2” XPS – Interior foundation walls Zone 5 – GreenBuildingAdvisor

In the early stages of finishing my basement. We will be in this home for a long time, potentially forever so I am willing to put in the work to get it A+ (within reason).

Basement was poured Summer of 2023, 8” thick poured walls with damp proofing (bituminous coating) from the bottom of footings to the top of the wall. So, the top 8-10” that are above grade have this coating on it.

Exterior perforated pipe at/below footing level that is connected to sump pump. We have been in the home for two full years now, the pit has never had water in it, and we have never had moisture in basement. We have clay soils that are graded well around home, the grade at our lot lines are 1-2’ lower than the grade around the home in all directions. Our basement slab elevation is 1’ above the retention pond normal water level behind our lot. I point this out to show we have a fairly elevated lot.

My plan based on (this site, building science, etc.) was 2” XPS glued to foundation, butt joints taped, all other joints spray foamed for air sealing. 2×4 framed wall in front with R13/15 in the cavities. The R10 continuous insulation satisfies code in my jurisdiction. Approximately 3/4 of my basement will be finished.

Spray foam/exterior insulation are not within budget so this seemed to be the best approach. However, I tend to overanalyze/research, and someone on these posts always seems to bring up that this method is a bad idea, concrete needs to breath, etc.