I’ve utilized XPS foam to air seal the top of a concrete block foundation wall and recently had to remove a section of the foam only to find large water droplets on the underside of the foam. The 2×6 non-treated sill plate showed signs of moisture and some mold growth.

Approximately two years ago I began to air seal my basement by sealing the top of a concrete block foundation wall with 3/4″ XPS foam. The foam was caulked to the block on three sides and also to the edge of the 2×6 sill. There is no capillary break or sealer between the sill plate and the concrete block. Drylok was applied to the interior of the concrete block per the manufacturers instructions. The basement temperature was ~ 54F. The house was built in 1982 in zone 5A.

This air sealing method seems to be prescribed quite often on GBA, but the detailing seems to apply most to poured concrete foundations with properly applied sill seals. The ultimate goal is to insulate the basement using foil faced polyiso and stud walls.

Questions:
1. Is the XPS foam approach suitable for concrete block foundations with open voids?

2. Instead of XPS foam boards, should closed cell expansion foam be used to fill the voids of the concrete block immediately below the sill plate, blocking the moisture of the block cavities from the wood of the sill?