably accumulates in permeable insulations. This not only reduces the thermal resistance of the insulation, it also accelerates condensation on the jacket surface with consequent dripping of water and possible growth of mold and mildew. Depending on local conditions, these problems can arise in less than 3 years, or as many as 30 years. Periodic insulation replacement should be considered, and the piping installation should be accessible for such replacement. Very-low-permeability insulating materials, sometimes in combination with vapor retarders, can be used to extend system life and reduce replacement frequency. This is normally done by using vapor retarders, insulation materials (e.g., cellular glass insulation), or both, with a permeance no more than 0.02 perm. However, some flexible closed-cell insulation materials have been used successfully without a separate vapor retarder material. The lower the insulation system’s permeance, the longer its life, given proper installation.

An alternative approach is to accept the inevitable water vapor ingress, and to provide a means of removing condensed water from the system. One means of accomplishing this is the use of a hydrophilic wicking material to remove condensed water from the surface of cold piping for transport (via the combination of capillary forces and gravity) outside the system where it can be evaporated to the ambient air (Brower 2000; Crall 2002; Korsgaard 1993). The wick keeps the hydrophobic insulation dry, allowing the thermal insulation to perform effectively. Dripping is avoided if ambient conditions allow evaporation. The concept is limited to pipe temperatures above freezing.

For dual-temperature service, where pipe operating temperatures cycle, the vapor-seal finish, including mastics, must withstand pipe movement and exposure temperatures without deterioration. When flexible closed-cell insulation is used, it should be applied slightly compressed to prevent it from being strained when the piping expands.

Outdoor pipe insulation may be vapor-sealed in the same manner as indoor piping, by applying added weather protection jacketing without damage to the vapor retarder and sealing it to keep out water. In some instances, heavy-duty weather and vapor-seal finish may be used. It is recommended that weather protection jackets installed over vapor retarders use bands or some other closure method that does not penetrate the weather protection jacketing, because other types have a high likelihood of also penetrating the underlying vapor retarder.

Underground Pipe Insulation. Both heated and cooled underground piping systems are insulated. Protecting underground insulated piping is more difficult than protecting aboveground piping. Groundwater conditions, including chemical or electrolytic contributions by the soil and the existence of water pressure, require special design to protect insulated pipes from corrosion and maintain insulation thermal integrity. For optimal performance, walk-through tunnels, conduits, or integral protective coverings are generally provided to protect the pipe and insulation from water. Examples and general design features of conduits, tunnels, and direct-burial systems can be found in Chapter 12 of the 2020 ASHRAE Handbook—HVAC Systems and Equipment.

Flat, curved, and irregular surfaces, such as tanks, vessels, boilers, and chimney connectors, are normally insulated with flexible or semirigid sheets or boards or rigid insulation blocks fabricated to fit the specific application. Tank and vessel head segments must be curved or flat cut to fit in single piece or segments per ASTM Standard C450. Head segments must be cut to eliminate voids at the head section, and in a minimum number of pieces to minimize joints. Prefabricated flat head sections should be installed in the same number of layers and thickness as the vessel walls, and void areas behind the flat head should be filled with packable insulation. Typically, the curved segments are fabricated to fit the contour of the vessel surface in equal pieces to go around the vessel with a minimum number of joints. Because no general procedure can apply to all materials and conditions, manufacturers’ specifications and instructions must be followed for specific applications.

Securing Methods. Insulations are secured in various ways, depending on the form of insulation and contour of the surface to be insulated. Flexible insulations are adhered to tanks, vessels, and equipment using contact adhesive, pressure-sensitive adhesives, or other systems recommended by the manufacturers. The insulation’s flexibility lends itself to curved surfaces. Rigid or semirigid insulations on small-diameter, cylindrical vessels can be prefabricated and adhered or mechanically attached, as appropriate. On larger cylindrical vessels, angle iron ledges to support the insulation against slippage can supplement banding. Where diameters exceed 10 to 15 ft, slotted angle iron may be run lengthwise on the cylinder, at intervals around the circumference to secure and avoid an excessive length of banding.

Rigid and semirigid insulations can be secured on large flat and cylindrical surfaces by banding or wiring and can be supplemented by fastening with various welded studs at frequent intervals. Springs may be necessary on banding to allow for expansion/contraction of the tank and insulation. On large flat, cylindrical, and spherical surfaces, it is often advantageous to secure the insulation by impaling it on welded studs or pins and fastening it with speed washers. Flexible closed-cell insulations are adhered directly to these surfaces, using a suitable contact adhesive.

