What is the best material for an air barrier?
Before I can ask my question, I should explain exactly how we’ll be using the material.
I”m in Zone 5 (Massachusetts).
From the inside out we’re in the process of building walls as follows: drywall, 2×4 structural frame, 1/2″ plywood, air-barrier/vapor-retarder material, 10″ lightweight “I-joists” packed with dense cellulose, 1/2 plywood, asphalt-impregnated felt paper, corrugated plastic spacers, cement clapboards. All of this sits on top of a 14″ Thermomass wall (5″ + 4″ + 5″) with EPDM structural gasket (Conservation Technology) below the inner 2×4 sill, and ordinary sill gasket below the outer 2×6 sill.
I considered air-tight drywall, but was concerned about supervising all of the necessary details and the failures that future interior modifications could introduce. The plywood sheathing (that combined with the 2×4 sheathing makes up the structural framing) is penetrated only by window and door ‘boxes’, and is the primary air barrier plane.
The plan is to frame traditionally, cover the outside of the plywood with an air barrier material, leaving extra material at the bottom so it can lay across the top of the foundation wall foam core and be secured between the outer sill gasket and outer sill, before screwing on the I-joists.
The dense-packed cellulose walls will be blown in from the outside, using fiberglass mesh netting, and will press the material tight against the plywood, the window and door boxes, and the top of the foam core in the foundation wall.
I don’t trust tapes or caulking to last long term, and stapling a sheet material over everything seems a lot less labor-intensive than priming the plywood and taping or caulking all of the joints. It also seems considerably less expensive than Zip Sheating.
We have planned to use Certainteed’s Membrain instead of Intello Plus because it came in wider widths and was expected to be less expensive being a US product. Surprisingly, it has not been easy to find locally in Massachusetts, and by appearance, I am concerned about how easy it is to tear or puncture it.
I’m not terribly impressed with its ability to reduce permeability when it is dry, as I expect my drywall paint layer to be my primary vapor retarder to combat diffusion. I really just need it to be permeable when moisture is present and first-and-foremost, to be an reliable air barrier.
I will greatly appreciate any advice for help in choosing a material to use for this, as well as places to buy it locally.
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