# Interior Concrete Foundation wall insulation



## amstaffterr (Dec 7, 2015)

Good day, after reading many post/replies here I am a bit confused as to what thickness xps fb to use against concrete foundation in my basement or if I should use it at all. 

I was planning on using 1.5" - 2" xps, then 2x4 stud wall filled w/ mineral wool, no vapour barrier and drywall. The reason I'm confused is i've read that insulation in stud wall may make Xps cold and create condensation depending on relative humidity and temperature.

I live in the greater Toronto area, Ontario. 

I have nearly finished insulating rim joist with Xps/foam. I would also like to put 1" xps under bottom plate of stud wall.

My question is, What thickness Xps should I be using? 

Also, my foundation walls aren't perfectly flat/even. 

Is it OK to have any space between Xps and concrete?

I will be fastening the Xps to concrete with plastic rigid board insulation anchors, drilling though Xps into concrete and hammering the anchor in. I will not be using adhesive.

Any help is appreciated.


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## Windows on Wash (Aug 30, 2011)

http://buildingscience.com/document...-foundation-8-xps-2x4-framing-with-fiberglass

http://buildingscience.com/document...ng-cold-weather-condensation-using-insulation

Good reading above. The 2nd link goes into more detail. 

No airspace between the XPS and the wall.


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## Gary in WA (Mar 11, 2009)

Welcome to the forums!

R-12 min, or R-19 (if electric heat). What Type of extruded; 2,3, or 4? Their density varies with the type... you could also use foil-faced -- leaving the bottom foot of concrete wall bare so you don't drive the moisture up into mud sill plate/framing to dry/rot. - though may not be allowed; to meet "air barrier" code required locally. Did you use ff XPS on the rims? If not, just add some radiant barrier (unperforated) to the inside face of foam now to stop vapor drives during winter/summer. Use caulking in 1 foot grid pattern to divide the air exfiltration into smaller sections not to compromise the whole wall (air space) resulting in condensation.convective loops; (from your neighbor!); http://joneakes.com/jons-fixit-database/743

Gary


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## Piedmont (Nov 1, 2007)

I think your original plan was perfect. You're not creating a science experiment everything can dry/breathe. 

Using the information from the post Windows on Wash gave I've conclude the following:

Average Coldest month temp in Toronto is -4.6 Celsius, using the chart and assuming a relative Humidity of your basement of 40% and your mineral wool is R14 you need at least R6.3 to be foam. Rounding up that's 1.5" of XPS foam (R7.5). 

I'm going to recommend you go with 2" though, besides the additional R-Value basements should be kept at 50% Humidity or less. If your basement gets or is 50% humidity you need 2" of foam not 1.5". Also it covers you if the temps are particularly cold (-10C instead of -4.6C). If the month averages -10C condensation will form in your mineral wool even at 40% Humidity, it won't if you install 2" foam.


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## Gary in WA (Mar 11, 2009)

Mine was from his city guide on basement insulation per local code; http://www.mah.gov.on.ca/Asset8275.aspx?method=1

Always good to more opinions, though. Nice to have a signed permit on the books when selling...

Gary


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## amstaffterr (Dec 7, 2015)

I have decided on 2" Xps Wallmate, 2x4 stud wall (24" o/c) cavity filled with R14 Roxul, (no 6mil poly), drywall and good coat of paint.

I am thinking about taking the Xps from the point where sill plate/concrete meet down to concrete slab and then spray foaming (Touch n Foam 2 comp. DIY) My thinking is a better seal at point where concrete, sill plate and Xps meet. 

Please let me know what you think.

Thank you all for your help.


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## Gary in WA (Mar 11, 2009)

Add some plastic to the interior side of rim insulation, at least- #6 *AND #10*; http://www.diychatroom.com/f103/insulate-rim-joists-canned-spray-foam-roxul-307313/

Gary


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