Insulation Finish. Insulation finish is often required to protect insulation against mechanical damage and weather, consistent with acceptable appearance. On smaller indoor equipment, insulation is commonly covered with tightly stretched and secured hexagonal wire mesh. Then, a base and hard finish coat of cement is applied, and sometimes painted. For the same equipment outdoors, insulation can be finished with a coat of hard cement, properly secured hexagonal mesh, and a coat of weather-resistant mastic. A variation is to apply only two coats of weather-resistant mastic reinforced with open-mesh glass or other fabric; however, this finish is limited to an operating temperature of about 300°F, because metal expansion can rupture the finish at insulation joints. Larger equipment may be finished indoors and out with suitable sheet metal.

Outdoor finish is generally metal jacketing with a 3 mil thick multilayer moisture barrier factory heat laminated to the interior surface, properly flashed around penetrations (e.g., access openings, pipe connections, and structural supports) to maintain weathertightness. Various outdoor finishes are available for different types of insulations.

For below-ambient operating temperatures, insulation is finished, as required, to prevent condensation and protect against mechanical damage and weather, consistent with acceptable appearance. In accordance with the operating temperature, the finish must retard vapor to avoid moisture entry from surrounding air, which can increase the insulation’s thermal conductivity or deterioration, or corrode the metal equipment surface.

Whenever a vapor retarder is required, all penetrations such as access openings, pipe connections, and structural supports must be properly treated with an appropriate vapor-retarder film, mastic, or other sealant. Equipment must be insulated from structural steel by isolating supports of high compressive strength and reasonably low thermal conductivity, such as a rigid insulation material. The vapor retarder must carry over this insulation from the equipment to the supporting steel to ensure proper sealing.

If the equipment rests directly on steel supports, the supports must be insulated for some distance from the points of contact. Commonly, insulation and vapor retarder are extended four times the thickness of the insulation applied to the equipment.

For dual-temperature service, where vessels are alternately cold and hot, vapor retarder finish materials and design must withstand movement caused by temperature change.

Ducts are used to convey air or process gases for several purposes. In general, their uses can be divided into process ducts and HVAC ducts. Process ducts can range from industrial hot exhaust to subambient process gases, and can be outdoors or indoors. They can need insulation for various reasons, including thermal energy conservation, personnel protection, process control, condensation control and noise attenuation. Because of the wide range of possible operating temperatures and environmental conditions, selection of duct insulation and jacketing materials for industrial processes requires careful consideration of all operational, environmental, and human safety factors. Issues of energy conservation, personnel protection, condensation control, and process control can be solved by careful analysis of heat flow. Computer programs are available for calculating heat transfer, surface temperatures for personnel protection, condensation control, and economic thickness.

HVAC ducts carry air to conditioned spaces inhabited by people, animals, sensitive equipment, or a combination thereof. Thermal and acoustical duct insulation is one of the keys to a well-designed system that provides both occupant comfort and acceptable indoor air quality (IAQ). These insulation products help maintain a consistent air temperature throughout the system, reduce condensation, absorb system operation noise, and conserve energy.

Typical air temperatures for HVAC applications are 40 to 120°F. Because of the more moderate temperature ranges associated with HVAC applications, there is a wide range of insulation materials available. Where acoustical and thermal considerations are significant, sheet metal ducts are often internally insulated, or the ducts themselves are constructed of materials that form both the air-conveying duct and the insulation. Where acoustical concerns are not significant, sheet metal ducts can be externally insulated with rigid, semirigid, or flexible insulation materials. Again, another alternative is to use ducts that incorporate insulation as part of their construction. The determining factor in duct construction and insulation materials selection is often a combination of performance criteria and budgetary limitations.

The need for duct insulation is influenced by the

All HVAC ducts exposed to outdoor conditions, as well as those passing through unconditioned or semiconditioned spaces, should be insulated. Analyses of temperature change, heat loss or gain, and other factors affecting the economics of thermal insulation are essential for large commercial and industrial projects. ASHRAE Standard 90.1 and building codes set minimum standards for thermal efficiency, but economic thickness is often greater than the minimum. Additionally, the standards and codes do not address surface condensation issues. These considerations are often the primary driver of minimum thickness in unconditioned or semiconditioned locations subject to moderate or greater relative humidity.

