Vapor permeable membranes in cold marine climates
I am building a new house in a cold marine climate with low temperatures reaching approximately 14 degrees F and outdoor relative humidity commonly between 80% and 95% in the winter. I am planning to place the insulation outside of the membrane in a rainscreen assembly, but am uncertain whether to use a vapor permeable membrane or a non-permeable membrane. With this high outdoor RH and low temperatures, I am concerned with vapor diffusing through the membrane and condensing within the insulation. Is this a valid concern and would a non-permeable membrane be a better choice, or is a vapor permeable membrane as good or better in these circumstances?
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Replies
George,
I'm not sure what you mean by "membrane." Are you referring to a water-resistant barrier (WRB) or an interior vapor retarder?
A WRB is always required. If you choose to use a plastic housewrap or asphalt felt, these materials are designed to be vapor permeable, which is appropriate. It is also possible to use rigid foam insulation as a WRB, as long as it is properly detailed. If you choose this route, the fact that the rigid foam may be vapor-impermeable should not cause any problems, as long as your wall assembly can dry to the interior.
In your climate, you shouldn't use a vapor-impermeable interior membrane like polyethylene. Appropriate materials for an interior vapor retarder include vapor-retarding paint, kraft paper, or MemBrain.
Finally, it sounds as if you are concerned that exterior moisture might somehow enter your wall assembly and condense. This shouldn't concern you. Remember that moisture will only condense out of air when the air contacts a surface at a lower temperature than the air. In winter, even if exterior air were able to enter your wall assembly, the surfaces within the wall will be warmer, not colder, than the exterior air.
It is rare for condensation to occur when exterior air contacts a wall. It only happens:
1. During the summer, when the house is air conditioned and a portion of the house is at negative pressure.
2. In some cases, on sloping walls.
In fall, winter, and spring, the coldest surface on the wall will usually be the siding. On cool mornings, condensation on siding is possible in some conditions, especially if the wall has a slope. This phenomenon is called "dew." It is harmless to the wall.
The reason that it happens more when the wall is sloped is that sloping walls face the night sky. That allows radiational cooling of the siding to space.