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Rim joist insulation

alawishy | Posted in Energy Efficiency and Durability on

I insulated and air sealed my rim joists using 2 and 3/4 inches of polyiso along with one part foam around edges plus an inch or a bit more of mineral wool to get me to my target of R20+. I covered the insulation with 1/2 drywall for a thermal barrier and caulked the top and side edges as well as around plumbing penetrations through the drywall. I did not caulk the bottom edge (at least as of yet) as it is hard to get a caulk gun to the area. This edge is screwed tightly against the sill plate. Given this construction, do I have any condensation issues to worry about? I live in zone 5 (NH).

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  1. Expert Member
    Dana Dorsett | | #1

    Short answer: "Nope!"

    Longer answer: At 2.75" even derated to R5/inch you're looking at ~R14 of air-impermeable foam outside of at most R6 of rock wool. The first condensing surface would be the interior face of the polyiso, but it's temperature will be roughly R5/R20ths (1/4) the room to outdoors delta lower than the room temp. Even if the interior temp only averaged 60F in mid-winter when the outdoors averaged 20F (the cold edge of a zone 5 climate), that's a 40F delta-T, and the interior surface of the would average 10F colder than 60F, or 50F.

    The only way the dew point of the interior air would be as high as 50F in winter would be if you were running a humidifier, bringing the relative humidity up to 70% RH @ 60F, or 50% RH @ 70F. Note 50% RH @ 70F is at the high end of the "healthy for humans" range recommended by health professions. The low end of the healthy range is 30%RH, and more likely where you will be during months with a binned hourly average outdoor temperature of +20F.

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