Movement and Sources of Basement Ventilation Air and Moisture During ASD Radon Control Additional Analysis

The original study in three Pennsylvania houses found that operation of active soil depressurization (ASD) radon control systems caused modest reductions in basement moisture levels. Additional, supplemental analysis has been conducted on the relationship between basement air and outdoor air moisture, and on the movement of airborne sources of basement air moisture under the influence of ASD systems. Results indicate that moisture levels in the air of the basement, first floor, soil, and outdoors are all strongly intercorrelated (correlation coefficients ranging from 0.6 to 0.9), with basement and outdoor air having correlation coefficients of approximately 0.77 and lagged responses for basement air of six to 12 hours, for all houses. Simple linear regression models found that moisture from the outdoor air directly, and indirectly from the other zones, explained approximately 70% of the overall variation in basement air moisture levels. Examining the movement of airborne moisture sources and basement air moisture revealed that all houses experienced large variations in interzonal air flows and ventilation, and that ASD operation increased the flow of outdoor and upstairs air into the basement. ASD operation virtually eliminated the minor, pre-existing flow of air and moisture from the soil, which may be a more important source in houses with larger soil gas entry. Lower moisture levels in outdoor air in winter and the upstairs air in summer (due to dehumidification by the HAC systems) have the potential to help dry the basement or offset moisture entry from other sources. The ASD systems extracted and exhausted large quantities of moisture from the house and soil around the house, implying that the systems may cause long-term drying of the basements. For one house, direct block wall depressurization (BWD) more effectively reduced wall moisture than sub-slab ASD. Results again hint that ASD/BWD may have a much larger, beneficial impact on moisture levels and conditions for microbial growth in and around the microclimate of moisture-sensitive, finish materials applied to basement walls and floors. ASD for effective moisture control may require higher air flow and PFE than is necessary for effective radon control. Although additional ventilation from ASD operation may help IAQ, its lack of control and predictability can cause over-and under-ventilation, and may be detrimental in some cases.