In the U.S. private sector, each new construction or renovation project is normally governed by state laws or local ordinances that require compliance with specific health, safety, property protection, and energy conservation regulations. Figure 1 shows relationships between laws, ordinances, codes, and standards that can affect design and construction of HVAC duct systems (note that it may not list all applicable regulations and standards for a specific locality). Typically, local building ordinances invoke NFPA Standards 90A and 90B or some variation thereof along with one or more of the national codes (see Figure 1) for HVAC duct system construction and installation.

Figure 1. Hierarchy of Building Codes and Standards

Because safety codes, energy codes, and standards are developed independently, the most recent edition of a code or standard may not have been adopted by a local jurisdiction. HVAC designers must know which codes apply to their designs. If a provision conflicts with the design intent, the designer should resolve the issue with local building officials. New or different construction methods can be accommodated by the provisions for equivalency incorporated into codes. codes. Local jurisdictions may also include amendments and additions to various sections of a referenced building code.

Duct construction static pressure classifications typically used on contract drawings and specifications are summarized by Table 1. The classifications are from SMACNA (2005) for sheet metal ductwork, and NAIMA (2002a) for fibrous glass duct board. Negative-pressure flat oval duct systems can be designed by using +10 in. of water sheet gages with the negative-pressure rectangular duct reinforcement members mechanically attached or welded to the duct. The most common flexible air ducts and air connectors are listed up to 10 in. of water maximum positive-pressure ratings and anywhere from 0.5 to 2.0 in. of water negative-pressure ratings, but there are listed flexible ducts with pressures as high as 16 in. of water and as low as −12 in. of water.

Air conveyed by a duct adds both static pressure and velocity pressure loads on the duct’s structure. The load from static pressure differential across the duct wall normally dominates and the mean static pressure is generally used for duct pressure classification. Turbulent airflow adds relatively low but rapidly pulsating loading on the duct wall.

Duct design is based on total pressure calculations as discussed in Chapter 21 of the 2017 ASHRAE Handbook—Fundamentals. From these calculations, the designer should specify the static pressure classification of the various duct sections in the system. All modes of operation must be considered, especially in systems used for smoke management and those with fire dampers that must close when the system is running.

Ducts can collect dirt and moisture, which can contribute to microbial growth. Design, construct, and maintain ducts and attached HVAC system components to minimize the opportunity for growth and dissemination of microorganisms. As described by NIH (2015), recommended control measures to reduce duct contamination include (1) locating air intakes away from contaminant sources, (2) providing and maintaining proper air filtration, (3) sealing ductwork during construction to prevent debris from entering, (4) inspecting new ductwork for oil or debris prior to system startup and cleaning if needed, and (5) implementing good housekeeping in occupied spaces.

Owners should routinely inspect ducts for cleanliness. If cleaning is needed, contaminant sources should be identified and controlled before cleaning is performed. NADCA (2013) and NAIMA (2002b) provide specific information and procedures for cleaning ducts. Fabric duct systems should be cleaned (washed) in accordance with manufacturer’s instructions. To minimize air leakage after duct sections are opened or disconnected, attention should be given to properly closing, connecting, and resealing them (see the section on HVAC System Leakage). Also, any duct insulation removed must be replaced, and vapor barriers (if used) must be resealed.

For the purposes of this chapter, ductwork includes straight duct, flexible duct, sheet metal and rigid fiberglass plenums, and fittings (e.g., elbows, transitions, tees, wyes) for distribution and extraction of air. It does not include duct-mounted components (e.g., terminal units, access doors/panels, attenuators, coils, fire/smoke dampers, balancing and control dampers). A system consists of the supply air handler, return fan, exhaust fan, plenums, and all ductwork that connects the air handler to the conditioned space. The system also includes duct-mounted components where leaks through joints or penetrations can occur. However, leakage from the occupied space into a return air plenum is not included.

HVAC system air leakage increases building energy consumption. It also reduces the system’s ability to control and deliver intended flows and pressures, and to manage the extraction and dilution of contaminants. Also, leakage can cause noise problems, drafts in the conditioned space, and dirt and dust deposits on the duct exterior. Leakage energy impacts depend on building and system type. For small buildings with single-zone air distribution systems served by equipment such as packaged rooftop cooling units and furnaces (e.g., houses, commercial buildings with floor area less than 25,000 ft²), 75 to 95% of the HVAC site energy is used for space heating and cooling (Thornton et al. 2010; Walker and Sherman 2008; Zhang et al. 2010), and the impacts are mostly on the thermal side. For large buildings with central multizone air distribution systems served by equipment such as central chillers and boilers (e.g., mid- and high-rise offices, supermarkets and retail stores with a floor area of 25,000 ft² or more), 20 to 80% of HVAC site energy is used by fans (Huang et al. 1991; Leach et al. 2009, 2010) and the impacts are also on fan power. All of these effects are strongly influenced by the location of leaks relative to conditioned space.

