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How to Insulate HVAC Ductwork

This Old House plumbing and heating expert Richard Trethewey helps a homeowner insulate his HVAC ductwork.

In this video, This Old House plumbing and heating expert Richard Trethewey helps a homeowner insulate his ductwork.

Steps for Insulating HVAC Ductwork

  1. Check the speed of the blower motor on the furnace. If necessary, switch the wires to reduce the blower to its lowest speed.
  2. Press a continuous strip of foil tape to all longitudinal seams along straight runs of duct.
  3. Use a paintbrush to apply duct mastic to the joints where an elbow connects to a duct.
  4. Use a utility knife to cut foil-faced fiberglass insulation to the proper size.
  5. Wrap the insulation around the duct, and then pinch the seam closed. Secure the insulation with short strips of foil tape.
  6. Apply a long strip of foil tape along the seam in the insulation. Repeat to insulate the remaining ducts.
  7. To install preformed duct insulation, start by disconnecting an elbow to expose the end of the duct.
  8. Snap a plastic cap onto the duct end, then slip the preformed insulation over the duct.
  9. Gently pull the insulation over the entire length of duct.

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