❖ Metal ducts are usually constructed of galvanized sheet steel. Duct size is based on required airflow, system pressure, flow velocity and pressure losses caused by friction. Duct material thickness is determined by duct size, static pressure of the system, distance between supports and whether the duct is reinforced. Metal ducts must be constructed with the minimum thicknesses specified in Table M1601.1.1, which bases the minimum required duct thickness on the geometry of the duct, the material used and the major dimension of the duct (the diameter for round ducts and the widest side for rectangular ducts).

Metallic ducts must be constructed to comply with the requirements contained in SMACNA HVAC Duct Construction Standards—Metal and Flexible. This standard contains information on duct reinforcement, joints, fittings, hangers and supports and other pertinent design information needed to achieve a stable, efficient and durable installation of ductwork. While this standard contains the minimum required thickness of duct materials, ducts that are installed within a single dwelling are allowed to be constructed with the minimum thicknesses specified in Table M1601.1.1.

Factory-made ducts must be tested and classified in accordance with the provisions of UL 181. Only Class 0 and Class 1 are recognized by UL 181. Class 0 indicates flame spread and smoke-developed indices of zero; Class 1 indicates a flame spread index not greater than 25 and a smoke-developed index not greater than 50, when tested to ASTM E84. The exception to the 25 flame spread index for nonmetallic ducts is Item 6 of this section, which is intended to allow the installation of plastic ducts above ground where such ducts have a flame spread index of 200 or less. However, it should be noted that there is an ICC committee formal interpretation that opines that plastic ducts are allowed to be installed above ground only if they meet the Class I flame spread index requirement of 25 and smoke development index of 50. Plastic rigid ducts are typically made of PVC that cannot meet a smoke development index of 50.

UL 181 requires that a nonmetallic duct be tested to determine its fire-performance characteristics, corrosion resistance, mold-growth resistance, humidity resistance, leakage resistance, temperature resistance, erosion resistance and structural performance. Air ducts that conform to the requirements of UL 181 are identified by the manufacturer’s or vendor’s name, rated velocity, negative and positive pressure classification and duct material class.

Fibrous ducts are constructed of a composite material of rigid (high-density) fiberglass board and a factory-applied facing, typically reinforced aluminum. The surface of the fibrous duct that is exposed to the airflow is sealed with a fiber-bonding adhesive that prevents erosion of the fiberglass material. The factory-applied exterior duct-board facing contributes to the strength and rigidity of the composite material, acts as a heat reflector, serves as a vapor barrier and is an integral component of the joining method used to construct fibrous ducts. The material is available in board form for shop or field fabrication into rectangular sections or into 10-sided duct form, which approximates a circular cross section. Factory-built round fibrous glass ducts are also available.

Fibrous ducts take advantage of the inherent insulating qualities of the glass fiber material. The air friction factors for fibrous ducts are higher than those for sheet metal because of the relatively rough surface finish of the former.

Construction of fibrous glass ducts must conform to the requirements of the SMACNA Fibrous Glass Duct Construction Standards or NAIMA Fibrous Glass Duct Construction Standards, which provide details for the design and fabrication of air distribution systems using fibrous glass ducts. The SMACNA and NAIMA standards referenced in this section and the previous section are enforceable extensions of the code.

The maximum discharge temperature permitted by industry standards for warm-air heating systems is 250°F (121°C). This section prohibits nonmetallic ducts from being used in applications in which the air temperature would exceed this maximum because the material has not been tested to withstand higher temperatures, and high temperatures will cause accelerated aging of the duct material.

Gypsum board is a composite material commonly used for the construction of air plenums and shafts. Gypsum board can reduce construction costs because it is a common component of building construction assemblies. By serving a dual purpose, gypsum board eliminates the need for independent duct construction. The use of gypsum board to form ducts and plenums is specifically regulated to prevent deterioration of the gypsum board material. Air temperatures that exceed 125°F (52°C) will, over time, dry both the paper facing and the gypsum of the gypsum board, leading to deterioration of the panel.

Gypsum board can also deteriorate when exposed to moisture, which will happen if the surface temperature of the gypsum board is lower than the airstream dew-point temperature, causing water to condense on the surface of the gypsum board. For these reasons, gypsum board cannot be used for air distribution systems using evaporative cooling equipment. It is further restricted to return-air system applications only, a maximum airstream temperature of 125°F (52° C) and an airstream dew-point temperature continuously below the temperature of the gypsum board surface. Evaporative cooling equipment, such as a “swamp cooler,” uses water as a refrigerant. The resulting addition of moisture to the airstream could cause deterioration of the gypsum board.

