Several mitigation techniques such as subslab depressurization and subslab pressurization have been used recently to reduce indoor radon concentrations to acceptable levels. Unfortunately, there is a lack of understanding of the mechanisms that influence the effectiveness of these systems and of their impact on the building and surrounding soil. As a result, no systematic approach has been established to determine which remedial procedure to implement in a given situation.
The objective of the work described here was to use a microcomputer program to make a comparative assessment of the effectiveness of subslab radon mitigation systems in an unoccupied house and to identify potential problems associated with their operation. For each of the mitigation systems, the following steady-state parameters were predicted: airflows into and out of the house (above-grade and below-grade), pressures in the house, soil airflow and pressure fields, interzone airflows, differential pressures, and radon levels throughout the soil and in the house. Comparisons of the program predictions with results obtained using hand-calculations, with predictions of other programs, and with experimental data were used to validate the program.
Program predictions for the unoccupied house showed that the lowest indoor radon levels occurred when the subslab depressurization system was used. Most of the air removed by this system came from inside the basement. Simulation results also indicated that the house was adequately ventilated when the subslab depressurization system operated. However, they showed the subslab pressurization system did not adequately ventilate the house, because most of the air exhausted from the house and supplied to the soil by this system returned to the house through the basement. These predictions were confirmed by field monitoring of the house and of ten other houses with subslab depressurization systems.
The results obtained indicate it is unlikely that cold air drawn through the soil by subslab depressurization systems could cause the soil to freeze near the footings. However, the flow of air from the basement could depressurize the basement enough to create two problems: (1) excessive inflow of cold outside air, which would waste heating energy and (2) furnace backdrafting.