What is ductwork?

What does ductwork mean?

Ductwork means the network of ducts that carries heated or cooled air from a home's furnace, heat pump or air handler out to the rooms, and back again. ENERGY STAR describes its job in forced-air homes, and notes how common ducted systems are in American homes: “In houses with forced-air heating and cooling systems, ducts are used to distribute conditioned air throughout the house.” Source: ENERGY STAR (U.S. EPA and U.S. Department of Energy), read 2026-10-29. “Ducted ASHP systems can be installed and connected to the conventional forced-air ductwork system that is typical of most American homes.” Source: ENERGY STAR (U.S. EPA and U.S. Department of Energy), read 2026-10-29.

What is ductwork made of?

Home ductwork is made from a few materials: bare sheet metal, rigid fiber glass duct board, sheet metal lined inside with fiber glass, and flexible duct. The EPA describes how the mix of materials in houses has changed over the years: “You may be familiar with air ducts that are constructed of sheet metal.” Source: U.S. Environmental Protection Agency, read 2026-10-29. “However, many modern residential air duct systems are constructed of fiber glass duct board or sheet metal ducts that are lined on the inside with fiber glass duct liner.” Source: U.S. Environmental Protection Agency, read 2026-10-29.

Where is ductwork located?

Ductwork usually runs out of sight, through walls, ceilings, attics, crawlspaces and basements, which is why most people rarely see it. ENERGY STAR names the usual hiding places, and the DOE Building America Solution Center explains why ducts inside the insulated part of the house lose less heat: “Because ducts are often concealed in walls, ceiling, attics, and basements, repairing them can be difficult.” Source: ENERGY STAR (U.S. EPA and U.S. Department of Energy), read 2026-10-29. “Ducted systems can eliminate these losses as well, however, simply by ensuring the air handler and all ducts are located within the thermal boundary (insulated walls, floors, and ceiling) of the home.” Source: U.S. Department of Energy Building America Solution Center (Pacific Northwest National Laboratory), read 2026-10-29.

How long does ductwork last?

How long ductwork lasts depends mostly on the material and on how well it was supported and sealed when it went in. A USDA Forest Service building-condition guide gives this rule of thumb for metal ducts: “Sheet metal ducts – galvanized or aluminum - usually last the life of the building” Source: USDA Forest Service, read 2026-10-29.

What is flex ductwork?

Flex ductwork is flexible, bendable duct, generally lined on the inside with plastic, that can bend around corners and framing on its way to a room. The EPA dates its rise in homes to the 1970s, and a USDA Forest Service guide notes how it fails when it is not held up well: “Since the early 1970's, a significant increase in the use of flexible duct, which generally is internally lined with plastic or some other type of material, has occurred.” Source: U.S. Environmental Protection Agency, read 2026-10-29. “Flex-duct may collapse if not properly supported.” Source: USDA Forest Service, read 2026-10-29. “Flex-ducts may sag over the years if not well supported” Source: USDA Forest Service, read 2026-10-29.

What is return ductwork?

Return ductwork is the half of the duct system that pulls room air back to the furnace or air handler, while the supply side pushes conditioned air out to the rooms. The EPA lists supply and return ducts among the parts of a forced-air system and defines the plenum where that air gathers: “Duct cleaning generally refers to the cleaning of various heating and cooling system components of forced air systems, including the supply and return air ducts and registers, grilles and diffusers, heat exchangers heating and cooling coils, condensate drain pans (drip pans), fan motor and fan housing, and the air handling unit housing (See diagram).” Source: U.S. Environmental Protection Agency, read 2026-10-29. “(Plenum is a space in which supply or return air is mixed or moves; can be duct, joist space, attic and crawl spaces, or wall cavity.)” Source: U.S. Environmental Protection Agency, read 2026-10-29. “The air handler has a larger blower motor to move air through the supply ducts and it also has a ducted return.” Source: U.S. Department of Energy Building America Solution Center (Pacific Northwest National Laboratory), read 2026-10-29.

Should ductwork be insulated?

Ductwork should be insulated wherever it passes through spaces that are not heated or cooled, such as attics, crawlspaces and garages. The EPA ties insulation to keeping condensation out of the system, and DOE's Building America Solution Center names poorly insulated ducts as a source of heat loss: “Make sure ducts are properly sealed and insulated in all non-air-conditioned spaces (e.g., attics and crawl spaces).” Source: U.S. Environmental Protection Agency, read 2026-10-29. “To prevent water condensation, the heating and cooling system must be properly insulated.” Source: U.S. Environmental Protection Agency, read 2026-10-29. “That’s because ducts are often sources of heat loss where not properly insulated and sealed, especially if installed in unconditioned attics or crawlspaces.” Source: U.S. Department of Energy Building America Solution Center (Pacific Northwest National Laboratory), read 2026-10-29.

