Radon mitigation in a difficult to mitigate school.

This paper describes radon diagnostics and mitigation in a school the U.S. Environmental Protection Agency (EPA) classified "difficult to mitigate." The school had subslab utility tunnels that served as the outside air and return air mixing chamber for the heating and ventilation system. The heating and ventilation system depressurized the tunnel, sucked radon from the soil, and distributed it to school rooms. Extensive diagnostics were conducted to test mitigation options and to provide mitigation design parameters. The final radon mitigation technique involved pressurizing the utility tunnel. The findings indicate that: active soil depressurization systems can be overpowered by heating, ventilation, and air conditioning (HV AC) operations; in some cases, increased ventilation can increase radon entry and indoor concentrations; and, if properly implemented, additional ventilation can reduce indoor radon concentrations without significant energy penalties.