I’ve been reading about adding rigid insulation to the exterior of a house to increase R-value. The discussion seems to focus on ensuring the sheathing is warm enough to ensure it will dry. The problem I see is that most installations seem to consist of faced polyiso or XPS, both which effectively act as a vapour barrier. Seeing as my house already has an interior poly vapour barrier, I’m not too convinced this is the way to go.
Has anyone ever used unfaced, or glass fiber faced polyiso such as Iko Enerair? Seems to me it would allow the wall to dry more effectively in case of moisture penetration.
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The problem is not "double
The problem is not "double vapor barriers", it's having the dewpoint rise above the temperature at some point in the wall profile, and this becomes more likely if materials towards the outside (cold side) are more impervious to moisture than they are to heat (and cold). The likelihood of the problem is reduced as moisture-impervious materials are placed towards the inside.
The "double vapor barrier" wives' tale got started when people foolishly placed plastic on the outside of their homes, as an air barrier. In fact, having a second vapor barrier on the inside of the wall will reduce the damage done by such a "feature", but not by much.
You're right to worry about having two vapor barriers: were moisture to get between them, it would have nowhere to go. It's good to keep the inside face of sheathing warm enough to prevent condensation in colder months. Similarly, moisture from the interior will condense on the inside face of your poly if it falls below dewpoint. Moisture that gets into your wall from the outside must dry to the outside because it cannot dry to the interior.
Please see: Lstiburek, RR-0410 "Vapor Barriers and Wall Design," November 2004, at buildingscience.com: Note the four principles listed on page 4, under the section titled "Assembly Characteristics," especially the second bullet point. Quoting: "Avoidance of the installation of vapor barriers on both sides of assemblies--i.e., 'double vapor barriers' in order to facilitate assembly drying in at least one direction." Then, quoting from page 6: "[T]wo important factors . . . . [1] How easy water vapor gets out of the assembly to the outside determines how much resistance to water vapor entry from the interior can be allowed. . . . [2] How cold the cavity side of the sheathing is determines how much moisture accumulates on the sheathing. . . . [T]his temperature . . . determines how much resistance to water vapor entry from the interior can be allowed. The illustrations and explanatory notes clarify how to think about factors involved.
See also Peter Yost's August 5, 2010 post at greenbuildingadvisor.com titled "Vapor Profiles Help Predict Whether a Wall Can Dry: to avoid moisture problems in walls, think about the permeability of all the wall’s components — not just the “designated” vapor retarder." His four-steps to a vapor profile are clear, concise, and non-technical. http://www.greenbuildingadvisor.com/blogs/dept/building-science/vapor-profiles-help-predict-whether-wall-can-dry