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Community and Q&A

Insulating foundation wall?

Greenbean390 | Posted in General Questions on

My name is Nate and i am currently in the final stages of finishing my home with leaving insulating the basement walls one of the last things i have to do. I am building in North Western, WI Zone 4 i believe and wondering the best way to insulate my basement walls?

I have insulated the exterior of the foundation with 1.5″ XPS all the way around with building codes requiring a R15 continuous and i have to have a energy wall on the interior. Also the Slab is insulated with 1.5″ XPS and will be heated.

My plan is to put up 1.5″ of Johns Manville CI Max Polyiso Foam taping all seams. Now from some of the threads i have read i should leave a space between the Polyiso and the concrete floor.

Is this a good solution or is there a better way i can/should be doing this?

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Green Bean,

    I know that some builders leave a gap to prevent the exposed edges of polyiso boards from wicking water. I have also seen builders wrapping the edges of polyiso with foal tape when using it on exterior walls, which you could do. But if you have a capillary break beneath your slab (poly sheeting), I doubt that you will have a problem with the foam wicking any significant amount of water.

    In case you haven't read it, here is an article on insulating basements:

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