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