Rigid polystyrene board on inside and outside of foundation?

[austin]

My basement already has rigid polystyrene board on the inside of a cinder block foundation. The house is about 40 years old.

Would it be a good idea to install rigid polystyrene boardon the outside of the foundation as well, above and below grade? Or would that create a problem because any water vapor in the cavity would have nowhere to go (assuming I seal the top of the cavity from the inside also).

[pushkins]

You cannot install it above grade, it would last one year and deteriorate very rapidly from exposure, let alone looking bad. Before you even try to install it below grade your wall must be water proofed with an asphalt product.
The only time insulation is installed on the exterior foundation is for basement climate control.

[austin]

I thought I would do it like this:
- repair cracks in foundation;
- install membrane
- install rigid polystyrene board
- install cement board above grade
- parging over cement board above grade

[pushkins]

Is there a basement behind this cinder block wall ?

[austin]

There is a basement behind the cinder block wall. One half of the basement is a heated, finished space. It is insulated on the inside with rigid poylstyrene board between the cinder block and the 2x4 framing. The top of the foundation is covered with fiberglass bats, but no vapor barrier. This is where I'm thinking of adding rigid poylstyrene board to the outside of the foundation.

The other half of the basement is an unfinished, unheated double garage with no insulation on the inside. There is below grade rigid poylstyrene on the outside against the cinder block, but I don't know if it goes down as far as the footing. This is where I'm thinking of adding rigid poylstyrene board to the inside of the cinder block and turning the garage into an insulated, heated space. The wall between the garage and the rest of the basement is insulated.