Stud spaces and joist spaces have commonly been used as plenums in residential construction (see Section N1103.3.5). These spaces are limited to return air plenums because the negative pressures within the return air plenum with respect to surrounding spaces will decrease the likelihood of spreading smoke to other spaces via the plenum. Also, the temperature and moisture content of heated, cooled and conditioned supply air could cause a fire hazard or deterioration of the construction materials exposed in the spaces.

Note that Section N1103.3.5 prohibits the use of all framing cavities as ducts or plenums.

The space must not be a part of a fire-resistancerated assembly because the ASTM E119 test does not consider the impact of air movement within the assembly on the fire-resistance rating. This restriction is a concession to the convenience and cost-saving potential of this method of moving air. The use of stud spaces inherently means the interconnection of different floor levels by the concealed space. Because of the hazard of such an interconnection, the use of this type of plenum is limited to return air from one floor level only for each independent stud cavity. All cavities not used for air movement must be isolated from the plenum by fireblocking constructed and installed in accordance with the code. The use of stud wall cavities as a plenum in outside walls is prohibited due to the difficulty in sealing the space to prevent outdoor air from infiltrating the cavity, which would increase the heating and cooling loads on the system. Note that Section N1103.3.5 prohibits the use of all framing cavities as ducts or plenums.

Commentary Figure M1601.1.1(1) shows an example of an acceptable stud and joist space installation. The bottom plate of the wall is cut away for the plenum to function, and fireblocking is installed in the joist and stud space to limit communication of the plenum with other spaces. The stud cavity shown is being used to return air from one floor level only, conducting it to the space below, where the air-handling equipment is located. Whether viewed as a shaft or as a duct, a stud-cavity plenum penetrates floor assemblies and is, therefore, subject to the floor penetration protection requirements of Section R302.4.1. Commentary Figure M1601.1.1(2) shows an example of an unacceptable stud and joist space installation. Because air is returned from more than one floor level through the same stud cavity, this section prohibits this type of installation because it creates a direct connection from one floor level to another by means of a concealed cavity. Such direct connections would act as a chase or chimney, allowing fire and smoke to spread quickly upward through the building. Thus, stud-cavity return-air plenums are subject to the same restrictions and requirements as floor openings and penetrations regulated by Section R302.4. Stud-space and joist-space plenums must be viewed as an exception to the fireblocking provisions of Section R302.11 because one or more fireblocks (wall plates) must be removed or relocated to construct the plenums. The code does not mention the type of materials allowed for “panning” the bottom of open joists to create joist-space plenums. Traditionally, sheet metal has been used; however, composite materials are also used. The building official must determine what materials are acceptable for joist panning.

Where stud cavities and joist cavities are used for return air, the negative pressure within such cavities can draw in outdoor air at any point where the cavities are adjacent to attic spaces, crawl spaces and outside walls. Sealing of the interface with attics, crawl spaces and outside walls will reduce unwanted infiltration and improve system efficiency. Because it would be nearly impossible to seal a stud cavity plenum space after the wall board is attached to both sides of the studs, sealing can be accomplished by applying a continuous bead of glue or caulking on the studs, on top and bottom plates and on any fireblocking so that the wall board will be sealed to the structural members, thereby preventing air from entering the cavity from adjacent cavities, attics and crawl spaces. Wood studs are not perfectly straight or uniform; therefore, there will always be some gaps where the wall board does not completely contact the stud.

❖ Ducts installed underground must be able to resist the forces imposed on them by the materials that encase them, the forces created by floodwaters in and around them and corrosion. The ASHRAE Handbook of HVAC Systems and Equipment recommends that all underground ducts and fittings be round to provide optimum structural performance. Unlike round ducts, square or rectangular ducts offer little resistance to deformation or collapse caused by the structural loads associated with burial.

Metal ducts must have either a protective coating to resist corrosion or be completely encased in concrete a minimum of 2 inches (52 mm) thick all around. Concrete-encased ducts may eventually corrode; however, the air passageway will be maintained because of the remaining concrete enclosure. All nonmetallic ducts and metallic ducts with factory-applied protective coatings must be approved, and all such duct installations must be in accordance with the manufacturer’s installation instructions. Application of any field-applied protective coating must be approved. Great care must be taken to protect underground ducts from damage prior to placing the concrete or installing the permanent structure above them. Plastic duct and fitting systems designed for underground applications are available and allow corrosion-resistant and waterproof installations.

