An owner-builder planning a new home in southern Ontario isn’t looking for a net-zero house, just one that’s well insulated and protected from moisture problems. The question is whether his proposed wall system is his best option.
“I’m unwilling to put foam board on the outside and vapor barrier on the inside,” writes User 6782048, whom we’ll just call Ontario, in a question posted on our Q&A page. “Just seems wrong.”
He does plan on some exterior foam — a 1-inch-thick layer over the studs, followed by plywood and then Tyvek housewrap. After rough wiring, the 2×6 wall cavities would get a 2-inch layer of closed-cell foam and then batt insulation.
The technique of combining spray foam with batt insulation is called “flash and batt”, and Ontario believes the total of 3 inches of foam would keep the exterior sheathing above the dew point in winter, thus reducing the chance of moisture condensation inside the walls.
“Thoughts, please!” Ontario writes. That’s the topic for this Q&A Spotlight.
(For more on how to change a user name from a number to a real name, a quirk in GBA’s registration process, follow this link.)
The downside of flash-and-batt
Filling a stud cavity with closed-cell foam provides an excellent air seal, but it’s expensive — and that’s why some builders advocate the flash-and-batt approach. But, writes Andy Chappell-Dick, it’s still costly, “it relies on the least ‘green’ insulation, and it’s interrupted (and thus substantially degraded) every 16 inches by a stud.”
The technique may be useful in tightening up an old house in an efficiency-inspired renovation, but for new construction it would be better to put all of the foam on the exterior of the house. Further, Chappell-Dick adds, it would be wise…
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