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

J S | 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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  1. GBA Editor
    Martin Holladay | | #1

    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. Torsten Hansen | | #2

    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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