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Rim joist, rigid foam, and thermal barriers

MallocArray | Posted in Building Code Questions on

I recently read that air sealing the rim joist can be an straight forward way to improve efficiency in your home and is something that can be done yourself.  That lead me to this site and the article here:

Insulating Rim Joists

It states: Most building codes require rigid foam to be protected with a layer of 1⁄2-in. drywall as a thermal barrier.

How do you find out what your specific building code says about this? I’m in Saint Louis County in Missouri but not sure where to even start looking in regards to this.

Currently I have just fiberglass batts stuffed in the cavity and I know they have had mice nesting in some areas in the past.  Cutting rigid foam and using expanding foam to seal it in sounds doable, but covering it with drywall adds a whole other level of complexity.  Can I add fiberglass batts over the rigid foam to provide the thermal barrier?

About 1/3 of the basement is unfinished laundry and storage area and 2/3 has a drop ceiling below the rim joist and no bedrooms.
Can you help point me to where I can find out what is required if I do this project?

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  1. Expert Member
    BILL WICHERS | | #1

    Building code allows rim joist insulation to remain exposed. There is a maximum thickness for exposed spray foam, I think it’s 3.5” but I can’t remeber for sure. The best way to be sure what your local requirements are is to call your local building department and ask them.


    1. MallocArray | | #2

      Thanks Bill. I did see that straight spray foam can be exposed, but my plan would be to use rigid EPS foam boards and some canned expanding foam around the edges to seal it in. Less mess and easier on time constraints. I'll see if I can find contact inform for the local building department.

  2. user-2642926 | | #3

    Here's why you may want to consider drywall.

    The fire risk is... I'll say non-zero. At least with drywall covering it the risks would be somewhat reduced. (I'm not trying to lay an ominous tone here. I live in a house with rigid foam on the exterior. I would personally not want to live in a house with interior rigid foam that doesn't have some degree of fire protection.)

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