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Community and Q&A

Rim joist foam and termites

Jeffrey S | Posted in Green Building Techniques on

I’m not sure if this would fit better ‘Energy Efficiency and Durability’ category, but I have a few questions about using foam to insulate rim joists. From an air-sealing and thermal performance perspective foam seems the clear choice. I’ve read Martin’s FHB article and spray foam and cut-and-cobble approaches make sense.

But what should be left exposed for termite inspections? Can the sill plate be covered, as in the picture below, if the top few inches of the vertical wall are left exposed? I have CMU walls and would like to seal the hollow cores, so this approach is ideal if it is sensible in termite-prone areas.

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  1. User avatar GBA Editor
    Martin Holladay | | #1

    As far as I can determine, termite inspection strips (that is, a visible section of the concrete foundation not covered by foam insulation) are not required by the IRC. For more information on code requirements, see 2012 IRC Section R318.

    Termite requirements are intensely local. The only way to know what is required in your jurisdiction is to talk to your local code authority.

    I welcome input from GBA readers, especially those building in geographical areas that fall into the "very heavy" infestation category.

  2. Jeffrey S | | #2

    Thanks, Martin. I talked with the building inspector and he said an inspection strip isn't required. He said he hasn't heard of a lot of termite problems in our town, but I don't know how much weight to place on that observation.

    Does anyone have experience insulating rim joists with spray foam in termite prone areas? We're on the border of zone 4/5 in southeast Pennsylvania.

  3. User avatar
    Dana Dorsett | | #3

    Local codes I've seen in the past typically require 3" minimum for inspection strips. It would be fine to spray-foam the foundation sill as long as you can access continuous 3" of bare concrete on both sides of the foundation for the entire perimeter. Cutting-in strips of rock wool that can be easily removed for inspection should be allowable, and would mitigate the low-R stripe in an otherwise moderate-R foundation insulation. In a zone 4/5 location 3" of rock wool in direct contact with the top of the foundation would not create a moisture problem and would perform about as well as the R15 foam/other below the inspection strip, if insulated only to code min on the interior.

    In that climate R4 foam trapped to the foundation with a non-structural 2x4/R13 studwall would meet code performance, but the inspection strip would have to be above the top plate of the studwall, and readily accessible for inspection.

  4. Jeffrey S | | #4

    Thanks, Dana. Those are very helpful details.

    I also spoke with our termite inspector and he was quite understanding. Because we have hollow block walls, he suggested capping that with aluminum or copper before applying any foam. He also said to keep the foam tight to the rim joist so the foam doesn't cover too much of the floor joists. This should be the case for a neat job, but I think it's a good point to consider.

    He also said that in addition to a leaving an inspection strip, it might be wise to leave access to areas with higher potential for moisture accumulation. He said areas near stairs and chimneys are high risk areas because of flashing failures where these parts meet the house.

    These recommendations mostly relate to moisture management, but I think they're helpful in thinking about wood boring insect management as well.

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