Smarter Framing and Vapor Barriers
Just read Joe Lstiburek’s article about smarter framing, then checked the code book and it looks like it works. Then I check his website for more details.
There I started thinking about vapor retarders and didn’t like what I found (being a Minnesotan). No vapor retarder required, since the idea is to let the wall cavity dry to the inside in the summer months. Permeable latex paint is recommended at the interior to allow that drying.
First, what happens to the permeability of the interior paint after five or six coats, perhaps of the wrong type? Will this adversely affect the wall system?
Would the author suggest a vapor retarder at bathrooms on exterior walls, creating a trap for driven vapor?
In Minnesota summer is short and humid, while winter is long and drier. If a house isn’t air-conditioned in spring,summer and possibly fall, the interior won’t be drier than the wet wall cavity, making drying difficult.
The problem seems to come from the idea of impermeable foam sheathing. Wouldn’t it make more sense to put the foam on the inside of the studs, use a vapor retarder, and let the drying happen to the outside in winter? That seems better given the impermeability or wicking properties of current foams.
Very interesting article. It would be great to hear from the author on this one.
Replies
The problem seems to come from the idea of impermeable foam sheathing. Wouldn't it make more sense to put the foam on the inside of the studs, use a vapor retarder, and let the drying happen to the outside in winter? That seems better given the impermeability or wicking properties of current foams.
The key to this is vapor drive, which is from warm to cold and from wet to dry, and drying ability. Whether or not your house is air conditioned, the sun in the summer is likely driving moisture inward. As long as the humidity inside isn't 100%, some drying will take place.
As to allowing the framing to dry to the outside in the winter, well, that's just not likely. Yes, the vapor drive is in the right direction, but once the moisture hits a cold outside surface, it's going to condense and probably freeze. Ice can and does evaporate when it's cold and very dry. The process is called sublimation, but it's slow compared to summertime drying. Effective evaporation requires energy input, and there's just not a lot of that available when it's below zero.
Another key is understanding just how little water, relatively speaking, enters framing cavities as vapor. It's minute, compared to the amount of moisture that can enter in air leaking through a one inch square hole. Air sealing is an order of magnitude more important than vapor barriers, and no where near as likely to create attendant problems.
I doubt Joe will comment here, but the next best thing would be to buy his Builder's Guide for Cold Climates.
Andy Engel
Senior editor, Fine Woodworking magazine
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Re: The problem seems to come from the idea of impermeable foam sheathing. Wouldn't it make more sense to put the foam on the inside of the studs, use a vapor retarder, and let the drying happen to the outside in winter? That seems better given the impermeability or wicking properties of current foams.
The article on the Building Science website (http://www.buildingsciencecorp.com) titled "Vapor Barriers and Wall Designs" addresses the question you pose about exterior sheathing. The answer is to balance the wall and sheathing insulation based on the particular local climate such that under the worst-case winter conditions the inside sheathing surface temperature never reaches the condensing temperature of moisture. The article above takes you through the procedure to calculate the proper design.