30% of insulation external in climate zone 5?
I’m working on building a house in climate zone 5 (Northwest Kansas).
If I understand my builders preferences correctly, he would like to run 2″ of closed cell spray foam insulation in the wall cavity and then R21 (for 2×6 walls) over the closed cell insulation in traditional 2×6 framing.
I could be putting words in his mouth but I think he said when he does it the batts are just barely compressed and I think he feels he is getting close to R35 in the walls with the two combined.
My preference would be to run comfortbatt in the walls for R24 and then run either 2 layers of 1.5″ externally for R 12.6 or so or two layers of 1.25″ for R10.5 or so.
3″ total external would get me 34.4% externally and two layers of 2.5″ total would get me 30.4% external.
How important is that external percentage? I don’t know of anyone building with external insulation at all in our area and I don’t hear of people with condensation issues in their walls. I wonder if it’s because it’s probably fairly uncommon to build to 3.0 ACH 50 around here so we leak enough to let it dry?
We’re shooting for under 1 ACH 50 and I’d really like to keep humidity in the 35 to 45% range in the wintertime so I’m a bit concerned about condensation issues.
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