RRNC 2020 Companion Guidance Section C: Understanding ASD CG 4 Active Soil Depressurization System ( ASD ): ASD is a family of radon mitigation systems involving fan-powered soil depressurization , including but not limited to sub-slab depressurization and sub-membrane depressurization . ASD is overwhelmingly effective for reducing indoor radon coftenoncentrations. The method stops radon from entering a building and exhausts the radon gas and other soil gases , including moisture, outside the home. An ASD System typically consists of a soil gas collection plenum under each foundation floor to collect radon and other soil gases . Piping connects between the soil gas collector to a specially designed radon fan that exhausts the soil gas outside the home. The radon fan runs continuously with a pressure monitor/alarm to serve as a means to notify the occupant of fan failure or the need for other system service. Systems can incorporate alternative materials, designs or methods not specifically prescribed herein provided that the design is performed by a certified / licensed mitigator and approved by the appropriate authority.