2021 IRC Chapter 16
Ducts move conditioned air where it belongs, and done wrong they also move garage fumes, crawl space air, and even fire from one part of a house to another. This lesson covers duct materials, sealing, support, the return-air rules that keep a duct system from pulling in air it should not, and the insulation requirements for ducts that run outside the conditioned space. GCHS staff see duct defects constantly in older rehab housing: an unsealed return in a crawl space, a supply register punched into a garage wall, or flex duct crushed flat behind a closet shelf. None of these show up on a utility bill the way a leaky window does, but each one is a real health or fire-safety issue, not just an efficiency one.
M1601.4.1 requires duct joints and seams to be sealed with a closure system listed and labeled to the standard that matches the duct type: UL 181A for closure systems used with rigid fibrous glass duct board, and UL 181B for closure systems used with sheet metal duct and with flexible duct. Ordinary cloth "duct tape" from the hardware aisle is not listed to either standard and is precisely the product this rule excludes, because its adhesive dries out and the joint reopens within a season or two. A leaky supply duct wastes conditioned air; a leaky return duct is worse, because return ducts run under negative pressure and every leak becomes an intake, pulling in whatever air surrounds the duct at that point, whether that is dusty attic air or contaminated crawl space air.
Factory-made ducts listed to UL 181, including flexible duct, must be supported in accordance with the manufacturer's installation instructions (M1601.4.4), while field- and shop-fabricated metal and flexible ducts follow the SMACNA HVAC Duct Construction Standards for metal and flexible duct. Flexible duct is installed as its manufacturer directs, fully extended and without kinks or crushing: a run squeezed through a tight framing bay and visibly compressed to half its diameter can lose a large share of its airflow, and the fix is to run it fully extended, at the listed support spacing, with generous-radius turns rather than tight bends. Thin wire hangers that have cut into the duct jacket also deserve a second look in the field, because damage at a support point opens seams and restricts the airflow the duct exists to carry.
Ducts that penetrate the wall or ceiling separating the dwelling from an attached garage, and ducts located within the garage itself, must be constructed of minimum No. 26 gage sheet steel or other approved material, and must have no openings into the garage (R302.5.2, referenced by M1601.4.9). A supply register cut into a garage wall and fed from the house system is a straightforward violation twice over: it breaches the fire separation between dwelling and garage, and it creates a path for carbon monoxide and fuel vapors from the garage to enter the duct system and circulate through the living space whenever the blower runs.
Return air is kept out of spaces that would contaminate the air stream. M1602.2 prohibits taking return air from closets, bathrooms and toilet rooms, kitchens, garages, mechanical or boiler or furnace rooms, and unconditioned attics. Two narrow exceptions apply: a kitchen return that serves the kitchen only and is located not less than 10 feet from the cooking appliances, and a dedicated forced-air system serving only the garage. Return air openings also may not be located less than 10 feet, measured in any direction, from an open combustion chamber or draft hood of another appliance in the same room or space, because a return that close can pull the burner flame off its port or pull flue gases straight out of a draft hood and into the duct system. The return air taken from any room may not exceed the supply air delivered to it, and a room that receives supply air also needs some path for that air to return, which is why a closed bedroom with supply but no return path is a problem. A direct connection between an unconditioned crawl space and the return side of a forced-air furnace is likewise not permitted, since it would circulate soil gases, moisture, and mold spores through the whole house on every heating and cooling cycle.
Underground duct systems must be built of approved concrete, clay, metal, or plastic (M1601.1.2). Metal duct in the ground must be protected from corrosion in an approved manner or be completely encased in concrete not less than 2 inches thick, which protects it from crushing under the slab and from soil moisture; ordinary sheet-metal duct laid directly in soil rusts through and then becomes a pathway for soil moisture and gas into the system. Ducts must be sealed, secured, and tested before they are encased or buried. Factory-made air ducts may not be installed in or on the ground, or within masonry or concrete (M1601.4.7), and other ducts must stay at least 4 inches from the earth (M1601.4.8). Round metal duct crimp joints must have a contact lap of not less than 1 inch and be mechanically fastened with at least three sheet-metal screws or rivets equally spaced around the joint (M1601.4.1), so the joint stays mechanically closed and the sealant is not left to do structural work on its own. Where a duct passes through a fire-resistance-rated assembly, such as a townhouse separation wall, the penetration must be protected by an approved method (R302.4.1); an unprotected penetration defeats the rated wall it passes through no matter how well the duct itself is built.
Ducts located outside the conditioned space must be insulated under N1103.3.1: supply and return ducts 3 inches in diameter and larger need at least R-8, and ducts smaller than 3 inches need at least R-6, whether they run through an unconditioned attic, a vented crawl space, or another unconditioned location. An uninsulated duct in an attic loses a substantial share of its capacity to the surrounding temperature extremes, and along the Gulf Coast the bigger problem in cooling season is condensation: a duct surface that drops below the dew point sweats and drips onto the ceiling below it. For that reason a cooling supply duct passing through an unconditioned space conducive to condensation needs a vapor retarder of not more than 0.05 perm, or foil at least 2 mils thick, on the outside of its insulation (M1601.4.6). Gypsum board is sometimes permitted as part of a return air duct or plenum, but only where the air temperature will not exceed 125 degrees Fahrenheit and the exposed surfaces are not subject to condensation (M1601.1.1), since paper-faced gypsum degrades with sustained heat or moisture and sheds particulate into the air stream when it does.
In the field
A GCHS inspector is called to a rehab house where closing a bedroom door makes the furnace "roar and then shut off." The bedroom has an undercut door but no return grille or transfer duct. With the door closed and supply air still entering, the room pressurizes and starves the system's return path, static pressure climbs, and the furnace's high-limit control trips exactly as it is supposed to. The fix a licensed HVAC contractor prices is a return path for that room, a transfer grille, a jump duct, or a properly sized undercut, not a larger blower or a disabled limit switch.
A worked example on concrete cover: a metal supply duct is completely encased in concrete beneath a slab with a measured cover of 1 1/2 inches on one section. Since the code requires not less than 2 inches of concrete around an encased metal duct, that section is 1/2 inch short of the minimum and needs to be re-poured or otherwise brought up to the required thickness before the slab is accepted.
1. Which closure system is acceptable for sealing joints and seams in sheet metal duct?
2. A supply duct passes through the wall separating the house from the attached garage. That duct must be:
3. Which of these is a permitted source of return air for a central forced-air system?
4. A long run of flexible duct is squeezed through a tight framing bay, visibly compressed to about half its diameter. This should be treated as:
5. Supply and return ducts 3 inches in diameter or larger running through an unconditioned attic require insulation of at least:
6. Why is a leaking return duct in a crawl space a health issue and not only an energy issue?
7. Ducts installed in the ground or in contact with earth must be:
8. Where a metal duct is protected by complete encasement in concrete beneath a slab instead of a corrosion coating, the concrete must be not less than:
9. Crimp joints in round metal duct must be lapped and mechanically fastened. The requirement is:
10. Return air openings must not be located less than 10 feet, measured in any direction, from:
11. A supply register is found cut into the wall of an attached garage, fed from the house duct system. The correct finding is:
12. Gypsum board used as part of a return air duct or plenum is permitted only where:
13. A homeowner reports that closing an interior bedroom door makes the furnace "roar and then shut off." The most probable duct-related cause to investigate is:
14. Factory-made ducts listed to UL 181, including flexible duct, must be supported: