Unconventional framing insulation strategy sanity check?
My buddy Don is building his 24’x48′ house in zone 4a North Central Arkansas.
He called me for a sanity check on his insulation strategy and I told him I was going to ask my vast group of building scientist friends on GBA.
UNCONVENTIONAL CONSTRUCTION DETAILS:
No code where he lives. He milled his own lumber and did not use any wall or roof sheathing.
Exterior walls, he used 2×4 studs, horizontal purlins 24″ OC, typar, vertical strips for rainscreen 9″ on center, and then metal roof panels for siding.
His attic is vented. He placed 1″ recycled XPS between roof metal and purlins for a condensation thermal brake.
His HVAC will be mini-splits, so no duct work.
CURRENT INSULATION STRATEGY:
2″ of closed cell in attic floor onto drywall.
1.5″ closed cell in wall cavity against housewrap.
He will have sides of 2×4 studs sporayed as well to get the R5 out of the 2×4 and purlin.
Currently he does not plan to add any other insulation in the cavities.
He said he can ad an inch of spray foam for a $1000 if need be, but he is thinking with the descent air sealing of the closed cell + R10 for walls and R14 for ceiling, he will be ok. This is one point where I disagree, yet concede it will be better than most houses in his area for the simple reason he will have descent air sealing, and most houses there leak like a sieve.
My opinion was he really need smore R value, but not sure the approach. I thought he might go with only an inch layer of closed cell and add cellulous on ceiling and fiberglass in cavity. This would give him and approx R20 in walls and he could easily get an R30-40 in ceiling. He hates the mess of cellulose when he has to work in attic.
As for fiberglass, he said it is really not much more $$ to go with thicker closed cell than to use fiberglass batts, so if it is unwise to go with his current strategy, he would rather add a few inches of closed cell to his current pan than to use cellulous or fiberglass.
Thank you for your help. I will send Don the link so he can read any responses.
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