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Is open-cell foam appropriate for spraying the bottom of a roof deck?

Allen Brown | Posted in Energy Efficiency and Durability on

We are building in Indianapolis, middle of zone 5. House will be on a slab, under-slab hydronic heat powered by geothermal, 4″ Thermocore SIPS walls with 2″ of rigid foam on outside of that. Truss roof. My homeowner wants the bottom side of the roof deck sprayed. My experience is typically spraying the attic floor, not the roof deck. However, in this scenario, we will have ductwork in the attic, so it does make sense to spray the roof deck, so the ducts are in conditioned space. My insulator is quoting us 5-1/2″ minimum open cell foam.

My understanding is that open cell has an r-value of roughly 3.6/inch. That would only give a total R-value of 19.8. Code minimum is 38, we typically insulate our attics to an R-50 using a combo of 2″ open cell on attic floor covered by 12-14″ of cellulose.

My insulator says that there isn’t really a defined R-number for open cell, and because it stops the heat transference, 5-1/2″ meets code.

I am not interested in “just meeting code”. Knowing the other things we are doing, we want this to be a high-end-performer on the efficiency scale. Can somebody tell me the best methods to use when insulating the bottom of the roof deck? Or even if you think this is a bad idea altogether?


Allen Brown
UBuildIt, Indy

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  1. Expert Member
    Dana Dorsett | | #1

    Your insulator is full of shi....

    ...nola. There IS an established ASTM C518 tested R-value for open cell foam, and it is published in the specification data from the manufacturer. (And yes, it's about R3.6/inch for most half-pound density polyurethane.)

    The issue with using open cell foam on the underside of a roof deck is it's comparatively high vapor permeance as compared to closed cell foam or other vapor retardent materials. It takes about 14" to get to R50 with an all open-cell solution (and it has to be applied in three lifts of 5" or less, or it'll have shrinkage isues and may even catch fire shortly after installation.) But the vapor retardency at 14" for most half-pound foam is between 1.5 and 5 perms, which could be risky in a zone 5 location with an OSB roof deck, and would even accumulate some moisture over a winter in the outer inch or so of the material.

    A better solution is to first put 1-2" of closed cell foam on the roof (R6-R12), with the rest of the R as open cell foam or damp-sprayed cellulose or damp-sprayed JM Spider. (Or dry blown behind gypsum.) This works fine in zone 5- see Table 3 in this document:

    An inch of closed cell foam runs between 0.8-1.2 perms, at 2" it's half that, which is vapor retardent enough to be protective of wintertime moisture adsorption, but not so vapor tight that the roof deck can't dry at reasonable rates.

    See also:

    It will be FAR more expensive to insulate at the roof deck than an attic floor insulation with cellulose, but don't let anybody try to con you with "Six inches is all ya need." BS, because is simply isn't true, and demonstrably so.

  2. Allen Brown | | #2

    Thanks Dana. That wasn't sitting right with me, which is why I asked.

    I just read Martin's article:
    Answers a lot of questions, but brings up more. I see now that we need to use closed cell foam. it seems to me that it will be a bit more difficult that we are using trusses. On the one hand, since our truss top chords will be 2x4, there is not the space to insulate between the rafters. It seems with trusses, putting several inches of closed cell on the bottom of roof deck AND covering the 2x4 top chords with same will eliminate most of the thermal bridging. The only thermal bridging I'll get is where the diagonal & vertical posts meet the top chords. Seems like a pretty minimal bridging scenario.

    If closed cell is R-6, I would need 6+ inches to get to code minimum, and 8+ inches to get to an R-50. That tells me that the spray foam would have to be installed in two sprays.

    If I am spraying closed cell in this scenario, is an R-50 overkill?, or should I stick to the plan of having it 8+ inches thick to get that R-50?

    Allen Brown

  3. Aaron Vander Meulen | | #3

    IMHO I would consider doing something different to get the ductwork out of the attic. At the end of the day it will probably be the most cost effective solution. 8" of closed cell is going to add up in a hurry. Alternatively, have you looked into doing SIP's for the roof system?

  4. Allen Brown | | #4

    Sips roof won't work on this house, as it is an all hipped roof. That makes it way expensive. Maybe I'll try to find a way to enclose the ducts.

  5. GBA Editor
    Martin Holladay | | #5

    If at all possible, it makes sense to install some rigid foam above your roof deck. In climate zone 5, the code requires at least R-20 if you go this route. Then you could install any type of insulation you want on the inside. (You're right, of course, that the truss roof makes it awkward to install loose-fill or batt insulation on the interior.)

    In general, keeping the ducts out of the attic is always preferred. For more information, see Keeping Ducts Indoors.

  6. Mitch Groves | | #6

    The insulation company I talked to said 6 inches of open cell on the underside of the roof is what they recommend in my area and I am from the Evansville, about 150 miles south of you. We are in zone 4. They do a LOT of work in my area. I at first was skeptical but my son is in construction and he has seen their work. He confirmed 6 inches was what they did. He has worked in the attic area in the midsummer heat and he said it was comfortable. I think the main benefit was the air barrier it provides. No air leakage you would have with cellulose or fiberglass.
    Just to warn you I have not been able to find any confirmation on the internet but it passes the codes here and is used in some high end houses. But you have to do what you are comfortable with.
    You might contact some of the building inspectors in your area and see what they say.

  7. GBA Editor
    Martin Holladay | | #7

    Here is an article that discusses the issue you raise: It’s OK to Skimp On Insulation, Icynene Says.

  8. Allen Brown | | #8

    OK, How about this method; we place the ductwork on attic floor. We staple netting to the joists, drape over the ductwork and staple to joists on other side. We spray 2" of open cell foam on attic floor & over the netting on the ductwork. We then add cellulose to an R-50 above that. That way we get our R-50, and the ductwork stays inside the thermal envelope.

    See attached sketch.

  9. GBA Editor
    Martin Holladay | | #9

    Q. "How about this method?"

    A. That will work.

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