How to Insulate HVAC Heating and Cooling Ductwork

By: Danny Lipford
Old inadequate insulation on ductwork.

Old paper faced insulation on HVAC ductwork in need of updating.

If the insulation on the central heating and cooling ductwork in your house 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, paying particular attention to any joints or connections in the ductwork. Mark any air leaks you find, then turn off the HVAC unit.

Finding, taping, and applying mastic to hole in HVAC duct.

Repairing a hole in HVAC duct using foil tape followed by duct mastic.

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.

HVAC ductwork with new foil faced insulation installed.

Install insulation with foil side out.

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 insulation is sealed securely with tape to prevent moist outside air from penetrating through the insulation and condensing on the ductwork.

HVAC ductwork with new foil faced insulation installed.

HVAC ductwork with new foil faced insulation installed.

Further Information

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14 Comments on “How to Insulate HVAC Heating and Cooling Ductwork”

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  • Louis Rosa Says:
    October 1st, 2016 at 5:18 am

    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



  • Karen Says:
    August 8th, 2016 at 10:51 am

    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?



  • Dan Shea Says:
    March 7th, 2016 at 7:56 am

    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



  • Mike Says:
    March 3rd, 2016 at 11:22 am

    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!



  • pat berry Says:
    December 12th, 2015 at 5:19 am

    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.



  • george Says:
    November 22nd, 2015 at 12:45 pm

    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?


  • Official Comment:


    Ben Erickson Says:
    April 29th, 2015 at 8:58 am

    Cornelis,
    The metallic HVAC duct tape I used was about 2″ wide.



  • cornelis regeer Says:
    April 28th, 2015 at 4:17 pm

    What is the recommended width of the sealing tape.



  • Paul Says:
    March 16th, 2015 at 11:29 pm

    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.



  • tmelonas Says:
    February 2nd, 2015 at 9:11 am

    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.


  • Official Comment:


    Ben Erickson Says:
    January 27th, 2015 at 9:19 am

    Harry,
    The return ducts on HVAC heating/cooling systems should be either insulated or made with insulated material.



  • harry Says:
    January 26th, 2015 at 3:09 pm

    Do you insulate return duct work. Thank you



  • Pamela Grahg Says:
    November 16th, 2014 at 8:17 pm

    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?



  • Tom Says:
    September 10th, 2014 at 6:02 am

    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.


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