Bill L. is planning a high-performance house in Massachusetts and is wrestling with options for the air barrier, that all-important building detail that enhances both energy efficiency and building durability.
Above-grade walls will consist of a 2×4 structural frame sheathed in 1/2-inch plywood, followed by I-joists packed with cellulose insulation, another layer of 1/2-inch plywood, a corrugated plastic product to provide an air space, and fiber-cement siding. The primary air-barrier plane will be at the plywood over the 2×4 studs.
“I don’t trust tapes or caulking to last long-term,” Bill writes in a Q&A post at Green Building Advisor, “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 sheathing.”
Although Bill had considered using CertainTeed’s MemBrain, it’s proving hard to get where he lives, and he’s not convinced it will be durable enough.
Where does this leave him? That’s the topic for this Q&A Spotlight.
Consider a liquid-applied WRB or high-quality tape
If Bill doesn’t trust tapes to go the distance, suggests Albert Rooks, use a liquid-applied WRB at the sheathing layer. Specifically, he points Bill toward FastFlash, a product made by Prosoco. Rooks says that was used in a Seattle Passivhaus project described in a 2011 GBA blog by Richard Defendorf.
“The approach is not inexpensive, but the quality is exceptionally high,” says Rooks. “We sell it at The Small Planet Workshop, but I’m sure you can find a supplier in your local market at Prosoco’s website.”
But in the end, Rooks adds later, “good quality tape is still (in my opinion) the simplest ‘go to’ material for turning sheathing into an air barrier.”