Subslab depressurization (SSD). SSD is widely considered the most practical vapor intrusion mitigation strategy for existing and new structures. EPA defines SSD technology as “a system designed to achieve lower sub-slab air pressure relative to indoor air pressure by use of a fan-powered vent drawing air from beneath the slab.” Thus, even if there are holes, cracks, or other pathways between the building and the subsurface, vapors flow downward, not upward. Thus, a well-designed depressurization system prevents any toxic vapors from intruding above.
In existing structures, installing an SSD system entails cutting one or more holes in the slab, removing a small quantity of soil from beneath the slab to create a “suction pit,” and then placing vertical suction pipes into the holes. These pipes are connected to a manifold containing an exhaust fan, and vapors are in turn vented outdoors. Experience has shown that one or two suction pits are adequate to depressurize typical residential homes. A large building needs more.
Where the potential vapor intrusion is minor, yet of concern, SSD systems can also be passive; that is, they do not have to rely on a fan to create a pressure gradient beneath the slab. In passive systems, the pipes are vented to the outside, relying on air currents and the “stack effect” to draw vapors up from below the house. The stack effect works on the principle that higher pressure gases rise through the path of least resistance to the lower pressure outdoor air. Passive systems are unpredictable, as they rely on changing outdoor air pressure to provide a negative pressure. In warmer months and climates, ambient pressure at the roofline may be greater than the subsurface, and passive systems may provide little help. In most cases, they do not create the same pressure differential between the sub-surface and the indoor air as an active system; they may merely vent harmful vapors intermittingly. EPA reported in 1993 that passive subslab systems are 30 to 90 percent as efficient as active systems.