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Musings of an Energy Nerd

Burying Ducts in Attic Insulation

Building codes provide guidance for contractors, but the details can still be tricky

If you bury attic ducts in cellulose or fiberglass insulation, you need to think about summertime condensation. In a hot, humid climate, moisture can condense on cool ducts and register boots when the air conditioning system is blowing cool air through the ducts.
Image Credit: Image #1: Home Innovation Research Labs

You shouldn’t install ducts in a vented unconditioned attic. As most GBA readers know, ducts belong inside a home’s thermal envelope, not in an environment that’s cold in the winter and hot in the summer.

Efforts by energy-efficiency experts to convince builders to move their ducts indoors haven’t been particularly successful. In states where slab-on-grade homes are common — in other words, in states where basements are rare — most builders still install ducts in vented unconditioned attics.

According to building scientist Joseph Lstiburek, the energy penalty associated with attic ducts is often in the range of 30% — that is, 30% of the home’s heating and cooling energy is lost to the attic. By performing two important steps — carefully air sealing all of the duct seams, and moving the air handler into the home’s conditioned space — the energy penalty associated with attic ducts can be lowered from 30% to about 10%.

Even better performance can be achieved, however, if the ducts are installed near the attic floor and buried in deep cellulose or blown-in fiberglass insulation. Historically, the main impediment to implementing this approach has been the concern that condensation can form on the exterior side of the ducts during the summer, making ceilings damp.

Why do attic ducts sweat?

If ducts aren’t buried with insulation, they usually don’t sweat. Most attic ducts are insulated flex ducts (that is, flex ducts with R-6 or R-8 fiberglass insulation, along with an exterior polyethylene jacket). When these ducts are exposed to exterior air, the plastic jacket never gets cold enough to permit condensation, because the hot attic air keeps the plastic jacket warm. But if the same ducts are buried in cellulose or fiberglass insulation, the added insulation lowers the temperature of the polyethylene jacket. If the jacket is cold enough, and…

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  1. Expert Member
    Dana Dorsett | | #1

    Diffusion venting
    According to Lstiburek:

    The best way to do buried ducts, unless you live somewhere where you have to worry about ice dams, is to close up the soffit vents, and replace the ridge vent with a diffusion vent.”

    I see prescriptive practices regarding diffusion venting have been addressed in the IRC 2018 (at least for US climate zone s1, 2, & 3.) See section R806.5, subsection 5.2:

    Lstiburek has been advocating putting this into code for awhile now!

  2. GBA Editor
    Martin Holladay | | #2

    Response to Dana Dorsett
    Thanks very much for your comment. I was unaware that Lstiburek's lobbying efforts on behalf of his vapor diffusion port idea had been accepted by code officials in the latest version of the IRC.

    Note to GBA readers: Most jurisdictions have not yet adopted the 2018 IRC, so it's premature to assume that your local code official will agree with an attempt to adopt the vapor diffusion port approach.

    I have edited my article to include information on the new code provisions for vapor diffusion ports.

    Below is an image showing the provisions of the 2018 IRC, section R806.5.5.2, cited by Dana. Click the image to enlarge it.


  3. Expert Member
    Peter Engle | | #3

    Science is cool.
    I was also not aware of the new section on diffusion ports. Very cool. And it shows that actual science can still make a difference. Joe understands the physics and proposed these as a potential solution for several different climates. Then, he did quite a bit of serious and careful experimental study to show that these work, and the code community took notice and adopted the techniques. It all actually happened relatively quickly, for the building code folks. Nice job. Can't wait until he's got rules for colder climates.

  4. Expert Member
    Dana Dorsett | | #4

    Code official signoff @ Martin
    Most code officials I've dealt with (OK, it's a small number, to be sure) usually go along with approaches & values that appear in newer versions of the code not found in the current state codes if you can point them to the relevant sections.

    I have a bit of concern about the minimum 20 perms requirement in subsection 5.2.3 if the house being built is in a hurricane zone. There is a hurricane resilience aspect to unvented roofs which is desirable, but materials north of 20 perms that are sufficiently rigid and can be fastened well enough to withstand hurricane force winds without ripping out (turning the unvented roof into a vented roof) are limited.

