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Community and Q&A

Rigid foam insulation and fireblocking

MikeCell | Posted in General Questions on

Hi.   I’m finishing my basement and was going to seal the rim joists with 2 inches of rigid foam with foam sealant around the edges then cover with unfaced insulation.   In the unfinished areas of my basement that I upgrade the rim joists do I need to them cover the joists with approved fire locking material?   Thanks,   Mike

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  1. plumb_bob | | #1

    Any expanded foam needs to be covered with an approved material, but joists themselves do not need to be covered. I believe this to be the case with most of the major codes.

    1. MikeCell | | #2

      Thanks Bob. So the areas of the rim joists don’t need to be covered? Just the walls?

  2. Expert Member
    BILL WICHERS | | #3

    You need to cover any exposed rigid foam that isn't rated to be left exposed (Dow Thermax, for example, is rated to be left exposed). For walls, this usually means putting drywall over the foam. For rim joists, you could use drywall, but it's likely to be easier to put in some mineral wool batts instead. You can use the thinner/cheaper Safe'n'Sound version of mineral wool for this purpose.

    When you're done, any exposed rigid foam should have a fire protective covering over it.


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