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Community and Q&A

Applying Thermal Barrier to Rigid Foam at Rim Joist

Erich Riesenberg | Posted in Energy Efficiency and Durability on

I am a homeowner planning to air seal the house. Starting with the basement because it will be easiest.

At the rim joist I am going to use rigid foam because it is cheapest and will be a good starter proejct.

I understand some locales do not require a thermal barrier to cover the foam, but I would like to add one for safety.

Here is a list of thermal barriers: http://www.foam-tech.com/theory/firebarriers.htm

A spray on sounded interesting, but none of those listed appear to be in current use, and they appear to be asbestos based.

So now I am thinking of gypsum board because it is relatively thin.

Would the boards be attached to the rigid foam with caulk, glue, or something else?

I have searched a lot for information on how to apply the thermal barrier and not found answers. Perhaps most locales do not require it, or the spray foam is more popular now? Is a thermal barrier not worth the effort if it is not required by code, i.e. it does not provide noticeable safety improvement?

Thank you.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Erich,
    If you want a thermal barrier, you might consider using Thermax polyiso (a type of rigid foam). Thermax has passed tests that show that the product resists ignition. It is accepted by many building inspectors for use without a thermal barrier.

    If you want to install a layer of drywall, that will also work. The drywall can be adhered with construction adhesive; choose a brand that is compatible with the type of foam you are installing. (Most building-supply stores sell foam-compatible adhesive.)

    If you don't want to use adhesive, you can use screws. Choose screws of an appropriate length -- long enough to extend through the foam into the rim joist.

  2. Erich Riesenberg | | #2

    Thank you Martin. I have bought 100+ books on home renovations and still there are multiple ways and small details.

    I have two houses now both from the 1940s - 1950s and am planning to upgrade them both as much as possible.

    Your blog is great reading.

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