Cut the duct wrap to fit, then install
1 of 4
Photo 1: Cut to size
Cut the insulation to match the circumference
of the duct, plus 2 in. Then peel
away the foil backing and cut off 2 in. of
2 of 4
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.
3 of 4
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.
4 of 4
Photo 4: Wrap round ducts also
Repeat the cutting, tabbing and taping procedure for each
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
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