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EPS exterior with CC SPF Interior

Canadarox12 | Posted in Green Building Techniques on


I am currently pricing out insulation solutions to achieve between R-35 and R-40 for my above grade walls.I am in climate zone 8. I have priced out adding 2 inches of CC spray foam then filling the cavities with fiberglass batts. This also included 0.5 inches of XPS outside to cut thermal bridging. However reviewing that I realize that is creating a sandwich with no path for the moisture to exit. I am still wanting to combat the thermal bridging concern. One spray foam company recommends going to 3 inches to ensure no condensation forms between the batts and the foam.

I am still wanting to cut thermal bridging if I can and am wondering if using 1 inch of EPS instead of XPS, and ensuring a rainscreen is in place would be enough to mitigate the OSB sheathing from being wet all the time?

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  1. Expert Member
    Dana Dorsett | | #1

    How deep are the studs? (2x8? 2x6?)

    In zone 8 ideally you would have more than 40% of the total center-cavity R to be on the exterior of the first condensing surface, which would be where the batts meet the closed cell foam. An inch of UNFACED Type-II EPS would be good for at least R4.5 at temperatures that matter, a couple inches of closed cell foam is good for ~R12-R13, so you're talking about R17 for the exterior R. With 2x6 framing you'd have room for R15 batts, for R32-ish total, which would put ~53% of the total R outside the condensing surface, which is plenty of margin.

    With 2x8s an inch of EPS and 2" of closed cell foam you'd have room for compressed R23 rock wool performing at ~R22, and a R39-ish total., with about 43-44% of the total R on the outside, which isn't as much margin, but also fine.

    If the studs are deeper than that you'll be needing more exterior R.

    An inch of unfaced Type-II EPS runs about 2.5-3 perms, which is a bit more vapor tight than standard interior latex on wallboard, which isn't bad. A half inch of XPS is similar, but lower-R.

    Putting 4" of EPS on the exterior of a 2x6/R20 wall is about R38 at center cavity and 47% of the total R outside the sheathing, which is plenty of margin. Even though it's center-cavity R is slightly less than the 1" EPS + 2" ccSPF + R22 batt solution, the performance level of the 4" EPS solution is substantially higher due to the fact that the EPS isn't thermally bridged by framing. With no closed cell foam on the interior of the sheathing it can dry relatively freely toward the interior through standard interior latex on wallboard. EPS is pretty cheap compared to closed cell foam too, but there are more details to deal with with 4" exterior foam compared to 1" exterior foam.

  2. Canadarox12 | | #2

    Thanks for the reply!

    The walls are 2x6 with 24” OC.

    One of the reasons I was considering the CC spray foam was for the air tightness as well. Wanting to be under 1.0 ACH at least, and the foam would definitely help that. I will have to love out the 4” of EPS.

    My concern with the EPS and 2-2.5” of CC is just the drying of the OSB in the

  3. Expert Member
    Dana Dorsett | | #3

    As long as the EPS has no facers and there is an air gap between the EPS and siding it will be able to dry toward the exterior.

    Detailing the sheathing as the primary air barrier has a bigger effect on air leakage than using foam in the cavities. Foam in the cavities doesn't seal between doubled-up framing (headers,jack studs, top plates, etc) or between the bottom plate & sub-floor, etc.. All of those seams will still need to be caulked, but taping the seams of sheathing, including the top edge over onto the top plate of the studwall and caulking/foaming the bottom edge gap is really the most important part.

    At the same wall thickness 2x4/R15 studwall 16" o.c. with 3" of exterior Type-II EPS (R12.6 in summer, R13.5+ in winter) puts ~ 47% of the total R on the exterior of the sheathing, and delivers a higher whole-wall R than 1" EPS + 2" ccSPF + R15 batts in a 2x6 24" o.c. framed building. The center-cavity R of the 2x4 wall will average about R28-R29, but the "whole-wall R" will be about R23 after thermal bridging. The 2x6 wall only comes in about R19-R20 whole-wall, despite a center-cavity R of ~R32, all due to the R6-ish framing fraction that has less than R5 of thermal break (the 1" EPS), for a total framing fraction-R of about R11. The 2x4 wall has a bigger, R4-ish framing fraction, but with R13-ish thermal break the framing fraction's total R is R17-R18. It's about a third less heat loss per unit area, but about 20% more area, for about 25% less heat moving through the framing fraction, and about 20% less heat moving through the wall overall.

  4. Canadarox12 | | #4

    Okay I was just pricing out what it would be for 4" of EPS then just use the included R22 fiberglass in the wall cavities. Seems like that would be similar price to the 1" EPS with 2" ccSPF. As you mention with the 4" of EPS the entire wall performance would be higher.

    Would you recommend going the EPS route in that configuration over XPS?

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