rim joist moisture issue


Old 01-06-13, 07:54 PM
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rim joist moisture issue

we built a new home two years ago (this is our third winter) and have had a n issue with rim joist condensation issues from the start. The rim joists were filled with r-13 unfaced fiberglass and I discovered the first winter while sealing up some pipes wth great stuff that the fiberglass was wet as well as the rim joist. The insulation company as well as contractor blamed it on the fact that it was a new build and the house needed to dry out. I pulled the insulation and sealed the inside peirmeter of all of the rim joists with great stuff and ran silicone along the seal plate. With the help of a de-humidifer the following winter the problem was not as bad until we ran into temps that dropped into the single digits. Once again I pulled the insulation and continued to work on lowering the humidity level in the basement. There is absolutely no water issue in the basement and as soon as the insulation is removed the rim joist becomes warm and dry. This winter I thought I had the problem licked...I had lowered the relative humidity in the basement between 38-42 % and the rim joist stayed dry. However, last week the night time temps dropped down to single digits and sure enough the rim joist and fiberglass insulation was wet. I have read where the fiberglass is a failed system and spray foam is the way to properly insulate the rim joist. I am having a blower test done tomorrow. I think that the insualtion is only good until the temp gets to a certain point and then condensation will occur. Any thoughts would be greatly appreaciated.
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Old 01-06-13, 08:54 PM
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The moisture is passing through the fiberglass and reaching the cold rim joist. Anytime you add insulation you will be removing the heat source from the surface you covered. As that surface gets cold, you must also install an air barrier. Even faced fiberglass would not be sealed well enough. Spray foam as you mentioned or some rigid insulation cut, fit, and sealed into place.
You can add other forms of insulation AFTER you install at least a couple of inches of foam. But the ratio needs to be more R-value from the foam than added after the foam, that keeps the inside surface of the foam warm enough to not form condensation.

The one note would be termite inspections do not like to dig through a lot of spray foam, but OH shouldn't be too bad for those little buggers.

Old 01-06-13, 09:08 PM
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The problem is that warm humid air can pass through the fiberglass and condense on the cold exterior surface. Spray foam (not the small DIY cans) is the best way to seal that space, since it can both be a vapor barrier and it will completely coat the cold surface, separating it from the warm moist air in the basement.

Even if the fiberglass insulation was faced, you would still have a condensation problem with no air barrier to prevent air leakage around the insulation.

If you don't want to go with spray foam, you could cut rigid foam to fit in the space, and then add more insulation provided you air seal all the edges with either a foamboard adhesive, tape (like Dow styrofoam seam tape) or spray foam. But you'll get better R-values and better air sealing if you go with the spray foam.

Edit: Sorry Bud, I see that I either type too slow or have too many replies going at once. LOL
Old 01-08-13, 09:57 PM
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Agreed that two part poly foam is the best way to do it. Keep one thing in mind though: Most DIYers do not properly foam the bottom of the sill and leave it exposed because it is tough to get the right angle to hit it properly. You have to make sure you insulate the entire sill surrounding. I would use 2" rigid foam cut a big large and friction fit into the bottom of the sill first and then cut another piece and friction fit into the rest of the sill. then you can take some great stuff and seal the seams (most important part). If you don't care as much about the aesthetics, then two-part the crap out of it and make sure you angle the gun down into the bottom of the sill first before you foam the rest of the sill. Keep in mind also that using two-part poly foam will require a fire retardant coating or drywall in most building codes because fumes are noxious if ignited. Good luck and throw that darn fiberglass away for good!


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