Duct Insulation Calculator

Under the 2021 IECC, supply and return ducts in an attic need R-8 when they are 3 in or larger in diameter (R-6 if smaller), and ducts in other unconditioned space such as a crawlspace or garage need R-6. Ducts inside the conditioned envelope need only sealing. Local codes can differ, so check yours.

Measure the perimeter, 2 × (width + height), then add about 6 to 8 times the wrap thickness for the corners and 3 in for the seam. An 8 × 12 in duct with 2 in (R-6) wrap is 40 in of perimeter plus 12–16 in for corners: about 52–56 in around, or 4.3–4.7 sq ft of wrap per foot of duct before the overlap and waste.

Always. Mastic or foil tape on every joint first — insulating over leaky ducts just hides the loss.

About 2.6 sq ft of wrap per foot of duct. With 2 in (R-6) wrap the insulated circumference is π × (6 + 2 × 2) in, or 31.4 in, so buy a roll at least 34.5 in wide to leave the 3 in seam overlap. A 25 ft run needs roughly 65 sq ft before waste, and about 33 ft of foil tape.

Seal first, every time. A leaking duct in an unconditioned attic dumps conditioned air straight outside, and insulating around the leak just hides it while it continues. Mastic on the joints then insulation over the top is the right order — and duct tape, despite the name, is not the material for the sealing step.

Any run passing through unconditioned space — attic, crawlspace, garage, unheated basement. Ducts inside the conditioned envelope lose heat into rooms you are heating anyway, so insulating them changes little. The attic runs are usually where the loss is worst, because that is where the temperature difference is largest.

Codes typically specify a minimum for ducts in unconditioned space, commonly in the R-6 to R-8 range depending on jurisdiction and location. Check your local requirement rather than assuming, since it varies, and remember that wrapping compresses the material — an R-8 wrap pulled tight performs as less than R-8.

About 3.1 sq ft with 2 in (R-6) wrap, because the insulated circumference is π × (8 + 2 × 2) in, or 37.7 in. With 3 in (R-8) wrap it is π × 14 in, about 3.7 sq ft per foot. Add 10% for seams and waste; a 60 ft run at R-6 needs about 207 sq ft.