How to Insulate Sheet-Metal AC Ducts


Air-conditioning ducts are usually part of an integrated heating, air-conditioning and ventilation, or HVAC, system that uses shared ducts to distribute cooled air from an AC unit and heated air from a furnace. In many systems, heating and AC ducts are essentially indistinguishable and both benefit from the same methods of insulation. Although several preinsulated duct materials are available, bare sheet metal remains the industry standard. To reduce temperature loss through the bare sheet metal, HVAC professionals cover the ducts either with rigid insulation boards or flexible insulation wrap. Both types of insulation are relatively easy to install, but rigid insulation board usually is more costly. Regardless of type, duct insulation increases your HVAC system's efficiency and reduces your home's energy requirements.

Things You'll Need

  • Tape measure
  • Calculator
  • Foil-faced duct wrap
  • Marker
  • Utility knife
  • Straightedge
  • Outward clinch staples
  • Outward clinch staple gun
  • Foil seam tape
  • Measure the perimeter of the duct that you are insulating with a tape measure. On square or rectangular ducts, measure and add all four sides to determine the perimeter. On round ducts, the perimeter is equivalent to the duct's circumference. To determine circumference, multiply the round duct's diameter by pi. The result is the round duct's circumference.

  • Determine the "stretch-out" value of the perimeter. Stretch-out values are units of length that you must add to the duct's perimeter to achieve the duct wrap's listed insulation value. Duct wrap manufacturer's include stretch-out charts with their products. Find your insulation's thickness and corresponding value on the stretch-out. Add the stretch-out value to the duct's perimeter. The result is the cut length for the duct wrap.

  • Unroll the duct wrap on a flat surface with its foil side facing down. Mark the cut length both sides of the wrap, lay a straightedge between the marks and draw a knife along the straightedge's side to sever the length of duct wrap from the roll. Mark roughly 2 inches in from one corner of wrap's cut edge. Create a corresponding mark at the opposite corner. Lay the straightedge across the marks.

  • Draw the knife along the straightedge's side to score through the insulation portion of the duct wrap. Do not completely cut through the wrap; leave the underlying foil intact. Remove the 2-inch strip of insulation from the end of the duct wrap to expose the underlying foil flap. This flap will overlap the opposite end of the duct wrap during installation.

  • Load outward clinch staples into an outward clinch staple gun. Drape the duct wrap around the duct with its foil side facing out. Lap the flap that you created over the intact, opposite end of the duct wrap. While holding the flap over the wrap's opposite end, fasten staples through the flap every 6 inches along the seam.

  • Cut and install successive pieces of insulation to cover the entire duct. Slip the intact ends of adjacent pieces beneath the integral flap of previous pieces. Cover both the stapled and butt-joint seams with foil seam tape.

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