# Insulating rim joist



## bseries (Feb 27, 2012)

Hello,

I'm in the process of insulating the rim joists in my house, and have noticed that my home's joist setup is different from what i've seen in pictures and guides online. (see picture)

Basically the joists are embedded nearly 3/4 of the way into cement along the tops of the basement walls. 

Is it a problem to have that chunk of cement in between the joists sealed in air tight with pink rigid foam? I'm thinking that due to cement being porous, it could wick moisture in there that wouldn't get a chance to dry.. am I way off with this?

The wood above the cement in the pic has water marks on it, but i'm 99% sure it's from when the house was constructed (1979), practically all the joists, and even sub floor have water marks and even chunks of grass and dried mud. Plus the rim joist is located a bit more than 2.5 feet above the ground. 

Any suggestions are much appreciated.


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## jklingel (Dec 21, 2008)

If the house has survived since '79, then I'd keep doing what it is used to, which is apparently "very dry". Normally, concrete and kiln dried, untreated wood do not mix well, but yours seems an exception. Can you insulate on the outside of the building instead? I'd be a little nervous of insulating inside and maybe stopping the drying to the inside that has likely been going on. Where are you located?


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## Windows on Wash (Aug 30, 2011)

jklingel said:


> If the house has survived since '79, then I'd keep doing what it is used to, which is apparently "very dry". Normally, concrete and kiln dried, untreated wood do not mix well, but yours seems an exception. Can you insulate on the outside of the building instead? I'd be a little nervous of insulating inside and maybe stopping the drying to the inside that has likely been going on. Where are you located?


+1

Changing the drying can mess things up if you aren't careful.

As inquired, where is the home?


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## bseries (Feb 27, 2012)

jklingel said:


> If the house has survived since '79, then I'd keep doing what it is used to, which is apparently "very dry". Normally, concrete and kiln dried, untreated wood do not mix well, but yours seems an exception. Can you insulate on the outside of the building instead? I'd be a little nervous of insulating inside and maybe stopping the drying to the inside that has likely been going on. Where are you located?


Hmm..I don't think it's particularly dry in the house, relative humidity is always quite high even in the middle of winter. 

I could insulate the outside, but it wouldn't look very good at all. Would it acceptable to caulk the joints where potential for air leakage might be, then just fiberglass insulation? Fiberglass stuffed in there is all there has ever been. 

I'm located in Canada (Saskatchewan).


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## jklingel (Dec 21, 2008)

Was it ever air sealed along w/ the fg? If not, then re-apply fiberglass, as it breaths real well. That is one reason it is a crappy insulation, but better than nothing. If the RH is high, you may want to consider bring it down a bit, too. Approx 30%. Always air seal cracks.


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## desus (Apr 2, 2012)

I am dealing with the exact same scenario. My rim joists are only embedded about halfway though, so about 3 inches of air space.

My plan was to use 1" rigid foam against the outside 2x6, atop the poured foundation and along the inside facing foundation that is between the joists. The thought was to leave this open vs sealing it off. Perhaps one could stuff fiberglass batt in the open cavity...

I think this would be a good balance between totally sealing it off and risking potential moisture/rot issues and the other option of just air sealing, as per the NRCan suggestion for dealing with embedded joists.

...sophisticated diagram attached.

Any thoughts on this approach.


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