Is EPS or XPS better for use in basement rim joists? Should faced or unfaced foam boards be used?

Thomas Dispenza Published: Sept. 8, 2019, 12:43 a.m.Last updated: Sept. 12, 2019, 1:16 p.m.

My home in south-central Pennsylavnia was built in 2001. The unfinished basement (walls and floor) surfaces are formed of poured concrete. One side of the basement has full-size regular casement windows, but the bottom of those windows is nearly at-grade. That wall is still full-height poured concrete. The rest of the basement is below-grade.  

We aren't going to be drywalling our basement soon (maybe someday), but I want at least to ensure ASAP that the rim joints are henceforth insulated properly. I am considering installing foam board insulation with circumscribing spray foam in each rim joist (or maybe using purely spray foam to cover each rim joist). However, I can't tell whether XPS vs EPS would be "superior" for our basement walls and/or rim joists.

I'm way less concerned about R-values or project cost than I am about moisture (and bugs, to be embarassingly candid about the matter).  I gather that I can always safely throw some fiberglass batting up in the rim joists to remediate any inadequacy of R-values acheived AFTER the use of spray foam (with or without foam board) to establish air seals.  However, despite numeous apparently-professional sources (who don't manufacture XPS) recommending EPS over XPS for use in basement applications, I can't seem to shake the to-me-offputting observation that EPS is more moisture-permeable than XPS.

When insulating rim joists, isn't the supposed superiority of foam board insulation over mere fiberglass batting due to the observation that fiberglass is more moisture-permeable, and therefore more likely to support mold growth on the rim joists? Therefore, by extension, shouldn't one prefer to use XPS--for its lower permeability--versus EPS when insulating rim joists?  Wouldn't that logic also translate to selecting a foam board insulation to cover the actual concrete walls of a basement like mine--at least when one considers that my basement concrete has already had many years to dry out (presumably about as much as it's ever going to dry out) following its initial pouring? 

Lastly, if I'm going to go out and purchase EPS or XPS boards for installation in my basement rim joists, should I be looking for faced or unfaced foam boards to install there--and which surface of the boards (faced or unfaced) should be installed toward the outside?


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