Posted this under a “musings” post, but it was an old one and thought I might get more traction here.

For basement walls; how thick is thick enough for EPS to act as a vapor barrier? I’m in the process of insulating my poured foundation walls. I had immediate access to some free left over materials, and have already tackled a few wall sections as follows;

2.5″ + 1.5″ of EPS Type 2
2″ + 2″ of XPS
2″XPS + 1.5″ EPS Type 2

These sheets are all lapped, and I ran the first layer horizontal and second layer vertical. I’m less concerned with these installed configurations but I’m against a budget and for the rest of the walls, I had hoped to just use two layers of 1.5″ EPS Type 2. Assuming a perm rating of 2.8/inch, are my calculations correct that 3″ would have a perm rating of 0.9333?  Should I just spring for a full 4″ EPS or 2″XPS+1.5″ EPS?

Overall, the plan is to install a service wall in front of the foam with R12 batts, then OSB to cover. Am I going down the right path? Atlantic Canada (US Zone 6?), ~noted as 4,200 heating degree days, but that seems to be trending lower past decade.