I have a early 1970’s split entry house in Anchorage, Alaska (zone 7) and am ready to apply 4 inches of rigid XPS insulation from the footers on up to the eves of the roof. Now that I have dug a trench around my house down to the bottom of the footer and pressure washed my foundation I have a question about how to go about it. The “traditional” way is to apply bituthene to the primed concrete, XPS foam,and then a 12″ section of drain rock enclosed in a type of landscape fabric to promote good drainage away from the foam.

The other way i have seen it done (June 2011 issue of JLC magazine “Tightening the shell from the outside”) is to apply the XPS directly to the concrete, and then cover the foam with a the waterproof membrane. This to me makes more sense, since you want both the foundation and the foam to stay as dry as possible. In the 13 years of living in the house we have never had a moisture issue with the foundation as we are on a slight rise and have good drainage.

My thoughts are if I do it the second way do I need to put any thing between the waterproof membrane and my backfill dirt? Since moisture has not been an issue, this way would save a fair amount of money as not having to buy the drain rock and fabric and would keep the foam drier. Can one use the low temp Grace bituthene to go from footer, over the foam to the sheathing?

Thanks in advance for any help.

Jon