    Weather resistant & rigid vapor permeable 3/4" asphalted fiberboard runs about 15 perms, half-inch about 19 perms, and wouldn't quite meet code. MDF without the asphalt, overlaid with a high permeance housewrap probably makes it at 3/4", but the vapor permeance of untreated MDF isn't normally found in the specs for MDF, since in it's usual applications vapor permeance doesn't matter.

    Weather resistant fiberglass faced gypsum board such as 5/8" DensGlass is specified at 17 perms (minimum). Half-inch DensGlass is 23 perms (min), but is half-inch gypsum board really rigid enough with sufficient fastener retention for the uplift of hurricane force winds at the roof peak mounted to 24" o.c. trusses/rafters? (I doubt it, but don't know.) It has to be better than housewrap, but is it good enough?

    Until diffusion venting becomes more common it's doubtful that products to solve this issue will magically appear. Perhaps future revisions of the code would allow slightly lower permeance if combined with slightly larger vent ports at the ridge (currently limited to 12" max from the ridge, per subsection 5.2.1.)

  5. Expert Member

    Diffusion ridge vents
    Does anyone sell a manufactured ridge vent big enough for a diffusion vent at the peak? Joe L's detail seems to show a fairy large area was necessary.

  6. Jon_R | | #6

    diffusion vent
    It would be interesting to know how the required 50 cfm to the attic balances out in terms of diluting humidity in the attic vs providing a little bit of positive pressurization (reducing humidity gain via summer infiltration).

  7. Randy_Williams | | #7

    Vapor diffusion port
    The way I'm reading the code is the vapor diffusion port, which is located at the peak of the roof, is the only part of the roof assembly that requires a minimum 20 perm. I'm in zone 7, so the code doesn't apply to me. I could be wrong.

    Peter Yost has an article talking about this code change on the JLC website.

  8. mfroberts | | #8

    I build in climate zone 4 (Bellingham WA) and the only ducts in the attic are ducts to the HRV. Do these ducts (both those going to outside and those going to indoor locations) have the same condensation concerns as those connected to an Heating/Cooling system?

  9. GBA Editor
    Martin Holladay | | #9

    Read the installation instructions for the HRV. The fresh air duct connecting the fresh air intake to the HRV needs to be insulated (to minimize the chance of condensation). If the HRV is located in a vented unconditioned attic (usually a bad idea in a cold climate, but perhaps possible in Zone 4), then you also want to make sure that the ducts conveying fresh air from the HRV to the registers are insulated, and you will also want to insulate the ducts conveying stale air from your exhaust grilles to the HRV (so that you don't lose heat scavenged by your HRV in the winter).

    1. mfroberts | | #10

      Sorry, I didn't mean to imply that the ducts are un-insulated entirely. We insulate the ducts or use insulated flex duct. What I was asking is if we can bury the ducts in blown in attic insulation without the concerns about the condensation issues addressed in your article.

  10. GBA Editor
    Martin Holladay | | #11

    The concerns about condensation addressed in this article arise with ducts connected to an air conditioner. These issues do not arise with ducts connected to an HRV, because ducts connected to an HRV never get cold enough to raise condensation concerns.

    1. drafthunter | | #24

      Hi Martin,

      I have 4” insulated flex duct (r6 for returns, and r8 for supply, not intentional, that’s just what I could get)connected to an ERV and going into an unconditioned attic with fiberglass batts on top of the Sheetrock and then blown cellulose on top of that. I’m able to bury the ducts about about 3/4 the height of the flex. I was thinking of then putting an R-19 23” wide fiberglass batt on top of each duct. I’m assuming unfaced, but not sure. I’m in the Northeast.

      Any advice?

      thank you.

  11. user-7527360 | | #12

    "Buy large diameter R-6 flex duct, and pull the R-6 flex duct over R-8 flex duct, as if you were sliding a sleeping bag over the R-8 duct. You end up with “flex duct inside of flex duct.”"

    Does code allow this? Isn't this method of achieving R-13 flex duct creating multiple concentric vapor barriers?

    1. Expert Member
      Dana Dorsett | | #13

      I'm not sure about code (I doubt it's even addressed) , but all flex duct is concentric vapor barriers- not a problem since it's air-tight.