Duct thermal efficiencies are generally regulated by local or national codes by specifying minimum thermal resistances, or R-values. These R-values are most often determined by testing per ASTM Standards C518 or C177, as required by the Federal Trade Commission (FTC) for reporting R-values of duct wrap insulations. Neither method allows for increased thermal resistance caused by convective or radiative surface effects. To comply with current code language, it is recommended that R-value requirements in specifications for duct insulation be based on Standards C177 or C518 testing at 75°F mean temperature and at the installed thickness of the insulation. Insulation products for ducts are available in a range of R-values, dependent predominantly on insulation thickness, but also somewhat on insulation density.

Temperature Control Calculations for Air Ducts. Duct heat gains or losses must be known for the calculation of supply air quantities, supply air temperatures, and coil loads (see Chapter 17 of this volume and Chapter 4 of the 2020 ASHRAE Handbook—HVAC Systems and Equipment). Heat loss programs based on ASTM Standard C680 may be used to calculate thermal energy transfer through the duct walls. Duct air exit temperatures can then be estimated using the following equations:

then, for warm air ducts,

and for cold air ducts,

where

tdrop	=	temperature loss for warm air ducts, °F
tenter	=	entering air temperature, °F
tgain	=	temperature rise for cool air ducts, °F
texit	=	exit temperature for either warm or cool air ducts, °F
q	=	heat loss through duct wall, Btu / h · ft2
P	=	duct perimeter, in.
L	=	length of duct run, ft
V	=	air velocity in duct, ft/min
Cp	=	specific heat of air, Btu/lbm · °F
ρ	=	density of air, 0.075 lb/ft3
A	=	area of duct, in2
0.2	=	conversion factor for length, time units

Preventing Surface Condensation on Cool Air Ducts. Insulation also can prevent surface condensation on cool air ducts operating in warm and humid environments. This reduces the opportunity for microbial growth as well as other moisture-related building damage. Condensation forms on cold air-conditioning ducts anywhere the exterior surface temperature reaches the dew point. The moisture may remain in place or drip, causing moisture damage and creating a potential for microbial contamination.

Preventing surface condensation requires that sufficient thermal resistance be installed for the condensation design conditions. Figures 7 and 8 give installed R-value requirements to prevent surface condensation on insulated air ducts. The first chart (for emittance = 0.1) should be used for foil-faced insulation products. The second chart is based on materials with a surface emittance of 0.9. The designer must choose the appropriate environmental conditions for the location. It may be financially imprudent to design for the most extreme condition that could occur during a system’s life, but it is also important to be aware that the worst-case condition for condensation control is not the maximum design load of coincident dry bulb and wet bulb. Rather, the worst case for condensation occurs when relative humidity is high, such as when the dry bulb is only very slightly above the wet bulb. This condition often occurs in the early morning in many climates.

For cold-duct applications, it is critical that water vapor not be allowed to enter the insulation system and condense on the cold duct surface. Once water begins to condense, loss of thermal efficiency is inevitable. This leads to further surface condensation on the surface of the insulation. To prevent this, exterior duct insulations must have a low vapor permeance. Permeance values of 0.1 perm or less are generally recommended for cold-duct applications. It is equally important that all exterior duct insulation joints are well sealed. Areas of special concern are often found around duct flanges, hangers, and other fittings.

Insulation Materials for HVAC Ducts. Insulated ducts in buildings can consist of insulated sheet metal, fibrous glass, or insulated flexible ducts, all of which provide combined air barrier, thermal insulation, and some degree of sound absorption. Ducts embedded in or below floor slabs may be of compressed fiber, ceramic tile, or other rigid materials. Depending on the insulation material, there are a number of standards that specify the material requirements.

Duct insulations include semirigid boards and flexible blanket types, composed of organic and inorganic materials in fibrous, cellular, or bonded particle forms. Insulations for exterior surfaces may have attached vapor retarders or facings, or vapor retarders may be applied separately. When applied to the duct interior as a liner, insulation both insulates thermally and absorbs sound. Duct liner insulations have sound-permeable surfaces on the side facing the airstream capable of withstanding duct design air velocities or duct cleaning without deterioration.

Abuse Resistance. One important consideration in the choice of external insulations for air ducts is abuse resistance. Some insulation materials have more abuse resistance than others, but most insulation materials will not withstand high abuse such as foot traffic. In high-traffic locations, a combination of insulation and protective jacketing materials is required. In areas where the insulation is generally inaccessible to human contact, less rigid insulations are acceptable.

