It depends on how cold it is in your region, but it may be better to use up to 2" of XPS and a 2x3 studwall. It's important that the sub-grade portion of the concrete be able to dry toward the interior to avoid upward moisture drive to the sill plate and efflorescence/spalling on the above grade concrete. XPS is semi-permeable at 1", but becomes semi-impermeable above 2". The fiberglass portion has to be UN-faced batts- facers placed on the exterior side would cause issues with the concrete, facers placed on the interior side would trap ground moisture in the studwall to create mold & rot conditions.
But if it's a cold/very-cold climate the above-grade portion would have condensing potential within the batts or on the studs with only 1" of XPS (R5) and 3.5" of fiberglass (R11-R13). But if the R-value is more evenly split between FG and foam, or predominantly foam, the region within the assembly where the dew point of the interior air occurs will be within the foam, not the fiberglass, and no problem occurs.
The sill & band joist similarly need to be insulated with foam, with seams & joints all foam-sealed to keep interior air from reaching cold wood and condensing there too.
Placing a highly vapor retardent layer like 6-mil poly anywhere on the interior side of the concrete below grade is a bad idea except in the most extreme cold climates. (Even though it's required by code in much of Canada, misapplication of vapor retarders below grade tends to cause as many problems as it creates. Simply using more foam, less fiber but more permeable foam unfaced EPS or fiber-faced iso and good air-barrier techniques is far less problematic than sub-grade poly in practice. ) Putting poly between the concrete and insulation may lower the mold potential of the studwall in your stackup, but will increase the rot potential at the foundation sill. |