Vapor barriers and air sealing: How much and where in a dry, heating climate?
We are in the process of framing our final wall on our small, single story slab on grade in Northern New Mexico, so it’s an important time to start thinking about air sealing and vapor barriers. So far we’ve done a bead of caulk above and below dow sill seal and around penetrations to seal the sill plate. A final bead will go along the inside edge. We are also planning to caulk between double top plates and california corners.
The question now is how to most efficiently and effectively address air sealing for the rest of the structure.
Our planned wall assembly, inside to out, is drywall, R-19 fiberglass batts, 5/8 OSB, 2″ polyiso, drainwrap, lathe, 3 coat stucco.
For the roof, drywall, 6 mil plastic, 2″ polyiso, R-38 fiberglass batts, 2″ vent, 23/32nd OSB, pro panel metal roof. This is a single pitch 4/12 roof.
This is a predominately cold and dry heating climate with big temperature swings. I was talking with my uncle who has been a builder here for 40 years. He was explaining the importance of dry heat for optimal performance of fiberglass insulation, hence his recommendation to put the 6 mil plastic on the ceiling. The heat moves through but not moisture and slow bakes the fiberglass. I was planning to tape the foil faced polyiso for this effect, but he thinks it’s a waste of time and effort vs the 6 mil poly.
He also advised putting poly under the drywall on the walls. This caused a bit more concern as it seems it could potentially trap moisture in the wall cavity, especially since I had planned foil faced polyiso under the stucco, also taped. He suggested leaving somewhere for the moisture to go. He also suggested using EPS instead of polyiso, both for the lower cost and I would assume moisture resistance. I’d prefer polyiso for it’s lower environmental impact and high performance.
Without a ground-up redesign, I’m curious how folks here would approach air sealing and vapor barriers for this wall assembly? My questions are as follow:
1) Poly or no poly behind drywall for the walls?
2) Should I skip the foil faced on the exterior wall foam / does taping the foam seams make sense for this assembly/climate?
3) Related to (2), is it a waste of time to seal all the osb and framing when exterior foam and drywall seem like a much easier place to address air leakage?
4) How do you plan for the condensation point with foam over the sheathing? Seems it would likely land somewhere in the middle of the wall assembly which is no bueno. A zero permeability interior and exterior vapor barrier (poly + foil) seems dangerous, but so does condensation anywhere inside the wall.
I understand this is very climate specific, so even if it does not apply to your particular building experience, I’m curious to hear a building science answer to at least have some theory wrestle with.
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