EPS instead of XPS
So I’m building a house in Chicago and I’d like to use continuous insulation outside attached to the sheathing. But I’d rather use EPS than XPS. What I plan to do is R15 cavity in 2x 6 framing, sheathing, WRB, 1.5” of EPS, .75” furring, then Hardie siding.
My thinking is that EPS has a much lower carbon footprint and holds its R value. If I used 1.5” of XPS, it would take close to 50 years to recover the carbon footprint to offset the heating cost. And my understanding of XPS is that it loses R value at a rate of about 10% in its first 5 years and then more gradually there after. So the thinking is, if I use 1.5” of EPS I’ll achieve an outside R value of about R 6. And with XPS at 1.5” it would start at 7.5 but drop to the same r value in about 10 years.
I understand that the recommended r value is 7.5 in my climate zone, but if I take the average temperature in the coldest months I’m safe from any condensation on the back side of the sheathing at r6. https://www.buildingscience.com/documents/digests/bsd-controlling-cold-weather-condensation-using-insulation Yeah, there will inevitably be a couple days that are colder that might allow the sheathing to drop below the dew point of the air reaching it, but the following days should be warm enough to allow it to dry. Also, we will have an ERV so that should help with removing excess humidity.
What do you all think? Is my reasoning reasonable and is EPS a viable alternative here?
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Replies
Ryan,
Please indicate your climate zone and more design details of the house (sq. footage, # of storeys, etc. Agree that EPS is a much greener choice than XPS and more vapour permeable, which is desirable. Did you consider ComfortBoard or other semi-rigid mineral wool? It would be considerably more vapour permeable than foam. It would be interesting to make a comparison between it and EPS for R value, material and installation cost as well as its GWP.
R15 is pretty light in a 2x6 wall. Normal insulation for a 2x6 wall would be R21 if using standard fiberglass batts, or R23 if using mineral wool batts. R15 would be standard in a 2x4 wall if using either mineral wool or high density fiberglass batts. Keep in mind that it is the ratio of interior to exterior R values that is important for moisture control, not the absolute R value. More information about this is available here: https://www.greenbuildingadvisor.com/article/rethinking-the-rules-on-minimum-foam-thickness
You are correct that XPS will gradually reduce in R value until it approaches EPS at around R4.2 or so per inch. That doesn't change code rules though, which will require you to use insulation rated at a particular R value for your application. R6 of exterior insulation would be a bit light with a 2x6 wall insulated properly, but probably reasonably safe if you underinsulate to R15 (which is probably going to be a problem at inspection time). You'd be safer all around by using 1.5" polyiso instead of XPS or EPS, since 1.5" of polyiso will give you about R9, and it will hold pretty stable over time. Even if you derate for cold weather (which is an often exaggerated issue with polyiso), you still have about R7.5 with 1.5" of polyiso.
Bill
Chicago is in climate zone 5A, near the border of zone 6: https://codes.iccsafe.org/content/IRC2018/chapter-11-re-energy-efficiency. You need at least 30% of the total, eventual R-value to be on the exterior (https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-100-hybrid-assemblies). With R-15 in the cavity, that means you need at least R-6.5 on the exterior. Since you're on the colder side of zone 5, and the ratios are for long-term resistance to moisture accumulation and resulting problems, I would not skimp on the ratio. That means at least 2" of EPS or 1.5" of XPS. I agree that EPS is a much better choice for environmental reasons, and it's vapor permeable so it also allows some drying.
Thanks for the reply. So I could use less cavity insulation then and get to the 30% ratio you suggest. Chicago uses 2018 code so the requirement is either 20 or 13+5. 1.5” of ESP meets that.
My assessment was based on this building science article. https://www.buildingscience.com/documents/digests/bsd-controlling-cold-weather-condensation-using-insulation
Which comes out close to the 30% anyway. But my main question is, in the long term is 1.5” of XPS really any better than 1.5” of EPS? I’ve read many places that XPS loses its R value over time (around 10% first 5 years) hole EPS doesn’t. So after 5 years I’d been in the same place with just the EPS.
Ryan, that's correct--there is little reason to use XPS if you can get EPS.
Thanks for the reply. I’ve been busy building this thing and just got back to the forums.
Or take a look at GPS - EPS with embedded graphite that gives it a nudge up in R-value. Slightly pricier but also better at insulating. Price difference is low in some markets. And don't forget to check Craig's list for local recyclers. Recycled foam would work fine for exterior wall insulation. The environmental cost has already been spent, the material is already aged, and it's cheap.
Thanks. I did look at GPS but like you said, a bit more pricey. We’re now at that stage where we’re gonna start running into budget constraints. Choices…..