How to Insulate Heat Ducts in a Crawl Space

If you have heat ducts in an unconditioned crawl space of your home, it's important to make sure they are insulated. Heat ducts can lose heat through the material it's made of (usually sheet metal) and through cracks between the joints, increasing your heating bill by 10-30%! If you notice your rooms are getting colder rather than warmer when the heat is on, it's time to insulate your heat ducts! Neglecting heat ducts simply because they are hard to reach in a crawl space can cost you hundreds of dollars each year on your heating bills. Here's how to insulate heat ducts in a crawl space:

Uninsulated heat duct.

Step 1

Inspect your heat ducts for air leaks. If you find an air leak, seal it with fasteners, mesh and mastic, or foil tape. Don't use duct tape - it doesn't hold up over longer periods of time.

Step 2

Figure out how much fiberglass insulation you need. Insulation is measured by its resistance to heat movement and rated with the letter "R" and a number after it. For example, 12 inches of insulation would be given a value of R-30, whereas 6 inches of insulation might be given a value of R-15 (heat will travel through 6 inches of insulation twice as fast as 12 inches). It is generally recommended that you use at least an R-4 insulation (the higher the better) to insulate heat ducts in a crawl space. It's important to note that different types and brands of insulation have different R ratings. Visit the site listed in the resources below to determine how much insulation you need for crawl space heat ducts.

Step 3

Cut the fiberglass insulation sheets and fit them around the heat ducts. Make sure the fiberglass side faces toward the heat duct and the foil side faces out (the foil prevents heat loss and protects the fiberglass).

Step 4

Use foil tape and/or a staple gun to secure the insulation seams together (staple the insulation together, don't staple to the heat duct itself).

Step 5

Take extra care when installing insulation near vents or electrical cables - leave about 3 inches between any potential heat source and the insulation.


  • Always wear face and body protection when installing insulation. This means wearing goggles, a face mask, thick work gloves and clothes that cover all exposed skin.