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Community and Q&A

Insulating Ducts

user-6765831 | Posted in Green Building Techniques on

I’m remodeling my house in SF Bay Area climate zone 3C.  Fujitsu ductless minisplit downstairs and ducted minisplit in attic, with my HVAC contractor supposed to deeply bury the ducts per our contract (though I notice now it states to R20, not R25 as one might expect).  Cellulose insulation to be installed to base depth of R49.  Ducts with R8 insulation, reflective vapor barrier, to 20 cfm25 leakage.

We have completed air sealing house and it is very tight (excluding windows and doors which are still taped, 0.22 ACH 50)

The boots of the ducts are bare metal, haven’t been foamed over from attic side.

I contacted my HVAC contractor in preparation for insulation install, and because he is subbing this out, asked him to ensure his guy  to cover all the ducts (except right next to air handler) with 3.5″ of insulation over the top per definition of deeply buried ducts.
He is from a reputable company who has a great reputation and has been recommended on this website, and he says the following:

“Just FYI there is often not 3 1/2 inches over all the ducts This is based in the world of averages. Fortifying the duct R-value to avg 20 is our goal The load calculations are based on R 16. The sides and bottom of the duct are infinite R. The larger ducts such as large returns will crown above the insulation level. R44 Total depth is the goal. That’s R14 above normal R30.

If a small amount of the duct sticks up on larger ducts above that is a very small percentage of the overall duct surface area. With R8 Exceeding the R 20 goal easily

The risk of condensation in California is dramatically low compared to most of the rest of the country. However raw metal in the attic will condense.

Duct work that has R8 insulation And a vapor barrier will not.

Generally the boots are taped off at the time of installing the register so we know the registers will fit.

We do things a little bit different in California because of the dry climate aspect. It changes a lot of those concerns. I understand you want to get this done the best but I wouldn’t worry too much about condensation.

It is possible with the tightening of the building envelope that some moisture will stay inside the shell but that’s what the ERV is for.”

Doesn’t seem like he is planning on spraying the exposed boots with closed cell foam, nor completely covering the ducts.  To me, the former seems risky, and the latter seems to not meet the terms of the contract and silly not to do.

Any thoughts or advice?  If he doesn’t want to do it, I could attach some fiberglass batts to the crowning ducts myself as well.  I think he does want me to be satisfied with install.


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  1. GBA Editor
    Martin Holladay | | #1

    It sounds as if the work will comply with new code requirements for buried ducts. For more details on code requirements, see this article: "Burying Ducts in Attic Insulation."

    You wrote, "I contacted my HVAC contractor in preparation for insulation install, and because he is subbing this out, asked him to ensure his guy to cover all the ducts (except right next to air handler) with 3.5 inches of insulation over the top per definition of deeply buried ducts."

    If you perceive this to have been part of the agreed-upon work, then you should discuss this with your contractor. That said, unless you have something in writing, this may be tough to enforce.

  2. user-6765831 | | #2

    I can’t find anywhere in the code requirements or article that the buried ducts can randomly jut out of the insulation- and I thought including the term “deeply buried ducts” on my contract meant something. Oh well. Thanks for replying.

    1. GBA Editor
      Martin Holladay | | #3

      If you have a written contract that includes language specifying "deeply buried ducts," that is, indeed, useful in any contract dispute.

      On re-reading the IRC, I am changing my opinion. Even in your climate zone, buried ducts must meet the requirements of R403.3.6 (if that code has been adopted in your jurisdiction). Good luck.

  3. oldbungalow | | #4

    Isn't the crowned part of the duct at attic temps? Even in a humid environment that shouldn't condense.

  4. jameshowison | | #5

    Yeah, I just went through a similar situation. I think it's almost impossible to avoid the crowning issue with cellulose. The ducts have to run above the joists and a little above the joists so that the wrapping insulation isn't crushed (and connections can be taped etc). Then the 90° attached the the register raises them even further (top of the boot is at the top of the joists already, vertical straight section of the curve is a few inches at least (more on larger ducts). And the take-off on the plenum is in the middle of the plenum, not at the bottom of the side, so that's also quite high (even if the bottom of the plenum is on the top of the joists, but they are raised on foam blocks). i.e. on a 20" plenum, a 6" take-off starts with the center at 13" (sheathing on floor, 2" foam block, 10" to center of plenum), so that puts the bottom of the duct at 10" above the joists ... larger ducts have centers at same place but bottoms lower. I dunno maybe they could do the takeoffs lower on the plenum, but I doubt that's common practice.

    So our metal ducts ended up running with the bottoms maybe 10" above the ceiling (joist depth + gap). We did R-38 of cellulose (probably should have done more, but barely makes a difference) which was about 13" deep ... so the "bury" was maybe 3 or 4" of the diameter. So, not much impact at all.

    Perhaps that is less of a problem with flex ducts allowed to lie on the joists, but then they aren't pulled tight and that adds static pressure? Maybe you can convince them to do some offsets to get a long run lower, but then you are adding like 4x45° (two down, two up) to the run ... If you try to run them down in the joist bays then you have them draped over joists when they have to turn, adding length and making flex even more likely to get crushed as people move around...

    Obviously any cross-over in the ducts (supply below return hopefully) raises things massively as well. And I got involved in routing the ducting to try to avoid cross-overs which I'm now convinced has lead to more effective length and air movement noise. So my effort to keep the ducts low to bury them was probably a net negative. I did stop any strapping of duct to the roof sheathing, and managed to get some refrigerant lines off the bottom of the rafters and into the cellulose. Understandably they want to route these in areas where they won't be stepped on etc, but some choices on the routing are hard (such as avoiding making it hard to crawl through parts).

    In any case only spots I saw any condensation (before the cellulose blow) even in hot humid summer with the supply ducts running very cold air, are the hand-dampers handle areas.(because they are disconnected from the vapor barrier). So the high quality job on the R-8 insulation is doing its job.

    1. oldbungalow | | #6

      My (perhaps mistaken) understanding has always been that the exposed R-8 duct in a warm attic has an outside surface temp the same as or close to the attic so it's unlikely to cause condensation. Once it gets buried it's more likely to get cold then condense? So it has to be buried enough to find that balance, depending on dew point.

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