The vapor permeance of exterior foam and other questions?
I am building in climate 4C Marine.
I’d like to apply a layer of exterior insulation on the outside of the WRB.
For a variety of reasons including ease of install, cost, minimal complications, etc. I want to use the minimum thickness, just enough to ge the benefit of the thermal break.
For a 2×6 wall I found an article that says I need R-3.75 for this in my climate.
This can be achieved with 3/4″ Foil faced Polyisocyanurate which is labeled as R-4.4. Climate 4C is pretty mild temperature wise, do we see the cold weather performance issues with Polyisocyanurate here? Or is that a concern only in much colder zones? It also feels counterintuitive to be putting foil on the exterior of house when we go to the trouble of applying a breathable membrane as a WRB.
What about EPS foam without foil? It is more permeable(correct?) and costs less. Is this a more suitable material for exterior insulation? In my climate what would be the minimum thickness I could use on the exterior walls? Even if it had to be 1 inch thick, this would not affect construction details too much.
thanks for your thoughts here
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