One important consideration for internal insulation abuse resistance is that, in large commercial units, it is common for maintenance personnel to enter and move around. In these instances, it is critical that structural elements be provided to keep foot traffic off the insulation.

Duct Airstream Surface Durability. One of the most important considerations in choosing internal duct insulations is resistance to air erosion. Each material has a maximum airflow velocity rating, which should be determined using an erosion test methodology in accordance with either UL Standard 181 or ASTM Standard C1071. Under these methods, the liner material is subjected to velocities that are two and one-half times the maximum rated air velocity. Air erosion testing should include evaluation of the insulation for evidence of erosion, cracking, or delamination.

Additionally, internal insulation must be resistant to aging effects in the air duct environment. Insulation materials have maximum temperatures for prolonged exposure, and some codes impose temperature requirements. Ensure that the material selected will have no aging effects at either the anticipated maximum duct temperature or the temperature specified by the code bodies for the local jurisdiction.

Another durability concern is ultraviolet (UV) resistance. Ultraviolet-generating equipment is used to mitigate microbial activity. Determine, both from the UV equipment manufacturer as well as the insulation manufacturer, whether the anticipated UV exposure poses a threat to the insulation.

Duct Airflow Characteristics. Internal duct insulations and ducts that have insulation as part of their construction have increased frictional pressure loss characteristics compared to bare sheet metal. Generally, duct dimensions are oversized to compensate for the increased frictional pressure losses and the decrease in internal cross section caused by the insulation thickness. However, frictional losses are only part of the total static pressure loss in a duct; dynamic fitting losses should also be considered in any required resizing. Generally, internal linings conform to the shape of the fitting in which they are installed. For this reason, the insulated fitting is assumed to have the same dynamic pressure loss as the uninsulated fitting of the same dimension. See Chapter 21 for further details on frictional pressure loss characteristics of internal linings and how they affect pressure drop and duct-sizing requirements.

Securing Methods. Exterior rigid or flexible duct insulation can be attached with adhesive, with supplemental preattached pins and clips, or with wiring or banding. Individual manufacturers of these materials should be consulted for their installation requirements. Flexible duct wraps do not require attachment except on bottom duct panels more than 24 in. wide. For larger ducts, pins placed at a maximum spacing of 24 in. or less are sufficient. Internal liners are attached with adhesive and pins, in accordance with industry standards.

Leakage Considerations. To achieve the full thermal benefits of insulation, air ducts should be substantially sealed against leakage under operating pressure. The insulation material should not be counted on to provide leakage resistance, unless the insulation is part of the actual duct. Using the case in Example 1, 10% air leakage from an unsealed duct represents an energy loss of 1.66 times the energy lost through heat transfer through the entire 65 ft of uninsulated duct. When that same 10% leakage is compared against an insulated duct, the energy lost through leakage is 15.3 times the losses through heat transfer through the insulation over the entire duct length.

Outdoor Applications. Insulated air ducts located outdoors generally require specific protection against the elements, including water, ice, hail, wind, ultraviolet exposure, vermin, birds, other animals, and mechanical abuse. Strategies for protecting externally insulated ducts located outdoors include protective metal jackets and glass fabric with weather barrier mastic. Note that most of these protective weather treatments do not replace the need for a vapor retarder for cold-duct applications.

The importance of a well-prepared specification to meet energy conservation objectives is paramount. Specifications should

Although each of these steps is important, identifying systems that must be insulated is critical. When defining the materials to be used, it is important to specify them exactly, while allowing for submission of generic equivalents or value-engineered materials. Overspecifying materials limits potentially good options, and underspecifying may allow underperforming materials to be used. A proper balance is needed, and specifying key properties is required. Specifying installation requirements is as important as specifying the correct materials. Specifying procedures for submittals and quality control at the job ensures correctness.

Reference standards from ASHRAE, ASTM, MICA, and others should be incorporated into the specifications; this practice saves time with respect to specification development, and shortens the specification considerably. Specifications that are not reviewed and updated periodically can perpetuate old technologies and obsolete materials, and fall out of code compliance.

Essentially all manufacturers of mechanical insulation products offer guide specifications for their products. These documents are insightful and offer credible information about specific products and the accessories commonly used with them. These documents are widely available on manufacturers’ websites.