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Dew point calculation vs location of air barrier.

dickrussell | Posted in Energy Efficiency and Durability on

The dew point at the first condensing surface often is considered important in judging the robustness of a wall design. In the case of adding external insulation, a minimum thickness is required to keep the sheathing above the dew point.

The dew point, as far as I have seen in discussions, has been based on that of inside air at room temperature and some appropriate relative humidity. This gives a “safe” answer and would seem to assume one of two things. The first is that the wall cavity is vapor-open to the inside air, as could be the case when the air barrier is on the outside and no particular care is taken to seal the inside boundary against flow of inside air into the cavity. The second is that there is a substantial vapor retarder on the exterior side of the cavity and not on the inside, in which case humidity in the cavity would approach that of the inside air over time.

I would think that, in the case of an inside air barrier with inside vapor retarder, using the inside air moisture content for a dew point calculation is far too conservative. Without flow (leakage) of inside air into the cavity, and the outward migration of water vapor via diffusion in winter limited by the vapor retarder layer, then there would be a gradient to the moisture content across the whole wall assembly. With the VR providing a huge fraction of the total resistance to diffusion across the wall, the moisture concentration at the sheathing layer ought to be far less than inside the house nearly all the time in winter.

Is this reasoning sound or flawed?

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  1. GBA Editor
    Martin Holladay | | #1

    You can use WUFI to test you proposed assembly if you want, but the use of WUFI has its own problems.

    I don't recommend installing a significant interior vapor retarder (in other words, something that is less vapor permeable than kraft facing or vapor-retarder paint) if your exterior insulation consists of rigid foam. Here's the reason: you want your wall assembly to be able to dry out in at least one direction. If the rigid foam prevents drying to the exterior -- and it does -- then you want the interior finishes to be relatively vapor-permeable.

    Of course, if you use mineral wool as your exterior insulation, your approach might be less risky.

  2. dickrussell | | #2

    I think you may have misunderstood my question. I am questioning the use of a dew point based on inside air conditions to judge potential for sheathing wetting in the case of properly detailed air barrier and vapor retarder being on the inside. I'm thinking more of the case without insulation outside of the sheathing, where the sheathing does approach outside air temperature more closely.

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