Airtight Wall and Roof Sheathingcomments (13) February 7th, 2013 in Blogs
Rosenbaum noted that at any sheathing edge that doesn't abut another piece of sheathing - for example, sheathing adjoining a rough opening or sill - needs to be sealed to the framing. To tape sheathing seams, Rosenbaum has used Vycor Plus for walls and W.R. Grace Roof Detail Membrane for roofs. (Since roofs get hotter than walls, they need tape with different specifications.) However, Rosenbaum has concluded that the Huber Zip System is faster and less expensive than using peel-and-stick tape.
Although it's possible to use closed-cell spray polyurethane foam as an air barrier on the interior of the sheathing - using the so-called "flash and batt" or "flash and cellulose" method - there are drawbacks to this approach. It's sometimes hard to maintain perfect access to all necessary areas that need to be foamed; spray foam is tricky to apply in cold weather; and spray foam can't address some common air leakage locations (for example, between the subfloor and bottom plates).
Rosenbaum sprinkled his presentation with interesting observations and useful advice:
- "You could have the builders caulk the sheathing when they put it up. The only problem with that is I can't tell if they did it or not."
- "I'm nervous about rigid foam and tape, because the foam moves."
- "Peel-and-stick tape on OSB doesn't work without a primer."
- "I don't use the Huber Zip System without a housewrap, and I don't use foam as a WRB. I do not believe that housewrap is an air barrier."
- "It's hard to use spray foam in really cold weather. If it's zero degrees out, you don't want them spraying against your sheathing."
- "You can always add more insulation on the outside of the sheathing to keep it warmer."
- "Cavity fill insulation must be very resistant to air movement. I prefer dense-packed cellulose. I am nervous about interior air going through fiberglass."
- "There are probably more infrared cameras being sold than there are people who know how to use them."
Rosenbaum's exterior air barrier details have achieved impressive results. On Martha's Vineyard in Massachusetts, Rosenbaum specified the Huber Zip System on an 8-unit residential project designed and built by the South Mountain Company. According to Rosenbaum, the single-family homes tested "Passivhaus-tight."
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