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In a mixed Midwest climate, is foam board insulation under siding a bad thing?

user-1035959 | Posted in General Questions on

I was talking to a contractor in my area about my siding options and I asked him if when re-siding my house I could add an inch of foam board insulation under the siding. My walls can get pretty cold in the winter months, after all.

He told me that it would be a BAD move and that my house would rot from the inside out due to condensation and moisture building up.

Now I already have some blown-in insulation in the walls, but I don’t think I have enough because my walls get pretty cold. I know that my house need to be able to “dry out” but what options would I have to improve the comfort of the house (cold walls) if I can’t put up foam boards?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Jason,
    If you search the GBA site for articles on foam sheathing, you'll find many articles and videos that answer your question. The best place to start is here: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    The short answer: installing rigid foam on the outside of your walls is a good idea, but it has to be thick enough to prevent moisture accumulation. Thick foam is safer than thin foam. The minimum R-value of the foam depends on the thickness of your existing wall framing and your climate.

    You might also want to read How to Install Rigid Foam Sheathing.

  2. Foamer | | #2

    Jason,
    Sounds like your contractor would prefer not to be bothered with a process that he doesn't understand. As Martin points out, exterior foam is a great idea, you just have to get the details right.

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