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Moisture trapped by rim joist foam

Posted by retiredron (My Page) on
Mon, May 4, 09 at 11:11

In the last 6 months or so I have read probably a hundred postings re: "how to treat rim joists". It's important to me since my 1700 SF basement has about 160 LF of rim joist (all except the daylight side in my 7 year old home). I have not seen my specific problem discussed.

Perhaps the climate here (40 miles north of Atlanta) is milder than most. Regardless, in this region, full basements are common, water infiltration through the basement walls seems rare, and every rim joist I have seen has fiberglass insulation, same as walls. My observations are limited and anecdotal, of course.

Problem: In three specific rim joist "bays" (the 14.5" space between floor joists) I found wet insulation and soaked particleboard and rim joist behind the fiberglass as I systematically removed it to seal air gaps. The source of the moisture was obviously rain infiltration due to poor flashing and caulking, and I had a roofer out last week to hopefully correct the problems.

Question: How would this have been discovered or repaired if sprayed on foam or even glued in XPS had been used? If I apply foam now, how can I check to insure the problem is corrected? The exterior siding at these locations is brick masonry, so I'm not anxious to remove it. If the builder had used sprayed on foam, I probably would not have discovered the leaks.

I'm leaving these areas open as I work on other parts of the basement, but will eventually have to bite the bullet.

Comments or reasurance please.


Follow-Up Postings:

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RE: Moisture trapped by rim joist foam

Fiberglass at the rims is a particularly poor choice in the South, where the pressure drive (warm to cold) is predominantly from the exterior to the interior. Judging best practice by looking at what other builders are doing is not the best way to improve the details on your own home.

Liquid water from leaking exterior sheathing will leak around and through foam boards and sprayed foam. But unlike fiberglass insulation, the foam board and spray foam are not moisture sensitive and will not lose R value or provide a home for mould growth.

Here is a link that might be useful: Understanding Basements--BSD-103


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RE: Moisture trapped by rim joist foam

...but it sounds like the rim will still rot if you can not identify the problem...I think this is teh question he is asking...

Good one...


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RE: Moisture trapped by rim joist foam

If you see water seeping through and around the foam, you know you've got a problem and you have to address it. But absent that, unlike other systems using PVC vapour barriers, water vapour is not trapped in the rim joist area causing silent mould and rot. As well, the insulation continues to fulfill its primary function.

Anytime you see water continuously coming in where it's not supposed to, you've got a problem.


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RE: Moisture trapped by rim joist foam

Obviously, fixing the leak is job 1.

Back to the foam, and to one more I-hope-not-too-dumb question. Do I understand correctly that we are only talking about using the foam where the rim joists sit right on the mud sill (where the basement walls are full height), and not in other situations, such as where the rim joists are 12" to 48" or more above the sill?


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RE: Moisture trapped by rim joist foam

The rim joist sits on the "mud sill" (plate) at the foundation. But in a multi-storey home it's not the only rim joist. Spray foam insulation (spf) works well on all rim joists.


Spray foam insulation used as block on
second floor rim joists.
Building Science Corp.


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