In Minnesota, Jeff Fredrickson is planning a new house, and his research has included lots of reading on the design and construction of exterior walls. His goals are twofold: a wall that will stay mold-free for decades, and one that is “somewhat energy efficient.”
Starting at the inside, the wall would go like this: drywall, a polyethylene vapor barrier, JM Spider insulation in 2×6 stud walls, 1/2-inch plywood sheathing, a water-resistive barrier (Benjamin Obdyke HydroGap), fiber cement and stone veneer cladding.
“Is this a good wall?” he asks in a Q&A post at GreenBuildingAdvisor.
Specifically, Fredrickson worries about condensation in this Climate Zone 6 wall, the potential hazards of volatile organic compounds in the insulation, and whether the blown-in Johns Manvillle Spider fiberglass insulation is really any better than plain old fiberglass batts.
That’s the focus for this Q&A Spotlight.
First, skip the polyethylene
Although it was commonly used in exterior walls at one time, polyethylene is now regarded as a potential moisture trap in all but the coldest climates.
“The interior polyethylene is more likely to cause moisture problems than solve them, because it prevents your wall from drying to the interior during the summer,” writes GBA senior editor Martin Holladay. “If you don’t want mold, then don’t install the interior poly (especially if there is any chance that the house will ever have air conditioning).”
Poly may present a higher-than-usual risk, Nate G. adds, if Fredrickson puts stone veneer right over the water-resistive barrier.
Holladay has the same misgivings about stone veneer on the exterior, telling Fredrickson, “Stone veneer over wood framing and plywood or OSB wall sheathing is probably the most problematic cladding ever invented. This type of cladding has more moisture-entry problems than any other. Lots of siding types are most…