Is 2″ XPS enough for Northeast Ohio (Zone 5A)?
Autopro76
| Posted in Energy Efficiency and Durability on
Hi I have a house in North East Ohio zone 5A. I was gonna install 2 layers of 1″ xps foam board on the exterior of the house over top house wrapped 1/2″ OSB for a total of 2″ R-10. I have 2×4 Walls with R13 faced batts no plastic barrier. And soon no wall paper. Just painted drywall. Is that enough to not worry about rotting my OSB sheeting? Cant find a straight answer, from my research it looks like that’s plenty. Thank you for any advice!
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Ricky,
That will work. All you need is R-5 of rigid foam. More information here: "Calculating the Minimum Thickness of Rigid Foam Sheathing."
Ya that's a good article, just wanted to make sure I was doing it right. One more quick question I put 4" XPS R-20 in my Rim joint. Will the R10 on the exterior be ok? It meets the at least 27% rule? Thanks Again Martin!
I'd use StuccoWrap over the OSB for better drainage and distribution of any bulk water. And of course the OSB would be taped (and tested) to be an air barrier.
Thank you I'll look into the Stucco wrap. No contractors around me are into any of this they are more old school think the extra insulation is a waste of money. Lol so I'm kinda on my own. That's why this site is awesome.
At 2" XPS is labeled R10, but if you read the fine print it's only warranteed for R9, and after full depletion of it's climate damaging HFC blowing agents after several decades it'll settle in at ~R8.4. From a lifecycle design point of view consider it R8.4.
R8.4 plenty of dew point control for a 2x4/R13 (or even R15) wall in US climate zone 5.
It's much nicer to the planet to install 2" of polyiso (labeled R11 -R11.5 if roofing iso, R12-R13 if foil faced sheathing iso), which is blown with far more benign hydrocarbons, instead of HFC. The predominant blowing agent in the mix for XPS is HFC134a, with a 100 year global warming potential of nearly 1400x CO2. The predominant blowing agent for polyiso are variants of pentane, at about 7x CO2.
The 4" of XPS on the interior of the band joist may meet the 27% rule, but it has reduced the drying path toward the interior to something like 0.3 perms- it's approaching true vapor BARRIER status. Temperature-wise you'd be OK, but if there isn't a capillary break between the foundation sill and foundation there may not be sufficient drying capacity overall if low-permeance foam is also installed on the exterior. How much above-grade exposure is ther on the foundation, and how deep are the roof overhangs?
+1 on avoiding XPS. I'd use EPS which will be around 1 perm.
Wow thank you for all the information! There is that blue foam stuff in between the rim joist and block. Is that considered a capillary break? The house is rectangular with the back block exposed 4,' one side exposed 3,' the front has a concrete porch up to it and the other side has a attached garage. The house has 12" overhangs all the way around. Will be 10" after the foam. Dana can I use the foil faced iso in my situation? I'm all for helping the environment especially when it's got the high r value benefit.
You’re probably thinking of the sill seal gasket material, which is usually blue for Dow or pink for Owens corning brand. While not necessarily the best material to use as a capillary break, it does serve the purpose.
Bill