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Old 09-20-2012, 11:06 AM   #1
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Rigid Foam in Rim Joists


I'm insulating my basement using 2" rigid foam on the concrete walls. I keep hearing to also put foam up in the rim joists, what is the purpose of this? I have plumbing and HVAC up there that fit ok with the existing fiberglass insulation, but there isn't room for 2 inches of foam in a lot of places.

Since the rim joists are above grade, why should I replace my existing insulation there?
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Old 09-21-2012, 06:01 AM   #2
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Rigid Foam in Rim Joists


The larger issue with the rim joists is air leakage.

Fiberglass is okay as long as it is properly air sealed at the framing (usually caulked and/or spray foamed).

If you pull the fiberglass out, you should see that it is tracked with dirt and will clearly show where the air movement is happening.
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Old 09-21-2012, 08:49 AM   #3
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Rigid Foam in Rim Joists


That makes sense, thank you very much.
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Old 09-21-2012, 09:34 AM   #4
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Rigid Foam in Rim Joists


MrAngles -

For a basement, the insulation above grade is more important than below ground level, so insulating the rim joist is very important, usually neglected.

I stumbled on a method that I used on several homes I lived in. -

I would rough pre-cut XPS foam(usually 1-1/2" or 2" thick) into pieces about 14-1/2" by the depth of the joists for joists that were 16" o.c..

I used the expanding foam and sealed around the cavity created by the joists and rim and the immediately shoved in a pre-cut foam piece. The foam expands to fill the voids, seal the cracks and glues the foam in place. If you have extra foam, you can repeat it before stuffing the worthless fiberglass to the desired thickness before finishing the wall off. A similar method can be used on the walls parallel to the joists and you can use the foam as an adhesive/sealer between off lengths of foam that may be cut to fit any length.

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Old 09-21-2012, 12:06 PM   #5
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Rigid Foam in Rim Joists


CM's way is perfect example of how to get it done properly.
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Old 09-21-2012, 05:38 PM   #6
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Rigid Foam in Rim Joists


“I'm insulating my basement using 2" rigid foam on the concrete walls. I keep hearing to also put foam up in the rim joists, what is the purpose of this?”---------


----- This is to air seal the wood rim from outside air infiltration because the wood shrinks/expands with the seasons: http://www.paintsource.net/pages/sol...ood_shrink.htm

See the "Calculating shrinkage" chart on page 51; http://books.google.com/books?id=iwS...rafter&f=false

“I have plumbing and HVAC up there that fit ok with the existing fiberglass insulation, but there isn't room for 2 inches of foam in a lot of places.”-----------


----- even 1/2" of foam is better than any amount of fiberglass (air permeable) insulation; read last paragraph (though the foil-facing would not permit any drying to the interior); http://www.buildingfoundation.umn.ed...oistphenom.htm


”Since the rim joists are above grade, why should I replace my existing insulation there?”----------


------ because the foam moves the condensation plane from the wood to the foam surface, to air dry inside (and won't rot when wet): "Condensation control"; http://www.buildingscience.com/docum...l_seal_rev.pdf

The foam is cold on one side, warm on the other; this is the "first condensing surface" and is warm enough to prevent condensation, IF the foam is thick enough for the location (temperature wise); http://www.buildingscience.com/docum...bout-diffusion
The wood rim is cold on both side with an R-value of 1.25 per inch; standard rim= R-1.9.

Gary
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Old 09-22-2012, 10:14 AM   #7
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Rigid Foam in Rim Joists


Okay, I assumed that condensation was more of a below grade issue than above grade. So in areas where rigid foam just won't fit in between the wood and pipes etc, should I just shoot as much Great Stuff in there as I can?
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Old 09-22-2012, 08:11 PM   #8
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Rigid Foam in Rim Joists


That will work, use the closed cell stuff, much better than fiberglass...

Gary
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