If supply air leaks to an unconditioned attic or crawlspace, or to the outdoors, the lost heating or cooling must be replaced. In this case, heating or cooling fluid flows or temperature differences must increase, or the system must run longer to meet the load. If, instead, supply air leaks to a ceiling return plenum adjacent to conditioned spaces, the fan must run faster or longer, and the largest impact may be on fan power. The heat associated with this added fan power also creates an additional cooling load. The effects of return leakage can also be significant. For example, leaks from a hot attic into a return duct heat the return air, which in turn reduces system cooling capacity.

Because the relationship between fan power and airflow is somewhere between a quadratic and cubic function depending on the system type (Sherman and Wray 2010), an increase in airflow to provide the desired service and compensate for system leakage means that fan energy consumption increases significantly. Field measurements by Diamond et al. (2003) showed that a leaky VAV system (10% leakage upstream and 10% downstream of terminal box inlet dampers at operating conditions) uses 25 to 35% more fan energy than a tight system (2.5% upstream and 2.5% downstream at operating conditions). For an exhaust system with 20% leakage, the fan has to move 25% more air to meet the specified flows at the grilles, which causes fan power to increase 95%.

 Leakage Testing

 Rationale.

 System leakage testing, including ducts and duct-mounted components, is recommended, because leakage data collected by researchers over several years (1998 through 2004) for 10 systems in nine U.S. large commercial buildings [see Figure 2 in Wray et al. (2005)] showed that seven had substantial leakage flows (10 to 26%). The other three systems had much smaller leakage flows (3 to 4%). These data include variable-air-volume (VAV), constant-air-volume (CAV), and dual-duct systems, and both high-pressure (main) and low-pressure (branch) system sections. The distinction between main and branch sections is their location relative to terminal box inlet dampers. For VAV systems, data include fan-powered and cooling-only boxes. Most systems supplied air through square diffusers, but some used slot diffusers. Ductwork pressurization tests conducted by Lawrence Berkeley National Laboratory (LBNL) on the same nine buildings at a test pressure of 1 in. of water also showed that, on average, the leakage area for branches was about three times more than for mains [see Figure 1 in Wray et al. (2005)].

 Scope.

 It is recommended that supply air (both upstream and downstream of the VAV box primary air inlet damper), return air, and exhaust air systems be tested for leakage during construction to verify (1) good workmanship, and (2) the use of low-leakage components as required to achieve the design allowable system leakage.

Systems may be tested at operating conditions and/or during construction before the installation of insulation and concealment of ductwork, but after the system section to be tested is fully assembled. As a minimum, 25% of the system, based on duct surface area, should be tested during construction and another 25% if any of the initial sections fail. If any section of the second 25% fails, the entire system should be leakage tested. Sections should be selected randomly by the owner’s representative. Leakage tests should be conducted by an independent party responsible to the owner’s representative.

 Procedures.

 Leakage test setups and test procedures should be in compliance with industry practice (e.g., AABC 2002, Chapter 5; ASHRAE Standard 126-2016: Section 7; EUROVENT 2/2 1996; SMACNA 2012: Sections 4, 6, and 7).

Figure 2. Relationship Between Leakage Class and EUROVENT (1996) Airtightness Classes used by European Countries

 Instrumentation.

 Temperature and pressure measurement should be in compliance with ASHRAE Standards 41.1 and 41.3. Flow measurement devices (e.g., fan inlet flow stations, flow capture hoods) should have an accuracy of 3% or better over the operating range of interest. The size of the tested system section should be large enough that the instrumentation can measure leakage flow with the required accuracy.

 Acceptance Criteria.

 The design engineer should establish the allowable system leakage rate for each fan system as a percentage of fan airflow at the design (maximum) system operating conditions. Recommended test pressures and maximum system leakage are shown in Table 2, except as noted otherwise (supply and return system sections that leak directly to/from the outdoors, exhaust systems that draw in air directly from the indoors, and air-handling units).

