Save Money by Insulating Crawl Space Ducts

Updated: Jun. 29, 2023

Special duct wrap insulation keeps ducts dry and mold-free

crawl space insulationFamily Handyman
Leaky, uninsulated ducts in crawlspaces and attics waste huge amounts of energy and money. Use duct wrap insulation for a quick fix with a big, immediate payoff.

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Cut the duct wrap to fit, then install

Photo 1: Cut to size

Cut the ductwork insulation to match the circumference of the duct, plus 2 in. Then peel away the foil backing and cut off 2 in. of fiberglass.

Photo 2: Wrap the duct

Remove several duct hanger brackets and slide the insulation around the top of the duct. Overlap the 2-in. foil tab at the seam and seal with aluminum tape.

Photo 3: Secure sections with wire

Relieve stress on the seam by wrapping wire around the insulation. Place two wires on each 4-ft. section.

Photo 4: Wrap round ducts also

Repeat the cutting, tabbing and taping procedure for each round duct.

Uninsulated ducts that run through unconditioned spaces can lose more than 30 percent of their heating or cooling capacity. So you’ll save money by insulating them. The Residential Building Code calls for R-8 insulation for these ducts, but check with your building inspector for local code requirements. Before you insulate, however, plug any air leaks by sealing all the joints with caulk or tape.

Look for “duct wrap,” a fiberglass product with an outer foil vapor barrier. The foil barrier prevents condensation (and mold) from forming on the duct.

Unfortunately, R-8 duct wrap can be difficult to find. We checked three home centers and found only R-3 duct wrap. That didn’t meet our local codes, so we contacted a heating equipment supply house. That company knew exactly what we were looking for and had it in stock.

We bought a roll of 3-in. by 48-in.-wide by 50-ft. duct wrap for our installation. We also bought a few rolls of UL181 aluminum duct tape to seal the insulation seams.

You’ll be handling fiberglass, so wear long sleeves, goggles, mask and leather gloves. Measure the circumference of the duct, add 2 in. to the total, and cut the insulation to that length. Remove a 2-in. strip of fiberglass to create an overlapping flap for taping (Photo 1). To finish the job, butt the insulation edges together along the bottom of the duct, overlap the 2-in. strip of foil, and secure it with aluminum tape (Photo 2). Wrap wire around the insulation to relieve seam stress and prevent the seam from separating (Photo 3).

You’ll save lots of cutting and fitting time on rectangular ducts by removing the hanging brackets one section at a time. Remove the screw from the joist and rotate the bracket. Once the insulation is in place and taped, cut a small hole near each bracket, rotate it back up through the hole and reattach it to the joist. Seal around the bracket with tape.

Video: Working With Mineral Wool Insulation

Required Tools for this Project

Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.

  • Caulk gun
  • Cordless drill
  • Dust mask
  • Knee pads
  • Lineman's pliers
  • Safety glasses
  • Tape measure
  • Utility knife
You’ll also need a trouble light and leather gloves

Required Materials for this Project

Avoid last-minute shopping trips by having all your materials ready ahead of time. Here’s a list.

  • Aluminum duct tape
  • Caulk
  • R-8 duct wrap insulation
  • Steel wire