Vapor retarder under slab insulation?
I am about to start work on finishing my basement. I have had some small water problems in the past that should be taken care of with the new foundation drain system that was recently installed but has yet to be tested by a heavy rain. The water table in my area is relatively high as both next door neighbors only have crawl spaces and one has major water issues. I know from the trench that was dug for the waterproofing that there is nothing but dirt under my slab. I plan on installing 1-2″ of rigid foam on top of my slab but i am not sure if i should install a polyethylene vapor retarder under the foam to let the slab dry to the outside or if i should just put the foam on the slab and let it (slowly) dry out to the inside? I am in the Philadelphia suburbs if that matters (climate zone 4A).
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If you plan on installing rigid foam on top of your slab, you shouldn't plan for any inward drying. Rigid foam is a vapor retarder. One inch of XPS has a permeance of about 1 perm, and 2 inches of XPS has a permeance of about 0.5 perm.
Installing a layer of polyethylene between the slab and the rigid foam is probably a good idea. If you have had an interior drainage system retrofitted under your basement slab -- and it sounds like you have -- you probably don't have anything to worry about.
For more information, see Fixing a Wet Basement.