I am thinking about putting foam board on the outside of foundation, at the beginning I was leaning towards polyiso foam because of it's higher r-value, but then I learned it could absorb water, since the foam board will be in contact with the ground, is XPS the better (only) choice here?

My second question is that for the garage section I have the choice of installing the foam boards either on the inside or outside of the foundation, in this case is one preferred over the other? My hunch is that I could now use polyiso on the inside since there won't be any moisture problem to worry about, and no insect problem either.

If the inside of the foundation wall has converted to a living quarter with vapor barrier installed then it would be a bad idea to put foam board on the outside of the foundation wall, correct? By the way, the foundation walls are completely above grade. Thanks!

My preference is always to put it on the outside. Why wait for the cold to get into the envelope before you stop it?

Either EPS or XP is acceptable from a performance point of view. Some would suggest eps is greener.

But if the inside is finished with drywall (I assume there is fiber glass insulation as well), then foam on the outside would create a double vapor barrier right?

Posted By seiyafan on 06 Dec 2013 08:17 PM
But if the inside is finished with drywall (I assume there is fiber glass insulation as well), then foam on the outside would create a double vapor barrier right?

ICF walls have rigid foam on the outside and inside, along with drywall on the inside, all with no vapor issues.

eps is not a vapour barrier but is a retarder. Most often vapour barriers area danger when used inside of concrete. Just air seal it well and don't use a plastic barrier.