High humidity within homes creates numerous problems. For residents of those homes, it can contribute to allergies and other respiratory issues, as well as compromise the perceived indoor air quality (IAQ). Moreover, it often increases the energy required for conditioning the home; fosters the growth of dust mites, molds and bugs; and in some cases leads to the damage of building components.
Despite the many home-related humidity issues, there has been little to no measured data available on actual indoor humidity levels in U.S. households. That is why the U.S. Department of Housing and Urban Development (HUD) funded Steven Winter Associates, Inc. (SWA) to collect data on interior humidity levels in homes.
"The study was aimed at aiding efforts already underway by the ASHRAE Standard Committee 160P on 'Design Criteria for Preventing Moisture Damage in Buildings' and others to develop moisture modeling tools and related technical standards," explained Lois Arena, a senior engineer with SWA, who recently authored a report for HUD, Data Monitoring of Indoor Air Quality (IAQ): Part II. "To support those standards initiatives, it was critical to have a much clearer understanding of where high levels of indoor humidity occur, as well as the contributing factors."
Under the HUD-funded project, SWA collected a full year of indoor temperature and humidity data from 60 homes within three different regions of the United States: the warm, humid Southeast, the cold Northeast and the Pacific Northwest. Additionally, SWA collected house and household characteristic data including occupancy levels, insulation levels, equipment efficiencies, envelope leakage, and duct leakage.
To monitor temperature and humidity levels, SWA engineers selected HOBO U12 data loggers from Onset. The battery-powered devices were used to record humidity levels around-the clock—even during power outages. SWA then used the accompanying HOBOware® Pro software to convert the data into time-stamped graphs that could be displayed on the company's personal computers.
"When we looked at monitoring options, we were impressed by the small, battery-powered HOBO data loggers, " Ms. Arena recalled. "The ease of installation, accuracy, and cost-effectiveness of these devices clearly outweighed the costs associated with the travel and man hours necessary to conduct an extra site visit."
The year-long analysis by SWA revealed several insights. While some confirmed some common industry assumptions, there also were a few surprises. For example, SWA's analysis of the data showed that homes in the marine climate consistently saw indoor relative humidity levels above 50%, which was higher than the levels detected in the other regions on average. However, the cold region had the highest occurrences of moisture problems.
Additionally, analysis showed some commonality among all three regions. For instance, the most common areas of visible mold or moisture damage were on or around the windows and in the bathrooms. However, SWA found clear regional differences as well. Notably:
- In the hot, humid Southeast, moisture problems seem to occur during the late fall/early winter in the newer, more efficient homes studied in that region. One thought is that because these homes are more comfortable and require less space conditioning during those months, this may be allowing the interior relative humidity levels to rise.
- In the cold Northeast, many of the homes had moisture leakage in the basement, and all had a portion of the basement that was unfinished. Homes with moisture problems in this set also had a much higher average air change rate compared to those without.
- In the marine environment of the Pacific Northwest, all the homes with moisture problems had either a full or partial crawl space without a well-sealed vapor barrier over the dirt.
"One of the important insights from our year of monitoring and analysis is that, in general, there are no statistically significant correlations across climate zones. Instead, each region has problems specific to that location, which need to be addressed," Ms. Arena observes. "Through this real-world data, we now understand that, rather than setting blanket recommendations for moisture control, it will be important to establish standards and guidelines that are applicable to the specific challenges of each zone."