Commentary: Five percent system leakage is attainable in large commercial buildings, as evidenced by the supply systems with low leakage (3 to 4%) reported by Wray et al. (2005). Also, Wray and Matson (2003) indicate that system leakage operating cost impacts are about $0.14 to $0.18 per square foot of duct surface area for a 20% leaky supply system compared to a 5% leaky system in a large commercial building with ceiling return plenums. System sealing costs vary with fitting-to-straight-duct ratio, pressure class, and other system variables. According to Tsal et al. (1998), an upper bound for the one-time cost of duct sealing is $0.25/ft2 of duct surface area. Using these costs, the payback period for achieving 5% leakage compared to 20% leakage is about 1 to 2 years. The recommended 5% value may be modified when further life-cycle cost analysis data become available.

Supply and return sections that leak to/from outdoors should be tighter than indoor sections, because the added thermal losses/gains from these leaks increase energy impacts. For example, an airtight VAV supply system with a design fan pressure rise of 4 in. of water, a design fan flow of 14,000 cfm, and a total efficiency of 60% will require about 11,000 W of power. With 5% leakage and applying an exponent of 2.4 to the flow ratio (1.05), the fan power increases by about 12% or 1400 W. Assuming that 55°F air is supplied to maintain a room temperature of 74°F, the supply air energy lost to outdoors by the 740 cfm of leakage is about 4600 W and the total energy impact of the leakage is about 6000 W. To achieve the same total energy impact as only the fan power increase with 5% leakage, leakage for the outdoor sections would need to be limited to about 1%.

However, this analysis is overly simplified, because it ignores the thermal impacts of leakage if the supply sections were located indoors instead. In some cases, the thermal impacts of leakage from indoor sections may be as much as the related fan power impacts. Accounting for these thermal effects is more complicated because of interactive system effects (e.g., supply-leakage-related cooling of a ceiling return plenum, which in turn reduces ceiling heat transfer to adjacent cooler conditioned spaces, and thus reduces supply airflows needed to manage space cooling loads). Energy simulation programs can be used to conduct detailed analyses of the thermal and fan power effects of supply leakage (Wray and Sherman 2010).

For buildings without static pressure control of the conditioned space (e.g., a midrise apartment building), exhaust sections that draw in indoor air through leaks need to be tighter than outdoor exhaust sections because the leakage flow also tends to depressurize the building and increase building air infiltration (assuming that the fan has been adjusted to still provide the design airflow at the exhaust inlets). The ratio of the change in infiltration flow to the change in exhaust flow from leakage depends on environmentally driven pressures, which can vary widely depending on weather. A reasonable average for this ratio is about 0.6 (Woolley et al. 2014). For example, an exhaust system with a design fan pressure rise of 2 in. of water, a design fan flow of 5000 cfm, and a total efficiency of 60% requires about 2300 W of power. With 5% leakage and applying an exponent of 3 to the flow ratio (1.05), the fan power increases by about 16%, or 310 W. Using a ratio of 0.6, the change in infiltration due to the 5% leakage (about 260 cfm) will be about 160 cfm. Assuming that the outdoor design temperature is 32°F and the conditioned space is heated to a temperature of 72°F, the energy associated with increased infiltration is about 3900 W and the total energy impact of the leakage is about 4200 W. To achieve the same total energy impact as only the fan power increase with 5% leakage, leakage in this case will need to be limited to about 0.4%. Even tighter systems are required for colder outdoor design temperatures, whereas outdoor temperatures closer to the conditioned space temperature will allow more leakage (to a maximum of 5%).

As a compromise so that the designer does not need to carry out additional calculations for supply and return sections that leak to/from outdoors and for exhaust sections that draw in indoor air through leaks, it is recommended that leakage for these sections be limited to 2%.

 Leakage Class.

 Leakage class, as specified by ASHRAE Standard 90.1-2013, can be translated to fractional (%) leakage using Equation (1) or Table 3.

(1)

where

Qleak,frac	=	leakage fraction of fan airflow, %
CL	=	leakage class, cfm per in. of water0.65 per 100 ft2 of duct surface area
p	=	system pressure difference, in. of water
Qfan,norm	=	normalized fan airflow, cfm per ft2 of duct surface area

For example, for a leakage class of 4 cfm per in. of water^0.65 per 100 ft² of duct surface area with a pressure difference of 3 in. of water and an inlet flow of 2 cfm/ft² of duct surface area, the fractional leakage is 4.1%. For a pressure difference of 6 in. of water, the fractional leakage is 6.4%.

