Insulating rim joists

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Dred
Posts: 5
Joined: Wed Dec 25, 2019 11:32 pm

Insulating rim joists

Post by Dred » Mon Jan 06, 2020 9:35 am

Hi Shannon, I've been reading conflicting opinions about using rigid foam in the rim joists. I read one where they recommended using EPS over XPS to allow for moisture to travel in case of condensation or water seeping through causing mold to the wood joist and plate. Others mention using XPS but no cautionary info provided. I think it's obvious no one wants damage to their home, but this makes it impossible for someone who isn't in the building industry to know who is right or if there needs to be concern.
I'm writing this because I am looking to insulate my rim joists and looking for a reasonable way to insulate these small areas. I have checked the depth of the rim joists and it's about 2".
Here is what I'm thinking so far after researching:

XPS form - seems most recommend 2" minimum. Concerns: water and mold building up which doesn't allow for it to evaporate quickly. Safety concerns: cannot be exposed to the inside of the house. There isn't more clearance for me to add fibreglass on the inside of it, what would you suggest I do?

EPS form - will provide the thermal break i need similar to XPS. Concern: similar to XPS but will allow vapour/water to travel through for quicker evaporation. Safety: none that I have read, so it can be exposed?

Roxul batts - will provide thermal break, but no measurement value whether it performs just as good as rigid foam? Concern: Not a tight seal? so there will be air seeping through between the joist and roxul? Safety: none based on the product information
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Shannon
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Joined: Fri Nov 25, 2011 11:58 pm
Location: Canada

Re: Insulating rim joists

Post by Shannon » Mon Jan 06, 2020 11:14 am

While I have seen the concerns by some as well I have never seen it cause the issues they say.
For your situation if you can not cover the foam to protect it from fire with Roxul insulation or 1/2” drywall then I would recommend Roxul insulation sealed with poly vapour barrier sealed to the perimeter wood.
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Sean of Chelsea
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Joined: Thu Jan 09, 2020 4:53 am

Re: Insulating rim joists

Post by Sean of Chelsea » Thu Jan 09, 2020 6:05 am

Hi Shannon, instead of fibreglass batts could I also use the Rockwool on the exterior side of the rigid foam? Also, in your video you said to place the Rockwool on the interior side rigid foam but if the rigid foam is on the lip of the sill how do I secure the Rockwool? Should I inset the rigid foam as much as possible and have some Rockwool protruding past the sill on the interior side?

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Shannon
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Joined: Fri Nov 25, 2011 11:58 pm
Location: Canada

Re: Insulating rim joists

Post by Shannon » Thu Jan 09, 2020 8:02 am

Generally if there is a framed wall inside I set the rigid foam just onto the top plate and seal all edges of foam with canned foam and the sit Roxul on the wall plate.
Not sure why you only have 2' of sill depth? I was expecting that you had concrete infilled sills but I see in the picture you do not. Usually the basement walls would be 8" of concrete?
Ohh I know why , this is a bilevel home so the joists are sitting n the 2x4 framed portion. You have basically two options IMO, insulate the rims with batt or roux and use a poly vapour barrier or spray them with 2" of closed sprayfoam. The spray foam needs to be protected from fire if there is no fire rated ceiling so then you could cover it with drywall and seal the edges with fire caulking.
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