How does ambient temperature affect wetting/drying cycles?
Hello GBA pros & community! I’ve been a lurker around here for awhile, but this is my first post. I’m enclosing a timber-frame in southeastern Manitoba (climate zone 7A), and plan to build a vented roof assembly with dense-packed cellulose insulation. For reference, the specific assembly I have in mind is as follows, from the inside:
-timber rafters
-1×6 T&G pine ceiling
-ice & water shield (interior air-barrier, class I vapour retarder, and temporary rain protection for the timber-frame)
-18″ deep non-structural ‘trusses’ dense-packed with cellulose
-3/8″ plywood sheathing (insulation containment & second line of defence against squirrels & bats)
-vapour-permeable self-adhesive air- and weather-barrier membrane such as Henry Blueskin VP
-3″x3″ false rafters to create overhang
-2″ rigid mineral wool between false rafters creating a 1″ vent channel
-1×4 skip-sheathing
-standing-seam metal roofing
In our climate, it seems to be common knowledge that building assemblies become saturated in the winter & dry out during the warmer months. At the same time, the winter-time vapour drive should be outwards with an interior RH of 35% (maintained by an HRV), and the summer-time vapour drive should be the opposite. If the vapour-drive during the supposed ‘drying season’ is inwards, should I be selecting a vapour-variable interior air barrier to permit drying to the inside?
Basically, I’m looking for an understanding of the relative impact of humidity & ambient temperature on the wetting & drying of an assembly, as well as a way to quantify the desired permeance (or permeability?) of an assembly’s components.
As a side-note, if someone knows of a vapour-permeable self-adhesive roof underlayment that performs as an air-barrier, is grippy, and can also act as temporary water-proofing until the final roof goes on, I’d love to hear about it! (That’s not the case with Blueskin, or comparable products made by Resisto, Soprema, Vaproshield or Cosella-Dorken). If it’s available in Canada and not heart-breakingly expensive, even better.
Thanks for your help,
Ben
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Replies
Benjamin,
Your assembly should work. I don't think that the cellulose will accumulate enough moisture to worry about.
Warm temperatures in the spring, summer, and fall, along with solar radiation, will keep everything dry.
Some green builders might worry that the Ice & Water Shield has an odor, and would prefer to see an air barrier between the tongue-and-groove boards and the Ice & Water Shield. I'm not sure whether that's a concern. Any GBA readers with experience are free to offer advice on this issue. (Different brands of peel-and-stick may have different odors.)
Before you settle on 3/8-inch plywood sheathing, get a price on 1/2-inch (or 7/16 inch) plywood. The thicker plywood may be the same price or even cheaper.
-- Martin Holladay
Thanks Martin, I will take your word that the assembly should work. Meanwhile, are there any resources you can recommend to help understand the physics behind it? I'm having a tough time squaring inward solar-driven moisture with outward drying of an assembly in the summer.
Benjamin,
Inward solar vapor drive is only a problem when there is a layer on the exterior of the building that is (a) porous, and (b) saturated with moisture. Your roof doesn't have such a layer.
The worst-case scenario for inward solar vapor drive is a brick veneer wall. In a driving rain, the bricks get saturated. When the sun comes out, the moisture is driven inward.
In your case, the metal roofing is a vapor-impermeable layer that prevents inward solar vapor drive.
-- Martin Holladay