High Indoor Humidity in New Buildings – GWC Mag

When well-meaning contractors make energy-efficiency improvements on an existing house, energy bills should go down. That’s good. But sometimes, even when energy bills go down, the indoor conditions or comfort get worse. For example, tightening up a building can lower indoor air quality—especially if the contractor doesn’t understand the need for a good mechanical ventilation system. Another example: energy-efficiency improvements can reduce air conditioner run time. While this lowers energy bills, it can also raise indoor humidity levels, leading to mold.

In other words, you can’t fiddle with the leg bone without affecting the knee bone. Contractors have to adopt “the house is a system” thinking to avoid these kinds of problems.

High indoor humidity can lead to mold problems

Ideally, a home’s indoor relative humidity won’t rise above 60% during the summer. That’s the goal—but not always the reality.

Kohta Ueno, an engineer and  principal at Building Science Corporation, has been studying the causes of  high indoor humidity for many years. (Back in 2013, I reported on his research on the topic in a GBA article called “All About Dehumidifiers.”) Ueno was one of three authors (the other two were Joe Lstiburek and Armin Rudd) of a groundbreaking 2002 study of homes in Houston that had high indoor humidity. As it turns out, the problems described by Ueno in 2002 aren’t limited to Houston—they also occur in many East Coast locations with hot, humid summers. Anecdotally, these problems are reported to occur more frequently in recently built energy-efficient homes than in older homes.

This past summer, I attended the Building Science Summer Camp in Westford, Massachusetts. During a meal break at the conference, I met a mold remediation contractor from Texas who told me that his business was booming. “The buildings with high…