An Energy Auditing Blog article on reducing mold risk by reducing the homeowner’s ‘water sports’ (water related activities in the house) was published at GBA last week. Allison Bailes, the self-described energy guy with the funny name, rightly pointed out that temperature plays a key role as well.

Temperature Makes Humidity Relative

Relative humidity is exactly that…relative. Relative to the current temperature and pressure conditions. Moisture exists in the air as water vapor, one of the gases in the atmospheric mix. As the temperature rises, water vapor can make up a larger portion of the gaseous mixture.

Relative humidity is the amount of water vapor in the air as a percent of the maximum possible humidity at that temperature. If you lower the temperature with the same moisture, the relative humidity rises.

And when humidity reaches 100%, well, $%^& gets real.

100% Humidity Means Temperature Problems

When air is saturated with moisture and the temperature drops further, it can no longer hold the moisture. The moisture is wrung out of the air mix as condensation. It’s the reason you see morning dew on grass. The temperature plunges overnight and can’t hold the same moisture it could 30 degrees ago. This temperature where water vapor condenses out of the air is the dew point.

Allison’s point was that under normal humidity conditions, cold surfaces can still grow mold. The temperature dip increases relative humidity and the likelihood of condensation. It recalled one of my favorite audit photos:


This picture showed some poorly installed insulation near the house’s shower. The high humidity would rapidly cool as it neared the uninsulated section of ceiling. The moisture condensed out of the saturated air and eventual grew that nice patch of mold.

What Does This Mean for Buildings?

What does this mean for buildings? Controlling moisture is one factor in curbing mold growth but controlling surface temperatures is another. The house’s air barrier needs to be wholly intact to keep interior air from contacting cold sheathing material. Properly installed insulation will warm potential condensation surfaces, keeping them over the dew point. Buildings sheathed with foam board insulation warm the interior walls, ameliorating condensation-based mold issues.

Mold creation has a few different factors. One is temperature. So control your surface temperatures and control the mold.