Table 2 Recommended Maximum System Leakage Percentages

Type of System	System Condition	Test Pressure,a,b in. of water	Maximum Fan Systemc Leakage
1. Fractional horsepower systems, small exhaust/return systems, residential systems	Operating	Operating pressure	5%
	During construction	0.5	5%
2. Single-zone supply, return, or exhaust systems	Operating	Operating pressure	5%
	During construction	2.0	5%
3. Multizone supply, return, or exhaust systems	Operating	Operating pressure	5%
	During construction	2.0	5%
4. Dual-duct supply systems, both hot and cold ducts	Operating	Operating pressure	5%
	During construction	6.0	5%
5. VAVd and CAVd supply systems	Operating	Operating pressure	5%
	During construction	Upstream box: 4.0	5%
		Downstream box: 1.0	5%
6. VAVd or CAVd return systems	Operating	Operating pressure	5%
	During construction	Downstream box: 3.0	5%
		Upstream box: 1.0	5%
7. Chilled-beam systems	Operating	Operating pressure	5%
	During construction	4.0	5%
8. High-pressure induction systems	Operating	Operating pressure	5%
	During construction	6.0	5%
9. Supply and return ductwork located outdoors	Operating	Operating pressure	2%
	During construction	3.0	2%
10. Exhaust ductwork located indoors	Operating	Operating pressure	2%
	During construction	3.0	2%
11. Air-handling units	Site test by manufacturer	Specified design pressure	1%
Notes: a Test pressure should not exceed duct pressure rating. b It is recommended that duct pressure rating equal fan shutoff pressure if possibility of fan shutoff exists either in VAV systems or in systems with smoke/fire damper control. c A fan system includes ductwork upstream and downstream of the fan. The fan system also includes components mounted to that ductwork where leaks through joints or penetrations can occur. However, leakage from the occupied space into a return air plenum is not included. Separate leakage specifications should be used for upstream and downstream sections. d Assuming primary air damper located at box inlet. If damper is at box outlet, then box should be included in upstream leakage testing.

Table 3 Leakage as Percentage of Flow

Leakage Class*	Qfan/AS , cfm/ft2 Duct Surface Area	Static Pressure, in. of water
cfm per 100 ft2 per in. of water0.65		0.5	1	2	3	4	5	6	8	10
12	2	3.8	6.0	9.4	12.3	14.8	17.1	19.2	23.2	26.8
	2.5	3.1	4.8	7.5	9.8	11.8	13.7	15.4	18.5	21.4
	3	2.5	4.0	6.3	8.2	9.8	11.4	12.8	15.5	17.9
	4	1.9	3.0	4.7	6.1	7.4	8.5	9.6	11.6	13.4
	5	1.5	2.4	3.8	4.9	5.9	6.8	7.7	9.3	10.7
6	2	1.9	3.0	4.7	6.1	7.4	8.5	9.6	11.6	13.4
	2.5	1.5	2.4	3.8	4.9	5.9	6.8	7.7	9.3	10.7
	3	1.3	2.0	3.1	4.1	4.9	5.7	6.4	7.7	8.9
	4	1.0	1.5	2.4	3.1	3.7	4.3	4.8	5.8	6.7
	5	0.8	1.2	1.9	2.5	3.0	3.4	3.8	4.6	5.4
4	2	1.3	2.0	3.1	4.1	4.9	5.7	6.4	7.7	8.9
	2.5	1.0	1.6	2.5	3.3	3.9	4.6	5.1	6.2	7.1
	3	0.8	1.3	2.1	2.7	3.3	3.8	4.3	5.2	6.0
	4	0.6	1.0	1.6	2.0	2.5	2.8	3.2	3.9	4.5
	5	0.5	0.8	1.3	1.6	2.0	2.3	2.6	3.1	3.6
3	2	1.0	1.5	2.4	3.1	3.7	4.3	4.8	5.8	6.7
	2.5	0.8	1.2	1.9	2.5	3.0	3.4	3.8	4.6	5.4
	3	0.6	1.0	1.6	2.0	2.5	2.8	3.2	3.9	4.5
	4	0.5	0.8	1.2	1.5	1.8	2.1	2.4	2.9	3.4
	5	0.4	0.6	0.9	1.2	1.5	1.7	1.9	2.3	2.7
2	2	0.6	1.0	1.6	2.0	2.5	2.8	3.2	3.9	4.5
	2.5	0.5	0.8	1.3	1.6	2.0	2.3	2.6	3.1	3.6
	3	0.4	0.7	1.0	1.4	1.6	1.9	2.1	2.6	3.0
	4	0.3	0.5	0.8	1.0	1.2	1.4	1.6	1.9	2.2
	5	0.3	0.4	0.6	0.8	1.0	1.1	1.3	1.5	1.8
*Consult Figure 2 for the relationship of leakage class and airtightness class used by European countries.

