How to Insulate Round Sheet Metal Air Duct

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How to insulate round sheet metal air duct with R-8 thermal insulation sleeve. This project is continued from How to Seal Sheet Metal Duct with Mastic – Part 1.

Project Index

Duct Wrap vs Duct Sleeve Insulation

I considered two methods for insulating sheet metal air duct insulation that are practical for the DIY homeowner:

  • Duct Wrap Insulation
    • Pros:
      • Insulation blanked applied after the ductwork is installed and sealed.
      • Faster overall installation with new ductwork.
      • Most popular option based on availability at HVAC supply stores in my area.
    • Cons:
      • May requires outward clenching staples.
      • More insulation joints to be sealed.
      • May require two layers to obtain R-8 thermal rating required by the residential building code.
        R-8 rated duct wrap was not available at local supply stores.
  • Duct Sleeve Insulation
    • Pros:
      • Slips over the duct pipe like a sock.
      • Fewer insulation joints.
      • Readily available with an R-8 rating as required by code (.pdf – see page 13 “Duct Insulation”).
    • Cons:
      • Requires installation planning to install the sleeve over the duct during assembly.

I chose the duct insulation sleeve because it was the better long term solution in my opinion. The extra ductwork assembly is a one time job and I prefer quality over speed.

How to Insulate Round Sheet Metal Air Duct

I purchased 10 roll box of 5 feet long by 12 inch diameter rolls of JP Lamborn MT-05 thermal sleeve R-8 rated duct insulation from a local HVAC supply store based on my estimate of ~40 feet of sheet metal duct to be installed, allowing extra for waste. A plastic starter cap is optional and fits over the duct pipe so the sleeve doesn’t catch on the sharp pipe end. In a pinch, a starter cap could be made out of cardboard.

J.P. Lamborn Duct Insulation: Thermal Sleeve MT-05

J.P. Lamborn Duct Insulation: Thermal Sleeve MT-05

See the JP Lamborn thermal sleeve installation instructions for details.

Sheet Metal Duct Insulation Sleeve Installation Options

I considered several options for installing the insulation sleeve over the sheet metal duct:

  1. Install the sleeve while assembling the ductwork.
    The challenge here is scooting the sleeve back and forth along pipe and elbows to seal the duct with mastic. The duct hanger straps get in the way and the sleeve will bunch-up (compress) only a certain amount, slowing down the mastic sealing work.
  2. Remove only one section of pipe and slide several sections of insulation sleeve down the entire length (over 25 feet) duct after sealing with mastic. Sliding the insulation sleeve over an elbow takes some effort – I know because I tried! I didn’t go this route because it seemed tedious.
  3. Remove the straight pipe sections and install the insulation sleeve.
    The duct pipe is easy to disassemble by removing the sheet metal screws at the crimp joints, and I was careful to not seal the pipe joints with mastic so I could go with this approach.

Which insulation method you choose will depend on the ductwork configuration, installation difficulty and your preferences. I have unobstructed access to the ductwork with plenty of space to move about.

Install the Sheet Metal Duct Insulation Sleeve

The mastic that was applied to the duct start collars, elbows and snap lock pipe seams has dried overnight. You don’t want to install the insulation sleeve over gooey wet mastic because it’ll make a mess and ruin the air tight mastic coating.

I disconnecting the straight pipe sections – handle the pieces gently so as not to split open the mastic duct sealant – placed the plastic starter cap on the pipe, then slipped on an insulation sleeve:

R-8 Insulation Sleeve on Sheet Metal Duct

R-8 Insulation Sleeve on Sheet Metal Duct

Make an alignment mark at the duct pipe joints to simplify reassembly, then back out the sheet metal screws and remove the duct pipe:

Sheet Metal Duct Alignment Mark for Reassembly

Sheet Metal Duct Alignment Mark for Reassembly

The sleeves are installed and the duct pipe reassembled with sheet metal screws. As I reassembled the duct pipe, the metal duct straps were lowered an inch or so to allow for the insulation thickness to prevent high spots that could stress the start collars. The duct disassembly, insulation and reassembly was completed in about 15 minutes.

The duct insulation is bunched up in this photo to expose the straight pipe joints to be sealed with mastic:

Insulation Sleeve Installation on Sheet Metal Duct

Insulation Sleeve Installation on Sheet Metal Duct

Because I planned to disassemble the duct pipe to install the insulation, I pinned some sections in place with only one or two sheet metal screws. No screws were initially set in the downward sloping duct pipe (far right in the above photo and closeup below) because it was held in place by the upper and lower elbows. I fastened the straight pipe sections to the elbows with a least 3 self piercing sheet metal screws equally spaced around the pipe and driven through the crimp joint:

Fastening Sheet Metal Air Duct with Screws

Fastening Sheet Metal Air Duct with Screws

Seal the Duct Pipe with Mastic

Scoot back the duct insulation to expose pipe joints, clean any dust or oils, then seal with a liberal coat of mastic. The blue low tack painter’s tape was helpful for holding back the insulation:

Seal the Sheet Metal Duct Pipe with Mastic

Seal the Sheet Metal Duct Pipe with Mastic

Mastic is applied to all duct pipe joints and over the sheet metal screws for an air tight seal. Wet mastic is a lighter gray compared to the darker cured mastic:

Seal Duct Pipe with Mastic after Insulating

Seal Duct Pipe with Mastic after Insulating

Inspect all joints for any cracks or gaps in the mastic and reseal if needed. Through careful handling, I had no problems.

