External foam board when using siding and stone / brick
I am embarking on having a home built in lucky zone 5. I was hoping to use some green building techniques where possible and not cost prohibitive. The current exterior is planned to be stone base and LP smart siding.
I mentioned the concept of 2″ XPS foam over 2×6 walls with blown in mineral wool to my architect and he responded that that would cause issues and/or extra expense because the depth of the wall with the stone would be almost 12″ and the depth with the smart siding would be 8 5/8″ the two main issues being 1) the foundation pour is 10″ and expanding to 12″ would add significantly to the cost, 2) windows would be much more expensive since we would need extended jambs and more framing around them to get them to fit correctly.
He recommended switching to 2×4 walls with 1″ of sheathing and closed cell spray foam to achieve a similar effect without a cost increase. It seems like the R-value would be similar but would I have moisture issues?
Any advice / thoughts?
I also mentioned 4″ of foam under the slab and he recommended foam only at the edges to keep costs down as well.
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