How to Insulate Return Air Ductwork

For maximum efficiency, ductwork needs to be insulated in certain circumstances. Ductwork warrants insulation when it is concealed above a drywall ceiling, above a suspended ceiling or in an attic. Basically, if you cannot visually see the ductwork serving the room you are in, it should be insulated. The main reasons for insulating ductwork are to maintain energy efficiency and to prevent condensation on the ductwork due to the temperature change. The above scenario always applies to supply air duct. Return air ductwork is insulated less frequently because it's basically room temperature air flowing back to the furnace. The only real instance in which it needs to be insulated is when the temperature variance is so extreme that moisture in the air will condense.

Return air ducts require insulation only when air moisture will otherwise condense.
  1. Measure the perimeter of your return air ductwork. The perimeter is the distance around the outside of it. So if your ductwork is 16 inches wide by 8 inches high, the perimeter will be 48 inches. Add around 6 inches to this measurement to allow for your insulation. This figure is your cut length.

  2. Stretch out your roll of duct insulation on a smooth, hard floor. Cut your insulation at the above calculated measurement with a sharp utility knife. Using the above example, if you had a perimeter of 48 inches and added 6 inches to that, you would cut your insulation at 54 inches.

  3. Place your insulation around your ductwork with the insulation side facing toward the ductwork and the foil vapor barrier facing outward. It's easiest to drape it over the top of the ductwork and allow gravity to hold it in place momentarily.

  4. Tape one of the ends of the insulation to the duct with foil tape. Stretch the insulation tight and tape the other end down with foil tape. This end will end up being taped down to the insulation jacket. Ensure tape runs along the entire seam to seal it properly.

  5. Repeat Steps 1 through 4 until your ductwork is completely insulated.

  6. Tape each seam where the separate insulation pieces butt together with your foil tape.


  • Dust masks, long-sleeved clothes and safety glasses are highly recommended when working with duct insulation. The fiberglass will irritate your lungs, skin and eyes.
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