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Cold, wet sheathing on double stud wall?

The exterior sheathing on double stud walls is known gain moisture during winters. The data I've seen is all about "conventional" sheathing choices. There are several brands of fiberglass reinforced gypsum sheathing in the market, being sold as exterior sheathing, all of these products require "bracing" by other means. Denseglass is one brand and it has a moisture permeability of about 20 perms. Does use of fiber glass reinforced gypsum exterior sheathing result in a more resilient assembly than plywood?

Asked by Jerry Liebler
Posted Tue, 05/27/2014 - 09:51

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3 Answers

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1.
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Jerry,
Yes, DensGlass sheathing is more vapor-permeable than OSB, and for that reason may be preferable for use on double-stud walls. GBA has already reported on this recommendation; in my blog titled Monitoring Moisture Levels in Double-Stud Walls, I quoted John Straube, who advised: "Choose a sheathing that is more vapor-permeable than OSB — plywood, fiberboard, or DensGlass Gold.”

Answered by Martin Holladay, GBA Advisor
Posted Tue, 05/27/2014 - 10:08

2.
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The real answer to cold sheathing is NO sheathing! I've just eliminated the DensGlass too! I am planning on a "thin brick" exterior finish. Thin brick is a "moisture reservoir" cladding and Greenguard DC14 is a fan fold XPS product designed to isolate the moisture that can be driven inward from such cladding by solar isolation. I plan on using DC14 under a metal panel system designed to facilitate the thin brick installation. At least one vendor of the metal substrates has tested them attached to framing 24" OC over 1/4" foam, with no other substrate. The resulting wall's layers, from outside in are: 1/2" thin brick, Ambrico "EZ-WALL" panels, DC-14 (1/4" xps), vapor open WRB (Tyvec), 2x4 24" OC with cavities filled with r15 mineral wool, 3 1/2" space filled with r15 mineral wool, 7/16 OSB detailed as air barrier, 2x4 24"OC cavities filled with r15 mineral wool, 5/8"drywall. An r40+ wall that's 12 3/8" thick with drying both ways from the OSB which has over 2/3 of the insulation on it's exterior.

Answered by Jerry Liebler
Posted Tue, 05/27/2014 - 13:40

3.
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Jerry, We had the same discussion about a year ago and you swore the real answer was fibreboard.

Answered by Malcolm Taylor
Posted Thu, 05/29/2014 - 20:17

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