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Need help with spray-foam insulation

Alexander_P | Posted in Energy Efficiency and Durability on

We are building the house, and I was going to put spray-foam insulation on external walls 2×4 covering 3″ with foam. But my builder advises against it, saying that there are potential mold issue. we leave close to Detroit (as weather goes).

Thank you in advance for your help.

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  1. homedesign | | #1

    We don't need no Stink'n Spray Foam

    What we need are Air Barrier Systems,
    mechanical systems INSIDE the Air Barrier,
    and Ample (well beyond code minimum) R-value

  2. GBA Editor
    Martin Holladay | | #2

    Although John Brooks appears in no mood to answer your question, I'll do my best. If I understand you correctly, you are planning to install 3 inches of spray polyurethane foam between your studs. There is no reason whatsoever to think that spray foam will lead to a mold problem.

    Three inches of open-cell foam will only give you about R-11 -- that's not much. Three inches of closed-cell foam will give you about R-19.

  3. homedesign | | #3

    Sorry for being obscure(and ranting)....
    you can build succesful assemblies with spray foam......

    I would urge you to look into other Air Barrier strategies.
    and to consider upgrading to more than a 2x4 wall

  4. Alexander_P | | #4

    Thank you VERY MUCH for your answers. I just talked to builder and decided to go with 1 inch spay foam and than fiberglass insulation (without paper). His concern was, "house not breathing" properly if he puts just spay foam. as I understand - mostly at areas where wall meats roof. Thank you again.

  5. GBA Editor
    Martin Holladay | | #5

    I'm not sure what you mean by "breathing" -- but it's vitally important to have a very good air seal where the wall meets the roof. That is NOT somewhere where you want to encourage air leaks, so be sure that your air seal is impeccable (that means EXTRA foam, not less foam) up there.

    Your contractor is planning to install only about R-15 of insulation. That's not much.

  6. Alexander_P | | #6

    Martin, thank you for the tip. I will talk to him tonight and will bring it up.

  7. Expert Member
    ARMANDO COBO | | #7

    Detroit is in Climate Zone 5A, so it means, to build you wall right you need 1'-2.5" of outsulation minimum to avoid thermal bridging and/or condensation in the wall cavity when the indoor temperature is 70°F and 30%-50% relative humidity; also, good moisture and sealing details. See the Dew Point analysis for Detroit; where possible condensation is related to indoor temperature and relative humidity. This is a conservative approach, but good enough for you to pay attention and decide how you are going to control the indoor temperature and humidity in the winter time. has the best information available on the subject. Here are 3 of their papers:

  8. Expert Member
    Dana Dorsett | | #8

    What Martin said- not only is R15 not much, it doesn't even meet code-min for R in Detroit.

    Putting 1" of closed cell foam on the EXTERIOR and putting R13 or R15 batts in the bays would meet code though, and it would outperform 3" of closed cell foam in the stud bays without exterior foam (which would be code min, but only if a product rated R7/inch was used.)

    The notion that the wall needs to "breathe" makes no sense. The sheathing needs to be able to dry and not accumulate condensation in winter. With 3" of foam effectively blocks moisture transfer from the interior to the sheathing in winter, but it also blocks drying toward the interior in summer. If the siding is back-ventilated to the outside the sheathing can dry toward the exterior all year without a problem. Vinyl siding is inherently back-ventilated, but for other siding mesh-type commercial "rainscreen" backing products are available, or furring can be installed between the siding & sheathing to provide a vent-space. The studs don't need to "breathe", since they transfer moisture via diffusion at about the same rate as closed cell foam.

    But the blowing agents for closed cell foam are far from green. Better to air-seal by other methods, and use 1-2" of foil-faced rigid polyiso on the outside of the sheathing, which allows you to use cellulose, open-cell foam, or unfaced batts in the stud bays without risk of excessive moisture accumulating in the sheathing.

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