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

Thomas Dispenza
Updated: Aug. 4, 2020

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?


Responses (6)

leo macinnes 4 years ago
Mike Reynolds 4 years ago

We always welcome 2 cents at any length Leo, thanks for sharing. 

I think you may be in the U.S. so I can't speak much to building code requirements there, but in Canada any basement / crawlspace of 4 feet and over has to have insulation covered by drywall for fire purposes. Under 4 feet doesn't.

I would also agree with you that you should meet 'certain' conditions before putting fiberglass or Mineral wool insulation in a basement, proper steps should always be taken. Mineral wool worries me less than fiberglass since it isn't harmed by moisture, but I'd still be managing it properly anyway no matter the type of insulation I use. 

I'm wondering though about the 'double foil faced EPS' if that's not actually Polyisocyanurate? That is not as resilient as EPS when met with exposure to moisture or water. There are a lot of rigid insulation panels, and foil faced rigid insulation boards make a great vapor barrier, it's just worth noting the characteristics of any particular product to make sure you're using it in the right application. Thanks for weighing in Leo. 

leo macinnes 4 years ago

Thanks Mike for heads up on ceiling height, and I am in Canada.

The insulation looks like foil but is a polymer. This  from the company website in Canada "Quik-Therm multi-purpose insulation{MPI} is a continuous rigid foam insulation consisting of closed cell Type 1,Type 2 or Type3 expanded polystyrene {EPS} layered on two sides with durable and flexible polymer facers. MPI meets code compliancy as an air, vapour and/or radon barrier, when connections are foamed or taped. MPI is available in rolls or sheets. Optional T&G connections."