How to Insulate Ductwork | Ask This Old House

Richard Trethewey helps a homeowner insulate his ductwork. (See below for a shopping list, tools, and steps.)

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Shopping List for How to Insulate Ductwork:
Foil tape [https://amzn.to/2kWBZ1s], for sealing ducts
Duct mastic [https://amzn.to/2mXAeSg], used to seal duct joints
Foil tape with mastic membrane [https://amzn.to/2lsjrpT], for sealing ducts
1-inch foil-faced fiberglass batt insulation [https://amzn.to/2kVm9UP], used to insulate ducts
Preformed fiberglass duct insulation [https://amzn.to/2lpqGyX], for insulating ducts

Tools for How to Insulate Ductwork:
2-inch paintbrush [https://amzn.to/2mrMB8Z], for applying duct mastic
Utility knife [https://amzn.to/2mWQ0gc], used to cut insulation

Steps for How to Insulate 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.

Additional advice was provided by Air Purchases, Inc. [https://www.airpurchases.com/]

Richard used mastic paste to seal the joints between ducts, in this case Hard Cast Duct Seal 321 by Carlisle HVAC [https://www.carlislehvac.com/].

He also used Nashua foil tape designed for sealing ducts, manufactured by Berry Plastics Corporation [http://www.nashuatape.com/].

Finally, Richard installed Owens Corning [https://bit.ly/2UxxHf4] 1.5 inch thick fiberglass insulation around the ductwork. This insulation has an FSK foil-facing and provides an R-value of 4.2.

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How to Insulate Ductwork | Ask This Old House
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