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Another basement question

polar_vortex | Posted in Green Building Techniques on

Sorry if this is a repeat question.  I tried finding the answer in the forum but was unsuccessful.  

Planning on finishing our basement in the Chicago area.  Home is 10 years old with 2” rigid on exterior basement walls. Do I need to insulate the interior?  If yes, am I limited to rigid foam?

thank you!!

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  1. GBA Editor
    Martin Holladay | | #1

    Chicago is in Climate Zone 5. In Zone 5, most building codes require a minimum of R-15 basement wall insulation. Your 2 inches of rigid foam has an R-value of R-8 to R-10, depending on what kind of rigid foam was used. That's not quite enough to meet code.

    I would advise you to improve the R-value of your basement wall by adding another inch or two of rigid foam on the interior -- 2 inches would be best. For more information on this work, see this article: "How to Insulate a Basement Wall."

    If you are sure that the exterior rigid foam was installed perfectly, and that the exterior rigid foam extends all the way from the top of the wall down to the footing, an argument can be made that your basement walls are warm enough and dry enough for you to consider insulating the wall on the interior with fiberglass batts. You can do that if you have a high appetite for risk. I'm conservative on this issue, so I would stick with rigid foam. That will reduce your chances of moisture accumulation and mold (compared to using fiberglass batts).

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