Plastic ducts have the advantage of being corrosion resistant. The required external loading properties of ASTM D2412 ensure that the pipe has the ability to resist deformation from the loads associated with direct burial. Plastic ducts rapidly lose strength as their temperature approaches their maximum service temperature. At temperatures above 150°F (65.5°C), PVC pipe is substantially weakened and deformation/collapse is possible.

Underground ducts must be sloped to drain to an accessible point in the event that water enters the duct through duct openings or from the subsoil. Water can cause corrosion, deterioration of duct materials, mold growth and duct blockages; therefore, sloping the duct to drain to a collection point will allow removal of water. The code does not require that ducts be watertight. This section requires that the duct be sealed, secured in place and tested before the concrete is poured or the trench is backfilled. It is obviously too late to seal ducts and reseal leaking ducts after concrete or backfill is placed.

❖ Duct sealing is commonly overlooked or poorly performed. The U.S. Environmental Protection Agency (EPA) estimates that 20 percent of the energy efficiency of heating and cooling systems can be lost due to duct air leaks located outside of the conditioned space. With the increased focus on lowering energy use in all types of buildings, substantial gains in energy efficiency can be obtained by making sure that ducts are “substantially” airtight. While approved duct materials have low permeability, the joints in these materials as well as the joints in the connections of these materials to fittings/equipment must be carefully sealed to achieve a “substantially” airtight condition.

Field experience has shown that poor workmanship and failure to follow manufacturer’s instructions has resulted in a high incidence of joint failure in attics and crawl spaces. It is not uncommon to find that air is supplied to or air is returned from an attic because of a separated duct joint. This is obviously a tremendous waste of energy.

In general, joints must be sealed using tapes, mastics, liquid sealants, gasketing or other approved closure systems. A “closure system” consists of the materials and an installation method used to make the joint substantially airtight. This section specifically addresses sealing for three types of duct material which can be accomplished as follows:

In addition to sealing, joints must also be mechanically fastened. In other words, sealing by itself is not a substitute for the mechanical fastening of a duct joint. Two examples are taping a flexible air duct to a fitting and “gluing” a round metal duct joint with caulking. Both practices are not acceptable fastening methods. Mechanical fasteners must be used. This section specifically addresses the mechanical fastening of three types of duct material as follows:

Exception 1 allows spray polyurethane foam insulation to serve as a duct joint sealing method. Spray foam insulation has strong adhesive properties and low porosity such that its application provides an equivalent air-tight condition as compared to other closure systems. Exception 2 allows the spacing of the screws or rivets on round metal ducts to be equally spaced on the portion of duct joint that can be accessed, if the entire perimeter of the joint cannot be accessed. The installation of duct work in close quarters such as in joist spaces and chases, often limits access to the complete circumference of the joint. The installation of three screws or rivets in the portion of joint that can be accessed is a reasonable accommodation for such conditions. For air systems with a static pressure of less than 2 inches water column (0.50 kPa), Exception 3 allows for longitudinal (including spiral-type) welded joints and locking-type joints and seams to not be sealed. A welded or locking-type (e.g., Pittsburgh lock) joint is sufficiently air tight at low pressures such that joint sealing would be of limited value. Snap-lock and button-lock type joints located outside of conditioned spaces (e.g., crawl spaces, attics, outdoors) do not fall under Exception 3 and must be sealed because such joints will leak and the leakage will not be into the conditioned space. Note that some duct manufacturers provide snap-lock joints that contain a factory-applied sealant in the locking groove that serves to seal the joint when assembled. S-slip joints, drive joints and lap joints must always be sealed. See Commentary Figure M1601.4.1(2).

❖ The crimped end of a round duct is the male end and it points in the direction of flow. The minimum required lap creates joint stability and allows for proper mechanical fastening. Crimped joints can be exceptionally leaky, depending on the length of lap and the depth of the crimp valleys. Section M1601.4.1 requires such joints to be sealed.

❖ Plastic ducts are typically installed underground. If plastic ducts are installed, this section requires the installer to follow the duct manufacturer’s instructions for making joints in the duct. Some manufacturers require joints to be solvent cemented while others require the joints to be caulked. Section M1601.4.1 does not address plastic ducts.

❖ These material requirements and the 10-foot (3048 mm) maximum spacing requirement prescribe structural support for metal ductwork to limit deflection and maintain alignment. Nonmetallic ducts must be supported in accordance with the manufacturer’s installation instructions because these ducts are produced in various configurations. See Commentary Figure M1601.4.1(1) for information on nonmetallic duct support.

❖ Section R602.8 refers to Section R302.11 and requires that all openings around ducts at ceiling and floor levels must be fireblocked. Fireblocking retards the spread of fire to other areas in the dwelling.

