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Double layering XPS to obtain required thickness?

nestea92 | Posted in Green Building Techniques on

I’m planning to use XPS rigid foam against my basement foundation wall. The existing wall is a 8″ thick concrete foundation with only the spray on ‘damproofing’ on the exterior. I am in Zone 4A (St. Louis, MO), where the climate is mixed-humid. I have read elsewhere that a 1-1/2″ XPS Layer is needed if I desire the XPS to act as a moisture barrier in addition to an air barrier.
See the 3rd Section titled, “Better Basement Insulation Detail”

With a bit of research I tend to agree by definition that the 1.5″ XPS is a Class II Vapor Retarder (although I cannot find a perm rating for 1.5″). I did quick plot of 1″ XPS (1.1 perm), 2″ XPS (0.70 perm) and 3″ XPS (0.60 perm) and then applied a trend line to fit (2nd order polynomial trend line). Using this trend line I find 1.5″ has a perm rating of about 0.86 perm.

I plan to do a wall section of XPS foam (1.5″ thick) sealed and taped, then studded with Roxul batt insulation, then drywall. I’ll be foregoing any sort of vapor barrier poly or ‘smart’ vapor membrane. Per several relatively recent discussions I’ve come across on this site. Needless to say the Roxul will be providing the R-Value, so I’m not concerned in using the XPS as an insulator, only as a moisture barrier.

My question lies in the method I take to get to 1.5″ thickness. I am having a difficult time getting 1.5″ XPS (Foamular Brand) with the Tongue & Groove (T&G) edge; everyone (the big box stores) sells square edge. Special order requires I buy a pallet of 64 (I need 12 sheets). I can, however, put my hands on 3/4″ T&G XPS rigid foam. Is there any pitfall to applying a double layer of 3/4″ XPS to obtain the 1.5″ total thickness. Part of me thinks it may be better in that I can stagger the seams, but another part imagines condensation in-between the panels. For what it’s worth, I want the T&G edge to create a better seal between sheets, and I also plan to apply a small bead of sealant in the grooves to supplement the tape. Any thoughts or lessons learned would be appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    In your climate zone, the 2009 IRC requires that basement walls be insulated with a minimum of R-10 insulation. So if you were using just XPS, you would need 2 inches (and that would be what I would normally recommend).

    Here is a link to an article that explains my recommendations for insulating basement walls: How to Insulate a Basement Wall.

    To answer your question: yes, you can use two layers of 3/4-inch-thick rigid foam. And yes, this will be better than a single 1.5-inch-thick layer, because you can stagger the seams.

  2. Expert Member
    Dana Dorsett | | #2

    In your climate you'll be fine with as little as 1" of EPS and a 2x4 studwall with Roxul. Put an inch of EPS under the bottom plate of the stud framing too, as a thermal and capillary break, otherwise you might need to use toxic pressure treated goods to keep it from slowly rotting from ground moisture draw through the slab.

    An 8" concrete wall with interior gypsum and a 24" o.c. 2x4 wall with only a single top plate to the framing with R15 Roxul comes in at about R12 after accounting for the thermal bridging. The inch of EPS brings it to about R16. If you went with double-layers 3/4" EPS with staggered seams you'd be at about R18.

    EPS is preferable to XPS due to it's more environmentally friendly blowing agent which is pentane, instead of HFC-soup, primarily HFC134a, that has an extremely high global warming potential. And in 50 years after most of the blowing agent is gone from the XPS it's R value will be about the same as the EPS was on day 1 and would still be after 50 years, since the pentane is mostly displaced by air within a few weeks of manufacture.

    Using t & g goods doesn't improve the long term performance appreciably if going with double layers. All XPS/EPS shrinks a little over time, and taping plus double-layering to thermally break the shrinkage gaps is about as good as you're going to do.

    You don't need or necessarily want a moisture barrier on the interior, but you DO want sufficient foam-R at the above grade section to limit wintertime condensation at the foam/rock-wool interface, and that's a function of the R-ratio. With R4 foam and R15 rock wool you have plenty of margin for a zone 4A climate, and won't end up accumulating moisture in the cavity from wintertime moisture drives.

    Type-II (1.5lb density) EPS runs about 3 perms at one inch, which is the same or slightly lower than a couple coats of latex paint (typically 3-5 perms), so any ground moisture wicking up the foundation wall from the footing can still dry toward the interior faster than it's getting into the studwall. If you went to 2 inches it would be about 1.5 perms, but it doesn't really need to go that low to be protective. XPS is typically between 1-1.5 perms @ 1", but it's a "who cares" situation.

    If you don't have a good capillary break (EPDM or metal flashing) between the foundation wall and foundation sill, if you also don't have sufficient above-grade exposure on the foundation for the foundation to dry toward the exterior (at least a foot, 18" is better) you may even NEED the higher drying rate of EPS to keep the moisture content of the foundation sill at a safe rot & mold-free level. There are a lot of factors at play, either XPS or EPS will work for 95% of the foundations out there, but EPS is less risky, cheaper, and more environmentally friendly.

    Most enviromentally friendly of all would be to use reclaimed roofing foam from commercial re-roofing & demolition (any type of foam but if polyiso keep the bottom edge off the slab). You get pretty good performance out of 3" roofing polyiso (R15-R18), which can be affixed using 1x furring 24" o.c. through screwed to the foundation with 5" TapCons 18-24" o.c. on which to hang the interior gypsum. It ends up eating up less interior space than a foam + studwall approach too. Foil-faced good can be used, but only if you're sure it won't threaten the foundation sill with much higher moisture levels in the concrete that has insufficient exterior drying surface.

  3. nestea92 | | #3

    Thanks for the responses; I’m open to options that make good sense. I looked at going with EPS originally for the reasons Dana outlined, but somehow I came to the conclusion/understanding that XPS was best for basements (forgive me, I didn’t save the link to that article I found some time ago…).

    Regarding the use of EPS: If it doesn’t act as a moisture barrier it seems unnecessary at all then. For this installation, The R-Value is coming from the Roxul (although I’m seriously considering/researching a thick foam only solution to leave the stud bays empty as Martin recommended… it would save me $$ on the mineral wool too). The way I see it, if the EPS against the concrete isn’t a moisture barrier, then I might as well have the Roxul/Stud wall directly against the concrete, which everyone seems to agree is not a wise detail as moisture will condense against the concrete and drain to the floor/bottom of the wall. I did find a quote Martin made about 4” of EPS providing an adequate vapor barrier (<1.0 perm):

    See Response #6:

    I’m not opposed to going 4” thick on the EPS; and keeping in line with the original question, I would likely look for a dual 2” layer in that case. I further understand that I should not use faced EPS in the basement walls. Then I asked myself can the 4” EPS Vapor barrier be sealed up? It seems that the Loctite PL 300 foam adhesive will still be OK, but then I’ve read that unfaced EPS cannot be taped:

    See Response #1:

    I’ve ordered the “Builder's Guide to Mixed-Humid Climates” book by Joe Lstiburek to try and further educate myself as to the need for a moisture barrier in my region, although I fear some of the recommendations in said book are now obsolete:

    The book should arrive today so I’ll be a bit smarter tomorrow with respect to the need for a MB in the Mixed-Humid, Zone 4A region...

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