How to Insulate an Air Duct

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Forced air heating and cooling systems are popular in many buildings and homes. A single blower forces the air through a series of air ducts to the separate rooms. Air ducts, because of the way they are made and joined together (thin sheet metal with seams) will lose 20 to 30 percent of the heating or cooling capacity of the air they carry through leaking out through the metal. You can stop heat loss (or cooling loss) by properly insulating an air duct.

Things You'll Need

  • Foil tape
  • Scissors
  • R-6 duct insulation
  • Wrap each seam on the air duct with foil tape. Make sure that the tape overlaps itself by 3 inches and smooth the edges onto the air duct so there are no bubbles or gaps between the tape and the metal surface of the duct. Foil tape is very hard to rip, so use a pair of scissors to cut the tape. When you cut the tape, leave the end of the tape that is still attached to the roll stuck to the scissors, if the foil tape sticks back to the roll it will be hard to peel it off.

  • Cut your R-6 duct insulation into 8-foot pieces, this will make it easier to handle. Duct insulation is fiberglass insulation that is completely encased in a thin plastic cover, so you don't have to worry about which side goes against the air duct.

  • Lay the insulation on the duct and use a piece of foil tape to attach one end to the metal of the air duct to anchor it. Wrap the insulation around the duct so the insulation overlaps itself by 4 inches. If you are working with large air ducts that need more than one piece of insulation, just make sure that as you add pieces of insulation that the new piece overlaps the first by 4 inches to make a seam. At odd shaped joints of the air duct (turns or where two ducts may join), cut and wrap the duct with insulation so it is completely covered. It is not important that the insulation be neat, but it is important that the duct is completely covered and all the seams of the insulation are 4 inches wide.

  • Tape all the seams of the duct insulation so they are completely sealed. What makes insulation work is how "air tight" you install it. There should be no gaps in the seams that would let air out or outside air in.

Tips & Warnings

  • Seal all the duct seams first, then go back and install your insulation. This way you won't miss any seams and the work will actually go faster because you will be focused on one task at a time.
  • R-6 is the maximum recommended insulation for air ducts, you can choose a higher 'R' rating but it would be an unnecessary expense.
  • Use caution when installing any kind of insulation around any duct that carries hot air. The exterior of the duct can become hot enough to cause burns.

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