# Insulating Basement Walls



## TheConstruct (Dec 8, 2017)

I have a customer who I've been doing small projects for around the house for a bit. 

The next project is refinishing a basement room. He has done all the demo and the room is currently down to framing and foundation walls. Two walls are full height poured foundation with one egress window that I installed last year. The concrete walls had two layers of strapping on them, 1x4 run vertically and then strapped horizontally with 1x4. Somehow they had insulation shoved into the wall in the horizontal run. I didn't actually see any of this but it sounded half assed.

The house was build in the 70's and doesn't have perimeter drains but the soil is very sandy and well draining. The foundation wall didn't show any signs of moisture but also didn't have a proper vapor barrier and could probably breathe pretty well. 

Now to my question. I would normally frame walls in front of the foundation leaving an air gap behind, insulate with batt insulation and vapour barrier as is common practice around here on new builds. The customer mentioned he'd been watching YouTube videos on using rigid insulation directly on the foundation wall which I've certainly seen done but haven't done myself. I'm hoping for some pointers on the benefits and drawbacks of both systems in this scenario so that I can deliver the best quality product while also considering material costs.

Thanks


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## Golden view (Feb 16, 2012)

I've found these details are very regionally specific. So careful where you get your suggestions.


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## TheConstruct (Dec 8, 2017)

That definitely occurred to me and I appreciate the suggestion. What may work in one climate may not in another and the last thing I want to do is create an issue. The customer was curious which sparked my curiosity so I'd like to look into it but maybe need to source some info locally.


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## asevereid (Jan 30, 2012)

TheConstruct said:


> That definitely occurred to me and I appreciate the suggestion. What may work in one climate may not in another and the last thing I want to do is create an issue. The customer was curious which sparked my curiosity so I'd like to look into it but maybe need to source some info locally.


I can't speak from long term experience, but I've stopped using a vapor barrier on the warm side of the wall for basement reno work.
I can't prove anything, but for a lot of our basements (either 4' or 9')....there needs to be a place for any accumulated vapor to dissipate.... So now I leave the gap and then use the batt insulation.
I think the last time I checked in to it, a vapor barrier is considered best practice in our area, against the concrete, and then acoustic sealed along the entire perimeter... But then where does any moisture go that accumulates behind the vb? 

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## TheConstruct (Dec 8, 2017)

That was my concern with using rigid directly on the foundation and whether I should treat it as the vapour barrier or not. I guess the benefit is that the rigid would stop any condensation but if the wall is ever damp it cant dry the same as with an air space. I'm sure there is some good literature out there I havent found yet.


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## Randy Bush (Mar 7, 2011)

Last one I did used 2" blue rigid then framed over that. passed inspection here. If would of done bats in the bays would of required more insulation. This was for here in Montana.


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## SLSTech (Sep 13, 2008)

Well it all depends... Well draining with no worries about bulk water coming in - foam applied directly against foundation is fine. Frame with a small air gap (because what foundation wall is ever plumb) & that should eliminate any concerns. The only other catch is make sure you seal the top off so that any moisture that might get through doesn't head straight up to your rim joists, etc... 

Bulk water issues you would put a dimpled mat behind & drain to interior French drain

If you are really nervous - check out Building Science Corp site & look at free papers there by Joe Lstiburek (as I recall he had at least 2 good ones on this)... Just make sure you are reading the newest ones because some things do change with time & new products come around.


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## META (Apr 9, 2015)

My understanding is it comes down to how one needs to handle vapor pressure, either inward, outwar, or both, and how that vapor can collect on barriers. Mitigating collection/ condensation is key when possible.


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## Fouthgeneration (Jan 7, 2014)

I have quit using any plastic foam insulation inside the home...

A few square feet smoldering = Dead people...

Rock wool or fiberglass, & less toxic paper based...

Prefect world, dig up the wall, install foam OUTSIDE, concrete walls inside rainscreen, and kept at nearly constant temp & ,dry state = very very long lasting walls, smaller HVAC plant needed.

No Paper drywall below grade, or in any room with plumbing.....

Stop drywall several inches from floor install tall mop board (plastic?)

Consider painting or plastering the Crete without plaster board....


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## TheConstruct (Dec 8, 2017)

Thanks for all the feed back so far guys. I've found some good reading on the application in our climate zone but the real wild card is the old house and not knowing what the concrete wall does. Its had such good air circulation and performed well that there is a lot to consider before sealing things up tight to it.


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## Morning Wood (Jan 12, 2008)

I believe XP’s foam is permeable enough up to 1” thick. You could also use up to 2” thick EPS foam. What I would do is fasten foam to the back of your wall and then stand the wall up leaving an air gap and then use rock wool in the studs. No vapor barrier. You assume the wall is wet because it is below grade . It will want to dry to the inside. Allow that to happen with vapor permeable insulation.


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## Mitch15 (Jan 7, 2020)

Good advice!


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