How problematic is XPS foam board applied against asphalt-based foundation coating, on exterior?
As we move forward with energy retrofit insulation work on our house, we finally decided, due to our moist clay soils and northern climate (Maine), to insulate the exterior of our foundation rather than the inside walls in the basement. Some fun.
At this point the plan is to keep the interior of the concrete wall exposed to the inside air.
We’re laying 2″ XPS foam boards against the foundation walls (Obviously that means we’ll have to deal with a protective covering above grade).
Today’s question has to do with foundation coating on the walls, vs. the XPS foam boards laid against it.
After rolling on a good coat of the asphalt-based (black tar-like, clean-up with solvent) foundation coating on one full facade of (old, well-cured) concrete , we learned that the XPS foam board would be expected to react with / be eaten by the coating. Oh great.
Given we’re laying against it foam boards that are a full 2″ thick, and the coating is dry to the touch, not still gooey, is the foundation coating really going to corrode that much of the XPS, or too little to be concerned about? I just can’t believe that it would really compromise the foam board to any appreciable degree.
If the former, then we need to figure out how to protect the XPS foam boards (not yet installed) from the tar coating that we already laid onto that one facade.
How about laying down a thin physical barrier between the foam board and the tar-coated foundation, of:
a) a sheet of weed cloth from the garden center
b) a layer of Typar house wrap?
Meanwhile, if it is a genuine problem, then we’re underway sourcing a foundation moisture coating that will work touching the XPS foam, for the rest of the job. If you have a favorite, would appreciate hearing about them. Some of those rubber coatings and self-adhering sheets are way expensive and not really necessary where we don’t have through-wall seepage; am thinking XPS boards plus foundation coating should be enough for practical purposes.
Your thoughts will be, as ever, much appreciated.
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