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.)

Shopping List for How to Insulate Ductwork:

Tools for How to Insulate Ductwork:

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.

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Homeowners have a virtual truckload of questions for us on smaller projects, and we're ready to answer. Ask This Old House solves the steady stream of home improvement problems faced by our viewers—and we make house calls! Ask This Old House features some familiar faces from This Old House, including Kevin O'Connor, general contractor Tom Silva, plumbing and heating expert Richard Trethewey, and landscape contractor Roger Cook.

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How to Insulate Ductwork | Ask This Old House

[rosewoodsteel]

Great video! Boy, have I done a lot of this in my day.. Couple of thoughts: Before taping the metal seams, use a dust brush to remove loose dust and debris before taping. (The metal needs to be clean and free of dust.) When cutting the fiberglass, remove at least 1" of insulation from the length of the wrap at one edge, to create a flap which will overlap the lateral butt joint before taping (industrial scissors are good for this). After installing with the aluminum tape, wrap high quality duct tape around the pipe/duct circumference every two feet or so (nylon ties work, as well) to help hold things into place. I'd put a few staples in the duct tape to ensure that it will stay there for years.
Make sure to keep surfaces clean before using any tape or it won't stick for long.

[blueseruser]

Nice to see you are standing up, try this on your back in the dark, with heat or cold and all ductwork in place!

[timb]

I saved at least 25% off my gas bill just by sealing all the ducts in the basement. I used both foil tape and duct seal. I used a plastic putty knife to apply the duct seal, brush wasnt working out well. Whoever installed the heating did a hack job so there was alot to fix. There were openings in the duct work everywhere that it wasnt even pulling air from upstairs, it was just sucking in all the cold air from the basement. The heat now works so much better, can feel the return air vent really sucking the air into it now. And no more basement smell coming up through the vents. Now the air filters arent getting nearly as dirty so quickly. I rent and did all the work myself, the money I spent on supplies really paid off with the savings on the gas bill in winter. Now im slowly buying rolls of insulation and wrapping the ducts to save even more on the gas bill.

[lilew]

Thank you for this video! It's very clear and concise. I'm going to be replacing all duct work in my old home. More than likely, it's over 40 years old. My family and I are constantly getting sick this winter in this newly purchased home that's from the 1880s. This will be the icing I can put on the cake after replacing the ducts. Thanks again.

[Josepadillamondragon]

im an isulator myself at construction sites and i would recommend wearing a 3m dust mask and long sleeve shirts to avoid inhalation and irrtiation to throat and skin. also grab a ladder and seal it on top to minimize getting insulation on yourself. also we use foil tape but put plenty of staples as tape tends to peel after a while. good video tho#

[nathensurman]

Thorough, clear instructions with brief explanations for the reasons why. Nice one!

[mattbeazer]

At 4:43 - Tommy would have a fit if he saw an HVAC guy notching that floor joist like that. Holy Moses.

[JohnSmith-iwky]

Good going next time show us how to do it in a attic with no room and 110 degree heat and blown in insulation.

[xmassan]

On larger ducts, foil tape may not hold. Use zip ties to secure the insulation and foil tape to cover the seams.

[undrachevr]

I learned so much from this episode. Thank you so much

[DordiHOTS]

Informative. The coal dump room for the furnace reminds of the old Tom and Jerry episode.

[metrazol]

"I brought along this furnace to show how..."
"Did... did you just have a furnace in your truck?"
"..."
"..."
"Anyway, the burner here..."

[kingkevin]

Why am I here, I already know how to do this. Oh wait so I can dream how this would already be done if I had a nice basement like this, and not a crawl space not even high enough to stand up in, and a dirt floor :(.

[Psychlist]

4:37 holy crap that giant notch in the joist.

[johnnichols]

Attention to detail. Great teacher as well

[sesteve]

Nothing brings out the weekend warriors and shade tree craftsmen saying they know better than a "This Old House" video

[TypeDarylBTeas]

If you just separate the foil tape from the paper before you tear off the tape and let the paper stay behind, you aren't stuck fighting the starting edge for every piece you need and when you go to clean up you have one long piece of paper and not 150 little ones.

[imtyrone]

I would cut the insulation at least 2” off at the end of the insulation so you don’t get insulation all over your tape when trying to stick it on

[kelvinlyons]

The basement is an conditioned space so a little leakage is ok, now if this was an attic nonconditioned space all that would be good

[servant]

Seems like you should insulate the return duct work also, to keep what heat it already has and not need to replace that part of the heat. But I could be wrong... It has happened before!