# Basement Rim joist: risky to insulate in the winter?



## Windows on Wash (Aug 30, 2011)

Not really. It can usually dry to outside. Even with the foil faced foam, its likely the fit isn't airtight and if the foam is part of a siding job, its probably micro-perforated as well. 

I would prefer to see 2" of foam here for dew point considerations. The fiberglass does insulate, but does nothing to stop moisture movement.


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## Bud9051 (Nov 11, 2015)

Although more is always better in your case you can add the 1.5" rim joist (R-1.9) plus the sheathing and that half inch layer of foil faced to your r-5. Call your total R-8 and you should be good. Air seal really well and seal the sill plate to the foundation. 

Bud


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## Nealtw (Jun 22, 2017)

I would not be worried about the moisture in the wood now.


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## jlhaslip (Dec 31, 2009)

Rims get insulated year round up here in snow country.

It will dry itself out the first chance it gets.


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## DarrenLevine (Aug 3, 2016)

Thanks all, sounds like i'm good to go. i'll beef up to 1.5" xps(just happened to have 1" lying around)

cheers
D


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## Windows on Wash (Aug 30, 2011)

Forgot about the 1/2" on the outside. Guys are right here that 1.5" with everything else should be very much adequate.


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## Dave Sal (Dec 20, 2012)

You're gonna notice some energy savings and extra comfort when you finish the job. The temps down there during winter were always just above freezing and I was worried that the copper water lines would freeze. After the rim joist insulation the temps are now 63 degrees all year round. I used 2" Owens Corning Foamular R-10, then sealed the edges with caulk. The floor above the crawlspace is no longer ice cold either. I have furnace ducts running through the crawlspace but have no registers blowing air in there, just the radiated heat from the ducts.


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## user_12345a (Nov 23, 2014)

It's very difficult to do a good tight vapour barrier with fiberglass at the rim joist. fiberglass is a lousy insulator without the barrier due to air movement and rim joists plus just below are known to leak a lot.

Use rigid foam cut to size, foamed in place with great stuff or sealed another way.

you'll be good moisture wise as long as you have drying potential to the outside. you do unless they built the house wrong or did something stupid like side the house later with a material impermeable to moisture.


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## jlhaslip (Dec 31, 2009)

user_12345a said:


> It's very difficult to do a good tight vapour barrier with fiberglass at the rim joist. fiberglass is a lousy insulator without the barrier due to air movement and rim joists plus just below are known to leak a lot.
> 
> Use rigid foam cut to size, foamed in place with great stuff or sealed another way.
> 
> you'll be good moisture wise as long as you have drying potential to the outside. you do unless they built the house wrong or did something stupid like side the house later with a material impermeable to moisture.


Not sure where you are in Canada, but where I have worked, (BC and Alta) your approach would require a layer of Drywall where the foam is exposed to the inside. Use fire retardant foam to install and seal the drywall. Regular foam is not rated for that situation.


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## SeniorSitizen (Sep 10, 2012)

DarrenLevine said:


> I'm eyeing up my empty rim joists in the basement, but in reading up on best practices, it would seem it's best not to do it in the winter when there may be more moisture currently in the joist. Is this a real concern?


I've always read that cold air is dry so why would moisture be a concern?


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## jlhaslip (Dec 31, 2009)

SeniorSitizen said:


> I've always read that cold air is dry so why would moisture be a concern?


Condensation occurs where the warm, moist. interior air meets the cold rim joists.


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## user_12345a (Nov 23, 2014)

> Not sure where you are in Canada, but where I have worked, (BC and Alta) your approach would require a layer of Drywall where the foam is exposed to the inside. Use fire retardant foam to install and seal the drywall. Regular foam is not rated for that situation.


in Ontario, and you're absolutely right - if it's uncovered you must have something that rated to stop flames over top of it.

They may even accept roxul safe and sound batts cut and put over top of the foam. 

Roxul is a much better fire stop than drywall. 

my signature does say to check local codes and to do at your own risk though.


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