How to Insulate Furnace Ductwork in a Crawl Space

Duct Sealing

• 1

  Remove any old duct tape that might be covering seals. Duct tape is a temporary solution, although it can be used over foil-surface ductwork.

• 2

  Use a commercially available vapor mastic if ducts are not foil on the surface. Mastic often will not adhere to foil surfaces. Paint on mastic over joints to create airtight seals.

• 3

  Embed fiberglass tape in the mastic before it dries to create a long-lasting and impermeable seal.

• 4

  Replace old duct tape with new duct tape if ducts are foil-surfaced.

Square Ducts

• 5

  Cut strips of the wrap-around duct insulation that are the length of the duct to be insulated.

• 6

  Use as many strips as necessary, butting the edges together, to cover all sides of the duct.

• 7

  Secure the insulation with duct tape along the seams.

Cylindrical Ducts

• 8

  Wrap insulation around the pipe, starting at one end. Butt edges close together.

• 9

  Spiral the insulation the length of the pipe.

• 10

  Secure the insulation with duct tape. You may secure it as you go to keep the insulation from slipping.

• 11

  Cover the seams with duct tape. The tape may be spiraled continuously the full length of the seam for best results.