# Xps board



## Canucker (May 5, 2011)

Are the joist bays on the top of your basement wall insulated and air sealed? If not, the draft could be coming from there. And no, you don't want your wall to "breathe", use a can of foam and seal it up.


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## Mijotter (Nov 14, 2012)

Ok it is my understanding that the 2" xps board allows some breathability through the board that is slow is this correct? So I seal around the entire foam board against the concrete correct?


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## Canucker (May 5, 2011)

Mijotter said:


> Ok it is my understanding that the 2" xps board allows some breathability through the board that is slow is this correct? So I seal around the entire foam board against the concrete correct?


 The warm indoor air holds more moisture in a vapour than cold outdoor air. If you don't seal the leak where you feel the air, then you could have the vapour condensing in that spot. Then you get wet. Wet is bad, especially if you insulated enough to keep most of the heat away so it can't dry out anymore. 
If you're interested, you can do a search and find all the technical details about insulating you could ask for here. Just look for posts from GBR in this section, he has good links in them.


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## Gary in WA (Mar 11, 2009)

No air should be allowed on the f.b./concrete joint. Air brings moisture form outside/inside (humidity). http://joneakes.com/jons-fixit-database/743 Foamboard installed air-tight to concrete will insulate *and* vapor retard any cold moisture coming through the concrete. If you air-seal the f.b. and glue as mentioned; with the *"closed grid pattern"* (horizontally and vertically) which limits the surface area of the air movement and wetness to diffuse through-- rather than drain-down to collect/pool at the bottom- possibly wetting wood framing (to mold, being organic) or cavity insulation from wicking- degrading it's R-value- if fiberglass; http://archive.nrc-cnrc.gc.ca/eng/ib...ling-heat.html

Seal the perimeter edges (canned foam) when installing against the closed-grid adhesive pattern. Mastic/tape the seams between panels, keep in mind f.b. will shrink over time (reason I typed mastic first); http://www.buildingscience.com/documents/published-articles/pa-foam-shrinks

Rim, as said already;
http://www.diychatroom.com/f103/insulating-rim-joist-caulk-spray-foam-around-edges-168849/

Gary
Thanks, Canucker, I was busy cut/paste my other links from other threads....!


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