  12. jonsacca | | #14

    Homeowner here getting ready to remove old/install new cellulose to R38 in unconditioned attic with joists with mix of R4.2 insulated flex duct and galvanized. Located in Sacramento, zone 2b. Is attic air dry enough to allow duct burial in this zone? Can I bury the existing R4.2 as well as the R8 insulated duct which will replace the galvanized? Attic probably needs to remain unconditioned. Roof is hi profile Spanish tile. Thanks

  13. bjh85 | | #15

    I'm installing bathroom fan in my home in Minneapolis, with ducts passing through the attic and exiting on a gable wall. I plan to blow in approx R50 worth of cellulose later. What's best practice for duct placement (buried or exposed) and insulation for the duct itself? My plan is to use PVC pipe and to angle it slightly towards the exit vent. If duct is buried, that seems to handle the insulation issue, but then there's Minnesota-cold air that's just a few inches (2x4 joists) above my bathroom to the extent that outside air penetrates the duct cavity when not in use. If I place the ducts above the cellulose, then what's a safe level of insulation to avoid condensation? R13? Thanks!

    1. GBA Editor
      Martin Holladay | | #16

      The best approach is to run the duct vertically -- straight up -- through the insulation to a 90 degree ell, and then at a slight angle downward to the gable wall termination. Lots more information is available in this article: "Bathroom Exhaust Fans."

  14. user-5621862 | | #17

    Hi Martin,
    Regarding this section of your excellent post: "Don’t forget that galvanized register boots must also be covered with duct insulation that includes a plastic jacket or closed-cell spray foam." Other than emptying a can of Great Stuff all around the boot, is there a good way to achieve this? Does someone make a jacket-type product yet, or publish a detail of the best way to DIY this? Thank you.

    1. GBA Editor
      Martin Holladay | | #18

      Several manufacturers, including CertainTeed and Owens Corning, make fiberglass duct insulation with an integral jacket. Here are links:

      CertainTeed duct insulation

      Owens Corning duct insulation

      Of course, the seams on the duct insulation must be sealed with a high-quality tape.

      Using canned spray foam (either single-component foam or one of the larger two-component systems) would also work.

  15. user-7682561 | | #19

    The ductwork I intend to put into my uninsulated attic space is strictly for range hood exhaust and makeup air. Would these codes apply to those?

    1. GBA Editor
      Martin Holladay | | #20

      User 7682561,
      The advice in this article concerns heating and cooling ducts. The advice does not apply to range hood exhaust ducts or makeup air ducts.

      That said, you don't want your range hood exhaust duct to be cold in winter. (Cold ducts encourage the accumulation of congealed grease on the inside of the duct.) It makes sense to insulate range hood exhaust ducts.

  16. user-7567093 | | #21

    How about an air handler in the attic? My central AC unit is in the attic, and while I can imagine finding a way to bury the ductwork in insulation, I haven't thought of a good way to deal with the air handler, short of constructing a foam box around it (or eventually converting my entire attic to conditioned space). And how much efficiency am I even losing from the air handler, versus all the duct work?

    1. GBA Editor
      Martin Holladay | | #22

      User 7567093,
      Having an air handler in a vented unconditioned attic is a very bad idea, and results in significant energy waste. The usual solution is to convert the vented unconditioned attic into an unvented conditioned attic. For more information, see this article: "Creating a Conditioned Attic."

      You probably also want to read this article: "Should I Build a Mechanical Room in My Attic?"

  17. mr_stop | | #23

    I'm having a condensation issue which I believe is caused by the attic ductwork. The home was recently remodeled by another contractor who moved the upstairs ductwork (ranch) from the basement to the attic. I was brought in to finish the basement space. I am getting a bunch of condensation on the ductwork where it turns up to go into the attic. The condensation is more prevalent on hot and/or humid days.

    The attic ductwork has only been wrapped in "bubble wrap" and is exposed to the attic (not covered in insulation. I'm not sure how well the ductwork is sealed beyond the bubble wrap "insulation". My hypothesis is that the attic is heating the ductwork and possibly allowing moisture in. At which point the hot, possibly moist air is entering the basement ductwork and causing condensation on the outside of the ducts. Is this plausible?

    I'm in climate zone 4. My thoughts are to reinsulate the ducts with R11 PSK and then blow in additional insulation over the top. I'm not sure whether to keep the "bubble wrap" or remove it. Does this sound right?

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