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ECM: Space Heating & Cooling

H9. Seal and Insulate Ducts


Photo: Leaky ducts can be the source of considerable heat loss.In buildings with forced-air heating (or cooling) systems, warm (or cold) air is distributed to each room through flexible or sheet metal ductwork. The air travels from the furnace, heat pump, or air conditioner through a supply duct to each room, and it returns to the furnace or heat pump through a return duct to be heated (or cooled) again. 

Forced-air distribution systems can lose energy in two ways. First, uninsulated ducts running through unconditioned spaces such as basements, crawlspaces, and attics loose energy through conduction. Second, ducts lose energy through leaks, or convection. Studies show that duct leaks typically raise a home’s heating and cooling costs by 20-30 percent. That figure can double in homes where ducts are not insulated. When supply ducts leak to an unconditioned space, less air reaches the room or apartment. To make matters worse, because not enough air is reaching the conditioned space, the room or apartment may become depressurized, which causes outside air to rush into the space through any path it can find, such as around windows or doors. The furnace (or air conditioner) then has to work harder to heat or cool the space.

Photo: Seal ducts to eliminate leaks.When return ducts have leaks, air from unconditioned spaces enters the return duct, reducing the amount of heated (or cooled) air that can enter it through the return grille. Because air cannot leave the room through the grille, the room or apartment becomes pressurized, and the air, seeking another escape route, squeezes its way to the outside. Not only do leaky return ducts waste energy, but they can cause indoor air quality problems as fumes from combustion appliances, vapors from household cleaners stored in the basement, and soil gases such as methane enter the conditioned space.

To cut energy waste, ducts should be sealed to eliminate any leaks, and then wrapped with insulation. The first step to sealing ducts is to diagnose where the leaks are. This process requires diagnostic tools such as blower doors or pressurization devices and should be done by experienced technicians.


  • Single-family and multifamily buildings with forced air heating or cooling 


  • Ducts
    • metal
    • rigid fiberglass
    • flexible
  • Duct insulation
    • fiberglass batts
    • blown rockwool, fiberglass, and cellulose
    • rigid fiberglass
  • Sealer
    • mastic


  • Pressurization tests help identify duct leaks.
  • Ducts should be sealed with mastic.
  • Duct sealing should only be done by a trained technician.


  • Duct insulation and sealing reduce heating and cooling costs between 20–30 percent.
  • Duct sealing reduces the amount of outdoor pollutants siphoned from the outdoors.


Energy Conservation for Housing – A Workbook, HUD, September 1998. Pages 7-75 through 7-77 address sealing and insulating duct systems.

Improving Energy Efficiency in Apartment Buildings, American Council for an Energy-Efficient Economy, 1995. ISBN 0-918249-23-6. Pages 86 through 88 address sealing and insulating electric heating duct

Duct Insulation, Part of the U.S. Department of Energy's Consumer's Guide. Discusses duct insulation R-values.