 Calculating Test Section Allowable Leakage.

 Determine from the drawings the airflow and duct surface area in each section and then calculate the allowable leakage (see Example 1).

If an entire system or section of ductwork is not to be tested, determine the allowed leakage in the test section. To do this, determine the surface area of the test section, and divide that by the total surface area of the entire section. Multiply this test section leakage percentage by the section airflow to determine the test section leakage airflow.

Example 1.

A system consists of three sections, with airflow and surface areas as summarized in Table 4. Determine the allowable leakage for a 5000 ft² surface area section of Section 1 identified by the owner’s representative. Branch 3 is located outdoors.

Solution. Table 5 summarizes the calculations. By test, the 5000 ft² test section cannot exceed 700 cfm at 4.0 in. of water.

Custom-designed air-handling units (AHUs) should be leak tested after the AHU is reassembled on site with all penetrations (e.g., for controls, electrical, and piping) in place and sealed. Leakage tests should be conducted by the AHU manufacturer in accordance with ASHRAE Standard 126-2015, Section 7, at design operating pressures (positive and negative) specified by the design engineer, and be witnessed by a representative of the owner. AHUs should be shipped with blankoffs and a round flanged opening for the flow meter/blower leakage test unit. It is recommended that packaged AHUs also be leak tested.

The casing leakage for AHUs should not exceed 1% of design airflow at the specified design pressure.

NFPA Standard 91 is widely used for design, construction, installation, and maintenance of duct systems conveying particulates and removing flammable vapors (including paint-spraying residue), and corrosive fumes. Industrial duct systems are generally classified as follows:

For contaminant abrasiveness ratings, see SMACNA’s (1999, 2004) round or rectangular industrial duct construction standards.Consult Chapters 14 to 33 of the 2019 ASHRAE Handbook—HVAC Applications for specific processes and uses.

Antimicrobial-treated ducts are coated (as a precoating or after fabrication) with a substance that inhibits the growth of bacteria, mold, and fungi (including mildew). Antimicrobial-treated galvanized steel, stainless steel, and aluminum ducts can be used when the service temperature of the antimicrobial compound is not exceeded. Prefabricated coatings allow metal to be pressed, drawn, bent, and roll-formed without coating loss. Imperfections in metal ducts, such as spot welds and welded joints, can be repaired with a touch-up paint of the antimicrobial compound.

Fabrics can be made inherently antimicrobial by combining the antimicrobial chemistry into the polymer fibers of the product. Glass fiber duct liners can be made resistant to bacterial and fungal growth using a biocide that protects the airstream surface from microbial growth.

All antimicrobial coatings or touch-up paints should be EPA-registered antimicrobial compounds, tested per ASTM Standard E84, survive minimum and maximum service temperature limits, and comply with NFPA Standards 90A and 90B. Coatings should have flame spread/smoke developed ratings not exceeding 25/50, and meet local building code requirements.

The Thermoplastic Duct (PVC) Construction Manual (SMACNA 1995) covers thermoplastic (polyvinyl chloride, polyethylene, polypropylene, acrylonitrile butadiene styrene) ducts used in commercial and industrial installations. SMACNA’s manual provides comprehensive polyvinyl chloride duct construction details for positive or negative 2, 4, 6, and 10 in. of water. NFPA Standard 91 provides construction details and application limitations for plastic ducts. Model code agencies publish evaluation reports indicating terms of acceptance of manufactured ducts and other ducts not otherwise covered by industry standards and codes.

Physical properties, manufacture, construction, installation, and methods of testing for fiberglass-reinforced thermosetting plastic (FRP) ducts are described in the Thermoset FRP Duct Construction Manual (SMACNA 1997). These ducts are intended for air conveyance in corrosive environments as manufactured by hand lay-up, spray-up, and filament winding fabrication techniques. The term FRP also refers to fiber-reinforced plastic (fibers other than glass). Other terms for FRP are reinforced thermoset plastic (RTP) and glass-reinforced plastic (GRP), which is commonly used in Europe and Australia. SMACNA (1997) has construction standards for pressures up to ±30 in. of water, temperatures up to 180°F, and duct sizes from 4 to 72 in. round and 12 to 96 in. rectangular.

