This paper summarizes recent research funded by the U.S. Department of Defense Environmental Security Technology Certification Program (ESTCP) intended to improve the cost-effectiveness of systems to protect building occupants from subsurface radon gas and volatile organic compound (VOC) vapors. Standard practice for sub-slab depressurization (SSD) uses a fan or blower to create a measurable vacuum below the building and has not changed in a few decades. ASTM E-2121-13 specifies a minimum target vacuum of 6 to 9 Pascals (Pa) everywhere below the building floor slab, but this can be difficult to measure compared to baseline fluctuations (a signal-to-noise challenge). A key variable that is not usually measured is the permeability of the material below the floor. If granular fill is below the floor (as specified in most building codes), high flow velocities can be generated with small pressure gradients, which can protect occupants via sub-slab ventilation (SSV). Alternatively, if the floor is well sealed, the venting system may be able to capture all of the available mass of VOCs or radon at a modest flow rate, in which case mass flux might be the most important metric. Using vacuum as the only performance metric will often result in an over-designed system that is not energy efficient because it draws an excessive amount of conditioned indoor air across the floor slab for discharge above the roofline and requires excessive electricity to power fans or blowers. Research was conducted at a 64,000 ft2 (5,950 m2) commercial building with an existing SSD system comprised of 27 suction points connected to 9 fans to demonstrate and validate new methods and criteria for system optimization and monitoring, including transient and steady-state pneumatic testing and mathematical modeling using the Hantush-Jacob model, sub-slab tracer testing, building depressurization testing, trichloroethene (TCE) mass flux monitoring, and confirmatory indoor air sampling and analysis. The results of this study demonstrate that the number of SSD extraction points can be reduced substantially and still maintain health-protective indoor air quality
