How does a vapor retarder work in a Northern climate?
I guess I’m asking a Building Science question here, but in a Northern Climate (e.g., 6A), What does an interior vapor retarder actually do, especially if the air barrier is at the sheathing layer for a typical 2×6 wall with fibrous insulation? I can understand an interior temperature of 68% with a relative humidity level of 35% (and the dew point that must be kept higher than any condensing surface, which seem to the real parameter of interest in all of these discussions), and I can understand the temperature gradient (and its associated “humidity” gradient increase with dew point change). But if there is no/little air moving into the wall assembly, except perhaps by diffusion, what does an interior vapor retarder/barrier do? And/or what problem/function is it mitigating/performing?
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Bob,
Q. "What does an interior vapor retarder actually do?"
A. Mostly, it satisfies the building inspector. You're right: vapor diffusion is almost never the cause of wet wall problems, and interior vapor retarders are usually unnecessary.
There are a few exceptions:
1. It's important to have a vapor barrier under any slab on grade (or any below-grade slab).
2. It's important to include an interior vapor retarder or vapor barrier if you are building a very humid room (a greenhouse or a room with a swimming pool) in a cold climate.
3. Vapor retarders become more important in northern Canada and Alaska.
4. A vapor retarder is a good idea if you are installing open-cell spray foam on the underside of your roof sheathing in a cold climate. It's a good idea to spray the underside of the cured foam with vapor retarder paint.
In any exterior wall, the most important mechanisms for harmful water transport into the wall are, external leaks (improper flashing around windows and doors, reverse-lapped WRB, etc), convective transport of water vapor (air leaks at outlets and drywall/floor & ceiling joints, etc), and lastly molecular diffusion through the interior wall surface. You don't worry about vapor diffusion, a very slow process, until you take care of the first two mechanisms.
To satisfy my curiosity about the selection of 1 perm as being what building codes typically require in heating climates, I did a calculation once for a wall section consisting of studs @16" oc and sheathing, plus drywall having a 1 perm VR applied in some way. It turns out that for inside air at 70 F and 40% RH and outside air having a zero F dew point (very dry air), and assuming all of the water diffusing outward is absorbed and retained by the studs and sheathing, it would take eight months to increase the moisture content of the wood from a fairly dry 10% to the 19% often considered a threshold for avoiding mold issues. Out winters don't last that long.
Of course, the above calculation ignores diffusion of absorbed water through the sheathing to the outside world and the moisture absorption capability of cellulose insulation (if used). One might argue that more typical leakiness of exterior walls introduces more moisture than diffusion does, so that if you can retard the contribution of diffusion to the total you are better off. The reality is that just a vapor retarder primer paint (0.5 perm) or even just a couple coats of plain latex paint (perhaps 2-3 perm total) slows down diffusion sufficiently that no further retarding is needed. Going further, with a polyethylene VR (0.06 perm or thereabouts) or vinyl wallpaper leaves the wall unable to dry to the interior on occasion. That can be harmful in some climates and especially if the exterior cladding is brick or something similarly water-retentive.
I just read a Canadian Mortgage and Housing Corp report (ftp://ftp.cmhc-schl.gc.ca/chic-ccdh/Research_Reports-Rapports_de_recherche/eng_unilingual/Vapour_Permeance_Volume_1_Web_sept5.pdf) and in the Conclusions section, one thing mentioned was that vapor barriers in the lower wall section, below grade, are unnecessary or retard drying. That seems in line w/ a Build Sci article, and what I find a general consensus on the Net. If true, why are vb's apparently required in Canada?
Martin is right IMO. And Dick is too....
Air barriers that have permeability are useful. Double vapor barriers are mostly NOT useful and very unforgiving!
John: Thanks for that report link; the information was very informative. After reading that report (I'll probably go through it again) I'm thinking that there is no simple explanation; I was envisioning a simple steady state explanation for a situation that is clearly a dynamic one. As the sun rises and sets during the course of the day, the delta T and the dew point in relationship to the interior temperature (and humidity) changes. And it's different based on season. (Having brick/water retentive cladding has its own set of issues). So the need for a vapor retarder/barrier is based more on experimental evidence gathered over numerous experiments in different climates.