No reduction in performance relative to what?
Foam installers have a tendency toward "You don't really need more than...", type advice, and it's not always well-founded. (It usually means: "If you insulated to a higher level than that with our premium but pricey product it's not cost-effective.)
2" of half-pound foam like Icycnene is only about R7- about half the recommended value for foundation walls for cooler parts of the lower 48. You'll still need to put a thermal barrier over it to meet code. If you did a 2x3 studwall afterwards with unfaced R8 batts (or 2x4 with R13 unfaced batts) to hold the gypsum thermal barrier you'd be in good shape.
For new construction in zip codes 622xx the ORNL sez R30 is cost-effective for band joists, R19 for crawlspace walls, R11.7 for masonry walls (if applied to the exterior- it'll be more like R15-R19 on the interior):
http://www.ornl.gov/~roofs/Zip/tmp/results13235.html
That would suggest to me that 2" of Icynene + a 2x4 R13/R15 batt studwall is in order.
Applying masonry sealer to the wall to slow down ground moisture vapor before applying the Icynene is a good idea. Icynene is highly permeable- more permeable than EPS or XPS, similar to unfaced polyiso. 2lb foam would give you about R13, @ 2", and would be a class-II vapor retarder (semi-impermeable) by itself, and somewhat preferable in this application. (I went with masonry sealer + R20 recycled unfaced iso board in my own place, and it performs well. It would have taken ~6" of Icynene or ~5" of EPS to match what iso or 2lb foam does in ~3".) |