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Community and Q&A

Polystyrene Board for Knee Wall Insulation

longfieldl | Posted in Green Products and Materials on

I want to insulate my knee wall space and bring it in the conditionned environment. For that I would like to put 2-faces 2″ polystyren boards to seal the underside of rafters. However, these 2-faced 2″ boards are hard to come by and I was wondering if I can achieve the same result by using two single-faced boards slapped back-to-back. For instance, I would use two 1″ garage door 1-faced SilveRboards.
Let me know what you think.
Thank you

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  1. Expert Member
    BILL WICHERS | | #1

    Have you considered 2" polyiso instead of the SliveRboards? 2" polyiso will get you a bit more R per inch (actually a fair bit more with a 2" thickness, R13 instead of about R10), and the most common polyiso has two foil facers. Box stores can typically order 2" polyiso without difficulty if they don't have any in stock.

    Assuming you have a vented attic space on the opposite side of that wall, I wouldn't expect any issues using rigid foam on the interior. Tape the rigid foam to get a good air seal, and remember that you're probably going to need to put drywall on the interior side to meet code.



  2. longfieldl | | #2

    Hi Bill,
    Thank you for your reply. In fact, I am reclaiming the knee wall space by building storage there and insulation behind is very poor. I am planning to follow the "bring the attic inside" option from this webpage: I will try to find some 2-faced polysio, thank you this advice. I found the 1-faced so far, but will keep looking.

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