Zone 4A 2×4 wall assembly
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Hey folks
We are building a house in zone 4A and are considering this assembly:
sheetrock
2×4 wall
R15 batt (either fiberglass or Rockwool)
1/2″ cdx
Tyvek wrap
1″ rigid foam
rainscreen
hardie shingle
Any thoughts or concerns? thanks!
Brad
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Replies
There are different types of rigid foam; EPS is about R-4/in, polyiso is about R-5.5/in and XPS is about R-5/in when new and about R-4.2/in eventually.
For climate zone 4, non-marine, the 2018 IRC requires at least R-13 cavity insulation plus R-5 exterior insulation; the 2021 IRC requires at least R-13 cavity insulation plus R-10 continuous insulation.
XPS is by far the worst for the environment, due to its blowing agents being potent greenhouse gasses. Newer formulations are much better than older formulations, but if you have to use foam, it's better to use polyiso or EPS.
Thank you Michael. I appreciate this input!
Hi Brad.
You'll need to determine what material in the assembly is your air barrier. Assuming that it will be to the exterior, mechanically fastened housewraps like Tyvek are air impermeable, but are difficult to detail as an air barrier. The rigid foam can be detailed as an air barrier, but may be difficult to make continuous at transitions like the foundation, windows and doors, ceiling or roof. It may be easiest and give you the best results to tape the CDX sheathing seams with a good quality air sealing tape. GBA has lots and lots of articles on air barriers and air sealing that may be helpful...
Consider GPS (Graphite infused EPS), R5 per nominal inch (1 1/6") so similar value to XPS but should not decrease with age and better for the environment.
Where I live it's a bit more expensive than standard EPS, but about 3x cheaper than XPS.