Air dispersion systems are designed to both convey and disperse air in the space being conditioned. Diffusion options include linear vents, nozzles, orifices, and porous fabrics. There are three typical cross-sectional shapes: semicircular D-shape, quarter round, and cylindrical (most common). D-shape and quarter round are normally mounted to a surface (wall or ceiling), whereas cylindrical are suspended from the ceiling. The cross-sectional area is based on the interior air velocity.

Typically, these systems are made of fabric, but also include sheet metal or plastic film, including porous and nonporous options. UL Standard 2518 is the recognized standard to evaluate fabric air dispersion systems and their materials for safety and building code requirements.

Figure 3. Fabric Duct with Nozzle Outlets

Consult the manufacturer for design criteria for selecting air dispersion type [linear vents, orifices, nozzles, microperforated (porous weave) fabric], fabric (porous or nonporous, color, weight, and construction), suspension options (cable or track options), and installation instructions. In design, consider velocities of 1000 to 1600 fpm at static pressures of 0.3 to 1.0 in. of water to ensure proper airflow performance. Excessively turbulent airflow (from metal fittings or fans) or higher inlet velocities can cause fabric fluttering, excessive noise, premature material failure, and poor air dispersion. Fabric airflow restriction devices are available to help balance static pressures, reduce turbulence, reduce abrupt inflation, and balance airflow into branch ducts.

Figure 4. Fabric Duct with Nozzle Outlets

No comprehensive standards exist for underground air duct construction. Coated steel, asbestos cement, plastic, tile, concrete, reinforced fiberglass, and other materials have been used. Underground duct and fittings should always be round and have a minimum thickness as listed in SMACNA (2005), although greater thickness may be needed for individual applications. Specifications for construction and installation of underground ducts should account for the following: water tables, ground surface flooding, the need for drainage piping beneath ductwork, temporary or permanent anchorage to resist flotation, frost heave, backfill loading, vehicular traffic load, corrosion, cathodic protection, heat loss or gain, building entry, bacterial organisms, degree of water- and airtightness, inspection or testing before backfill, and code compliance. Chapter 12 has information on cathodic protection of direct- buried conduits. Residential installations may also be subject to the requirements in NFPA Standard 90B. Commercial systems also normally require compliance with NFPA Standard 90A.

Figure 5. Fabric Duct with Porous Material as Air Outlet

Exposed ducts and their sealant/joining systems must be evaluated for the following:

In addition, supports must be custom-designed for rooftop, wall-mounted, and bridge or ground-based applications. Specific requirements must also be met for insulated and uninsulated ducts.

Seismic analysis of duct systems may be required by building codes or federal regulations. Provisions for seismic analysis are given by the Federal Emergency Management Agency (FEMA 2009). Ducts, duct hangers, fans, fan supports, and other duct-mounted equipment are generally evaluated independently. Chapter 55 of the 2019 ASHRAE Handbook—HVAC Applications gives design details. SMACNA (2008b) provides guidelines for seismic restraints of mechanical systems and gives bracing details for ducts, pipes, and conduits that apply to the model building codes and ASCE Standard 7. FEMA (2013a, 2013b, 2014) has three fully illustrated guides that show equipment installers how to attach mechanical and electrical equipment or ducts and pipes to a building to minimize earthquake damage.

AWS (2012) covers sheet metal arc welding and braze welding procedures. It also addresses the qualification of welders and welding operators, workmanship, and the inspection of production welds.

Insulation materials for ducts, plenums, and apparatus casings are covered in Chapter 23 of the 2017 ASHRAE Handbook—Fundamentals. Codes generally limit factory-insulated ducts to UL Standard 181, Class 0 or 1. Commercial and Industrial Insulation Standards (MICA 2014) gives insulation details. ASTM Standard C1290 gives specifications for fibrous glass blanket external insulation for ducts.

Master specifications for duct construction and most other elements in building construction are produced and regularly updated by several organizations. Two examples are MASTERSPEC by the American Institute of Architects (AIA) and SPECTEXT^®by the Construction Sciences Research Foundation (CSRF). MasterFormat™ (CSI 2014) is the organization standard for specifications. These documents are model project specifications that require editing to customize each application for a project. It is the design engineer’s responsibility to create clear construction specifications.