❖ Where a duct is installed outdoors or in an unconditioned area (such as an attic or crawl space), it can be exposed to humidity and temperature differentials that can create condensation on the outside of the duct. A vapor retarder must be installed on the exterior of the duct to protect the duct and/or insulation from damage caused by moisture. Duct insulation and coverings must not penetrate fireblocked assemblies. If the exterior insulation and vapor retarder burn in a fire, a pathway will open for the fire to spread through the penetration in the structure for the duct.

The exterior of cooling supply ducts that are insulated with spray polyurethane foam which has a maximum water vapor permeance of 3 perm per inch does not require a vapor retarder or aluminum foil. Spray polyurethane foam has been successfully used without a vapor barrier in other applications, such as exterior walls. The natural low permeance of the installed material makes a vapor barrier over the spray foam insulation redundant.

❖ Factory-made air ducts must be listed and labeled for underground installations. Ducts not listed for inground installations might not have sufficient strength to withstand the loads applied by these types of installations and might not be suitable for high-moisture areas.

❖ A physical separation from earth is the best method of protecting metallic and nonmetallic ducts from the effects of corrosion and moisture.

❖ Ducts in garages and ducts penetrating separation walls or ceilings between garages and living spaces must be designed to prevent fire and smoke from easily entering the living spaces (see commentary, Sections R302.5.2 and M1601.6). Section R302.5.2 is intended to protect the penetration of the fire separation between a dwelling and an attached garage, and also to prevent contaminants in the garage from entering the living spaces. The code assumes that the 26-gage steel ductwork and the furnace/air-handler casing will offer a significant impediment to the spread of fire from the garage to the dwelling interior. Placing furnaces, air handlers and ductwork in unconditioned spaces is becoming less common because of energy saving efforts. Concern has been expressed by some that having a furnace/air handler with nonmetallic access doors to the blower or filter compartments creates a weak spot in the steel duct barrier. It is the author’s opinion that Section R302.5.2 is violated where any air-handling equipment in the garage has access doors that are less resistant to fire than 26 gage steel ducts. The code official can approve materials other than steel in accordance with Sections R302.5.2 and R104.11.

❖ In buildings and structures located in flood hazard areas, ducts must be installed above the design flood elevation or must be capable of preventing water from entering the ducts and capable of withstanding the forces of buoyancy and moving water (see commentary, Section R322.1.6).

❖ Loose combustible scrap must be removed from an under-floor space used as a plenum. The space must be substantially airtight because the furnace will place the plenum under a slight positive pressure to force conditioned air out of the registers into the rooms above. If there are leaks in the plenum walls, the conditioned air will escape to the outdoors and result in energy loss and the creation of negative pressure in the living space.

Efforts to rod drains through cleanouts located in under-floor plenums could create spills and splashes of sewage in the plenum and cleanout openings might be carelessly left open allowing sewer gases to enter the plenum. Because this would create an insanitary air condition for the building occupants, plumbing cleanouts are prohibited in these spaces. An issue that often comes up is whether fuel gas piping is prohibited in under-floor plenums. Chapter 24 contains the requirements for fuel-gas piping and does not prohibit gas piping in such spaces.

❖ The enclosing materials of an under-floor plenum, including the sidewall insulation, are limited to materials having a flame spread index less than or equal to 200 because the plenum space could introduce smoke and fire to the living space.

❖ A noncombustible receptacle, (i.e., a pan) must be installed below each of the supply openings in the floor. This receptacle (pan) must be suspended from the floor members and must be located not greater than 18 inches (457 mm) below the floor opening so that items and debris dropped into floor openings can be easily retrieved by reaching down through the opening. The receptacle (pan) must extend 3 inches (76 mm) beyond the opening on all sides and have a vertical lip at least 1-inch (25 mm) high to keep items from sliding or rolling off the edges of the pan. The requirement for the pan material to be noncombustible anticipates the potential for burning material being dropped into the opening.

❖ An 18-inch by 24-inch (457 mm by 610 mm) or larger opening in the floor is required to provide access to the under-floor plenum to permit maintenance and repairs in the crawl space.

❖ Because exposed wood joists and flooring often form under-floor plenums, the outlet air temperature from the furnace must be limited to 200°F (93°C).

Furnaces must also be equipped with an automatic control that starts the air circulation fan before the air temperature in the furnace at the heat exchanger exceeds 150°F (66°C). This requirement serves to limit plenum air temperature. Note that replacement furnaces will not likely be available that can meet these control requirements and altering the furnaces would violate the listing of the appliance.