Soil depressurization

To facilitate the use of soil depressurization, it is suggested that a permeable layer of material be placed beneath the slab, all major foundation penetrations be sealed, and a passive stack be run from... 

In theory, the application of radon barriers should be adequate to avoid elevated radon levels in houses. In practice, however, a backup radon mitigation system has been found essential for maintaining indoor radon concentrations below 4 pCi/L in most homes studied. In the recent radon-resistant residential construction projects conducted by U.S. EPA and/or private builders, several of the homes designed to be radon resistant have contained radon concentrations above 4 pCi/L. In each of those houses, a backup system consisting of an active (fan-assisted), or passive (wind-and-stack-effect-assisted), SSD system was installed at the time of construction. When mechanical barriers failed to adequately control radon, the soil depressurization methods were made operational. 

A thorough discussion of slabs is included in the section on foundation materials as mechanical barriers and should be referred to. However, when installing a soil depressurization system, it is more... 

Active soil depressurization (ASD) is the most common technique used to mitigate high radon levels in homes, and typically lowers concentrations by 50 to 99%. ASD relies on a simple, controlled alteration of the air-soil pressure differential. A suction point (open-ended pipe) is installed underneath the slab or floor of the house in a small excavated area where radon can collect. This pipe is connected by additional pipe to a continuously operating exhaust fan with a rooftop outlet. The vacuum effect of the fan pulls the radon-containing air from the excavated area into the pipe and ejects it above the roof, bypassing the interior of the house (and the lungs of its inhabitants). 

The white pipe in these photos is part of an Active Soil Depressurization system to mtt ate radon in a New Jersey home. The U-tube manometer liquid level differential in the photo indicates that the exhaust pump is functioning properly. 

When the blower door was turned off the soil probe pressures returned to zero, indicating that the observed pressure field in the soil was directly related to basement depressurization. 

It is believed that the basic mechanism that brings soil gas into the house is the pressure difference between the indoor and the outdoor environments. This pressure-driven flow (movement from a high- to a low-pressure area) is enhanced in the winter by a stack effect , similar to that encountered in the chimney. This chimney effect is created by the continuing rising of heated air as shown in Figure 2. In addition to the stack effect, contributions to the house depressurization arise from wind effects, the use of appliances that consume indoor air, etc. 

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