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Building Science

Two Rules for Humidity

To prevent moisture damage from humid air, just do these two things

Mold is growing on the interior of a wall because the drywall was below the dew point.
Image Credit: Andy Bell

Because I’ve written so much about moisture in buildings, I get a lot of questions on the topic. Some are about walls. Some are about the attic. Some are about windows. Some are about the crawl space (which generates the most questions on this topic).

The key to answering a lot of those questions boils down to an understanding of how water vapor interacts with materials. Once you know that, it’s easy to see the two rules for preventing damage from humidity.

How water vapor interacts with materials

The first thing to understand is that water vapor floating around in the air gets pulled in by the materials in contact with the air. Let’s ignore the issue of hygroscopic materials here and focus on the effect of temperature. The dividing line is the dew point temperature. When the temperature of a material is above the dew point, we don’t get condensation. When it’s below the dew point, condensation happens. And the lower the temperature of a material, the more water vapor it will pull out of the air.

We use that fact to our benefit with dehumidifiers, which run humid air over a cold coil, condensing out a good deal of water vapor. When we’re talking about parts of a building, though, we’d rather not have water vapor condensing on (or absorbing/adsorbing in) materials, whether we’re talking about bathroom windows, crawl space band joists, or vinyl-covered walls. So here are the two rules.

Rule 1: Keep humid air away from cool surfaces

When you’re looking at plans for a building or trying to understand what went wrong in an actual building, a good place to start is identifying where the humid air is and what parts of the building it’s in…

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