Should ductwork be taped?

Ductwork joints and seams should be sealed, but ENERGY STAR is specific about which materials hold up: mastic sealant or metal tape, not ordinary duct tape. Its guidance on sealing materials reads: “Start by sealing air leaks using mastic sealant or metal tape and insulating all the ducts that you can access (such as those in attics, crawlspaces, unfinished basements, and garages).” Source: ENERGY STAR (U.S. EPA and U.S. Department of Energy), read 2026-10-29. “Never use duct tape, as it is not long-lasting.” Source: ENERGY STAR (U.S. EPA and U.S. Department of Energy), read 2026-10-29.

What is ductwork sealing?

Ductwork sealing means closing the leaks, holes and loose joints in a duct system so the air the equipment heats or cools reaches the rooms instead of the attic or crawlspace. ENERGY STAR puts a number on typical losses and names a safety reason, and the EPA separates it from coating the inside of ducts: “In a typical house, however, about 20 to 30 percent of the air that moves through the duct system is lost due to leaks, holes, and poorly connected ducts.” Source: ENERGY STAR (U.S. EPA and U.S. Department of Energy), read 2026-10-29. “Leaky ductwork in your heating and cooling system may cause “backdrafting,” where these gases are drawn back into the living space, rather than expelled to the outdoors.” Source: ENERGY STAR (U.S. EPA and U.S. Department of Energy), read 2026-10-29. “Note: Use of sealants to encapsulate the inside surfaces of ducts is a different practice than sealing duct air leaks.” Source: U.S. Environmental Protection Agency, read 2026-10-29.

Can ductwork get mold?

Ductwork can get mold when moisture and dust collect inside it, and the EPA lists the inside of ductwork among the places hidden mold grows. Its guidance explains the warning signs and why lined duct is harder to recover once mold takes hold: “The presence of condensation or high relative humidity is an important indicator of the potential for mold growth on any type of duct.” Source: U.S. Environmental Protection Agency, read 2026-10-29. “Once fiberglass duct liner is contaminated with mold, cleaning is not sufficient to prevent re-growth and there are no EPA-registered biocides for the treatment of porous duct materials.” Source: U.S. Environmental Protection Agency, read 2026-10-29. “Other possible locations of hidden mold include areas inside walls around pipes (with leaking or condensing pipes), the surface of walls behind furniture (where condensation forms), inside ductwork, and in roof materials above ceiling tiles (due to roof leaks or insufficient insulation).” Source: U.S. Environmental Protection Agency, read 2026-10-29.

Why is ductwork sweating?

Ductwork sweats when cold air inside the duct chills its outer surface below the dew point of the surrounding air, so water condenses on it, much like a cold glass on a humid day. The EPA describes how moisture gets into the system and the role of condensation: “Moisture can enter the duct system through leaks or if the system has been improperly installed or serviced.” Source: U.S. Environmental Protection Agency, read 2026-10-29. “Research suggests that condensation (which occurs when a surface temperature is lower than the dew point temperature of the surrounding air) on or near cooling coils of air conditioning units is a major factor in moisture contamination of the system.” Source: U.S. Environmental Protection Agency, read 2026-10-29.

Sources

• Duct Sealing — ENERGY STAR (U.S. EPA and U.S. Department of Energy); read 2026-10-29.
• Benefits of Duct Sealing — ENERGY STAR (U.S. EPA and U.S. Department of Energy); read 2026-10-29.
• Ductless Heating & Cooling — ENERGY STAR (U.S. EPA and U.S. Department of Energy); read 2026-10-29.
• Should You Have the Air Ducts in Your Home Cleaned? — U.S. Environmental Protection Agency; published 2026-02-19; read 2026-10-29.
• A Brief Guide to Mold, Moisture and Your Home — U.S. Environmental Protection Agency; read 2026-10-29.
• Ductless Mini-Split Heat Pumps — U.S. Department of Energy Building America Solution Center (Pacific Northwest National Laboratory); published 2023-04-25; read 2026-10-29.
• Building Condition Assessment and Documentation, Part IIIb (Mechanical, Electrical and Plumbing), slide 36: Ductwork — USDA Forest Service; read 2026-10-29.