How to Insulate HVAC Ductwork

Air leaking from ductwork in the attic
Your heating, ventilation and air conditioning system needs insulation for optimal performance.

If the insulation in your house’s HVAC ductwork has seen better days, consider replacing or upgrading it to improve energy efficiency and reduce your utility bills.

This is particularly true if the ductwork is located in the attic, where the intense summer heat can make your air conditioner work overtime to cool your home.

When working with fiberglass insulation, always wear:

  • Goggles
  • Protective clothing
  • Approved dust mask

Gloves are a good idea, too, but it can be hard to do the delicate fitting and taping required while wearing them.

Repairing Leaks in HVAC Ductwork

Start by turning your heating/cooling system on and feeling along the ducts for air leaks. Pay particular attention to any joints or connections in the ductwork, and mark any air leaks you find. Then turn off the HVAC unit.

Woman repairs air conditioner ductwork with metallic foil tape
Repair a hole in HVAC ductwork using foil tape followed by duct mastic. (©Kuchina, Adobe Stock Photos)

Cover any air leaks with metallic foil tape made for sealing ductwork. Don’t use standard cloth duct tape on HVAC ductwork, since the adhesive won’t hold up to extreme temperatures.

For an added layer of protection, you can apply duct mastic to the tape and duct.

Insulating HVAC Ductwork

When insulating HVAC ductwork, use a foil, faced fiberglass insulation with an R-6 or higher R-value. Use the type of metallic foil duct tape recommended by the insulation manufacturer to seal and hold the insulation in place.

Make sure the foil surface of the insulation is free of dust before applying the tape. Peel the paper backing off the tape as you apply it to prevent the tape from sticking to itself.

If your existing duct insulation is in fairly good condition, an additional layer of duct insulation can be applied over it. If the old insulation is in poor condition, remove it and replace it with new insulation.

Cut the insulation to width and length, using a square and a sharp utility knife, so that it fits snugly around the duct without compressing the fiberglass.

Apply several small pieces of tape across the insulation seam to hold it in place, then seal the entire length of the seam in the insulation with a long strip of tape.

Where the sections of insulation join together, apply tape all the way around the joints by sliding the tape under the duct, then peeling off the paper backing once it’s in position.

Carefully cut and fit the insulation together where the branch lines and registers come off the main trunk line, making sure there aren’t any gaps.

Check that every seam and joint in the foil facing on the HVAC ductwork insulation is sealed securely with tape. This will prevent moist outside air from penetrating the insulation and condensing on the ductwork.

HVAC ductwork with new foil faced insulation installed.
This is what HVAC ductwork looks like with new foil faced insulation installed.

Further Information


  1. My A.C. ducts look like that. Do you know if there is asbestos in that kind of insulation.My house was built in the 50,s but i dont know when the A.C. was first installed.

  2. I got brand new duct work in my basement with a brand new 95% efficient heater (we haven’t started insulating the basement just yet and were wondering what are good products to use in a basic nonfinished basement. To help get the most house more efficient. Also will be looking at insulating our attic and have read blow in insulation would be best?

  3. Hi, I have the metal HVAC in my house and one side of house stays cold in winter and hot in summer. When they placed the metal duct work the put it under flooring “I have a basement” and on some rooms on that side the ductwork is bent a lot. I think that is why the air does not get to that side of the house. Is this a possibility and if so how do I fix it.

  4. Okay you want to check your duct grading depending on where you are and what you are using it for. Check with your ducting supplier he will know this.
    The bending in your duct work will slow the air flow down. You can fix this by putting in manual dampers to restrict the airflow to the areas it travels freely. They are like a butterfly valve in the duct. Check for leaks first. Also check if they where added later and the unit can handle the amount of outlets in your house.

  5. We moved to a town house built in ’90s. In the basement we found polystyrene insulation in between the Heating duct and the joists; Is that OK? What material we can use for that and what we cannot use?

  6. Need to add ventilation to my basement ceiling, for air and heat, have a nice unit but the vent they add doesn’t provide properly air or heat to room.

  7. The duct work in our basement is clicking and banging. Would insulating the duct work help cut down on noise? Or any ideas on how we can stop this noise? It’s a newer gas furnace. The noise is driving us crazy!

  8. I want to add a/c to my hot air heating system. the duct work is not insulated and the basement is finished. I can get to some places but the duct work is butted up against the main carrying beam and that side would not be covered. Is this a problem? It is all in the basement and a/c removes moisture (humidity) from the air, so do I really need to insulate the duct work? thank you, Dan

  9. I live in a condo, we are having remediation done to our HVAC units. I had mine done approximately 6 years ago. Fiberglass with a covering over it. The company that is NOW doing our HVAC told me the product I have is not good, their product is much better. I have seen this product, It”s about 1/4 inch thick, looks like black mating. HELP!!! Is what I have alright, or am I being BAMBOZZELED?

  10. hi Mr. Danny, I live in the state of Maryland and my question to you goes as follows. It is required for all of the a/c ducts on a brand-new home to be insulated. And why. Please advise thank you much

  11. I own a 1954 built home and an attic furnace. The furnace has been changed and there has been much traffic up and over the duct’s therefore tearing off of a lot of insulation. I need a price to reinsulate the attic ducts.

  12. I have air handlers for my ac in my attic’s. One of the systems has flex ductwork for the cold air return the other has sheet metal and neither plenum’s are insulated. Question, should they have insulated wrapped on both the plenum and the cold air return for each and does it really work. What kind of insulation wrap would you suggest. I have lived with it this way for 15 years not really knowIng about this issue.

  13. Is it appropriate for a consultant to check Insulation of duct by cutting duct wrap 1 ft square . is it as per SMACNA rule s. what do you think so

  14. Hello. After removing a piece of drywall in my laundry room, I discovered that the previous homeowner had installed fiberglass installation underneath a heat vent (duct?). He installed with the paper side touching the metal duct. Is this dangerous? I’m concerned because the basement is finished, and the ceiling is drywall. I’m concerned he installed fiberglass this way through the entire basement (with the paper touching heat ducts.) websont have the money to cut into and refinish the ceiling, let alone fix the insulation. Is this safe? Thanks

  15. I have a single level home with big open attic it has blown insulation and very narrow crawl space but my furnace (electric) has the single return air duct running about 4 feet from furnace it is in lowered ceiling from bathroom to kitchen ,, my question is can I use a flexible hvac pipe duct an d run a network to living room and two bedrooms the way it is now is very in sufficient, furnace runs almost constantly in cold months , I would appreciate your professional opinion and tips what to do thanks

  16. I live on a 3rd floor condo with forced air heating. Two bedrooms in the unit are noticeably colder in winter and warmer in summer by about 5°. I have accordion style vents in the attic. Could it be possible these vents aren’t well insulated causing heat/cooling loss? Would insulating them help the situation? Furnace and ac units are brand new and appropriate for the unit size. Thanks for any help/suggestions.

    • Hi, Kenneth,
      Great question! We have forwarded it to the Today’s Homeowner Radio Show’s producer.
      He will contact you soon to discuss featuring it during an upcoming show.
      Take care!

  17. A part of me has to wonder how the cost-benefit breaks down when buying pre-insulated piping. Of course, you have to know ahead of time that you’ll need it.


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