The duct insulation sleeve is butted together and sealed with HVAC metal tape in the How to Insulate Round Sheet Metal Air Duct – Part 2.

Take care,

Bob Jackson

Copyright © 2018   Reproduction strictly prohibited.


  1. Tom February 23, 2015 at 10:41 am - Reply

    I cant find were to buy this can only find the flex

  2. John Phillips December 28, 2015 at 3:07 pm - Reply


    I work for the manufacturer of the thermal sleeve mentioned above (JP Lamborn Co.). Thermal sleeve and insulated flex duct are two of our top sellers because of the ease of installation.

    Please contact me if you’d like the name & number of a local distributor in Kansas City.


    • Jt woytek February 10, 2017 at 6:10 pm - Reply

      I’m a hvac contractor looking for a easier way to wrap residential metal duct in the attic bin looking into the sleeve approach, will these sleeves slide over metal 90’s every picture I see there sliding over 45’s which I understand for better air flow. But can they be used with 90’S

      • Bob Jackson February 11, 2017 at 2:34 pm - Reply

        A best practice is to avoid 90 degree elbows if possible due to high friction losses. That’s why I installed two 45 degree elbows instead of single 90.

        I don’t see why a thermal sleeve wouldn’t make it around a 90 degree elbow. It will require some extra effort to coax it over the elbow. The MT-05 Thermal Sleeve installation instructions don’t mention elbows. Try asking JP Lamborn.

  3. Lee Farm June 20, 2017 at 3:40 pm - Reply

    The ductwork In my attic, needs some repairing. A small amount of it is the duct sleeve & they do not have a duct metal sleeve inside. So my first question is, are they okay without the metal sleeve, or do I need to have them redone?

    Next, the rest of the ductwork in my attic, does have the metal sleeve, but it’s old and the insulation covering it is showing it’s age & is very brittle to touch, about to fall apart in places. In this case, what would you recommend for repairing it? It’s difficult to work in my attic, because of limited space, ductwork takes up most of it, and it’s extremely hot, most likely because this roof doesn’t have a ridge vent. I have thought of putting mastic over all of it, or using the wrap insulation, and leaving the old insulation on it. This roof covers 3 bedrooms and 1 bath. FYI; The ductwork to each bedroom, comes down into the closets of each room, and then goes to the wall for vent. I’m guessing this was done in order to avoid having a ceiling vent. Very stupid way to of done this, plus takes up closet space. Again, just FYI

    I appreciate any help or support you can provide. I just need to get it addressed, the sooner the better.

    Thank you,

    • Bob Jackson June 20, 2017 at 6:40 pm - Reply

      You mention metal sleeve but I believe your have flexible duct. Correct?

      > The ductwork to each bedroom, comes down into the closets of each room,
      > and then goes to the wall for vent.
      Hmm, how is your house? Makes me think the central AC was a retrofit after the home was built.

      Send pictures of the ductwork that needs repairing to bob[at] (replace the [at] with the @ symbol) so I can’t better understand the problem and provide more specific advice.

  4. Lee Farm June 29, 2017 at 11:15 pm - Reply

    Sorry for the delay, had surgery on my hand & probably not good for me to be up in my attic. I think my AC was a retrofit, I disconnected ceiling heat in my home when I bought it, it’s close to 50 years old, it’s a trilevel. Most of the ductwork is a metal sleeve that has a very stiff layer of insulation around it, about 1 1/2″ thick and looks old. This is what I was talking about above. If I was to reinsulate it, do I just wrap additional insulation over what’s on it or do I have to take off what’s on it now, just not sure what needs to be done. There are a couple of ducts in my attic that are the flexible duct. For those, I’m assuming they are efficient the way they are. I hope I am correct, if not, please let me know. I hope this is okay for now, I’m in too much pain to be in my attic snapping pics. I really appreciate your help.

    • Bob Jackson June 30, 2017 at 8:48 am - Reply

      OK – the sheet metal duct insulation is cracking and deteriorating. Inspect the duct joints for any breaks or separation, repair if needed, then apply a new layer of insulation wrap over the old insulation.

  5. Davud October 19, 2018 at 9:16 pm - Reply

    Just had our metal duct put in our crawl space. The HVAC contractor had to manually wrap all pipes with insulation because there was no source of supply for insulation sleeves. They wrapped all the sections of straight pipe individually before going under the house to install them. Not sure how they connected the pipes since the insulation was all the way to the edge from end to end on each pipe section and there was no metal duct showing on any ends to connect the pipe. They used foil faced wide tape to seal the insulation wrapped. Don’t know if mastic was used at all but heard them using sheet metal screws.

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