- 1). Measure the circumference (round duct) or perimeter (rectangular duct) of the duct you wish to insulate. Circumference is measured by either taking a flexible tape measure around the outside of the round duct or wrapping a string around the outside and then stretching it out along a ruler. Perimeter is measured by taking the measurement of each of the four sides of the rectangular duct and then adding them together.
- 2). Unroll your duct insulation on a flat surface. Take the measurement from Step 1, add a few inches for overlap and cut a piece of insulation at the calculated measurement.
- 3). Drape the insulation over the top of the duct. Tape one edge tight to the duct with foil tape along the insulation's entire length.
- 4). Pull the other end of the insulation tight. Tape its edge tight to the duct. The insulation will overlap slightly and this is fine.
- 5). Measure and cut the next piece of insulation and install in the same fashion as above. Tape along the seam where each piece of insulation meets the next with foil tape to ensure a tight seal.
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