Rim Joist Insulation

Old 08-03-23, 02:06 PM
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Rim Joist Insulation

Hi All

Zone 5 Illinois

Basement rim joist have about 3/4" of closed cell spray foam (builder sealed the rim joist perimeter and field) with no batt insulation.
I plan on drywalling the basement after insulating the rim joist.

1. Will I gain any Insulation advantage, if I install unfaced R19 batt insulation in the rim joist, but the exterior walls on the 1st and 2nd floors have R15?
There are only 180 SF of rim joist, so, cost is not a major factor.

2. Is Rock Wool better than Fiber Glass in this case?
I know that RW has more health issues when installing. I will wear a respirator with either FGlass or RWool.

Thank You!
Old 08-03-23, 04:06 PM
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R value is R value, type of material is whatever you are willing to work with.

Will it make a difference, in the grand scheme of a homes insulation, probably not, as we all know heat goes up, out and down so that small area isn't going to be a huge factor.

But, your there and material delta cost is minimal so go for the max!
Old 08-03-23, 04:16 PM
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If insulation is compressed, it loses r value. So of you squeeze r-19 into a 3.5" space it will probably only achieve R15.
Old 08-03-23, 05:16 PM
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The rim joist is in an open joist bay, no drywall to compress the insulation. If anything, the insulation is fully expanded.

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