The most common way radon enters a home is when air pressure differences move soil gases containing radon through the open spaces within the soil to the building foundation. Pressure differences will make radon-laden soil gases move from an area of higher pressure to an area of lower pressure. Most buildings tend to maintain themselves at an air pressure lower than that of the surrounding soil. This situation results mainly from weather-driven parameters such as indoor/outdoor temperature differences and wind. The use of exhaust fans and combustion devices will also create a negative pressure within a home.
sub-slab depressurization system alters the pressure beneath the concrete
slab, making the sub-slab pressure less than the indoor pressure. It is
the altered air pressures that keep radon from entering the home.
OVERALL DESIGN CONSIDERATIONS FOR SUB-SLAB DEPRESSURIZATION
Many design considerations are common to both active and passive systems. Placement of the exhaust and removal of condensation are extremely important. Water collecting in an elbow or other low point of the system can effectively block a pipe, and reduce or disable the system. A pipe system should be designed in such a way that it will allow condensation to run back through the pipe to the sub-slab aggregate. This can be accomplished by ensuring that the pipe is vertical the entire distance from the basement to the exhaust outlet. A completely vertical pipe with no bends or elbows will also provide a pipe system with lower static pressure losses which enhances the effectiveness of both active and passive systems. If an elbow or low joint is incorporated in the design, a condensate pump can be used to drain the water away.
The exhaust outlet should be located above the eave line of the roof. When the exhaust point is chosen, the reentry of radon-laden soil-gas into the home through open windows and doors should be avoided. The location of the outlet close to a chimney that could backdraft and draw the exhausted soil-gas into the home should also be avoided.
A low pressure area being developed beneath the slab will draw radon out of the soil, up the pipe, and exhaust it outdoors. If the sub-slab material consists of tightly packed soil or contains coarse gravel, the pressure field may not extend to all areas of the soil surrounding the foundation and, thus, allow radon to enter the home where the pressure field does not exist. If an appropriate medium is installed beneath the slab prior to pouring of concrete then there would be a proper extension of the pressure field.
Crushed rock or gravel is a good material to use, as it provides a drainage bed for moisture and a stable, level surface for pouring the slab. The material preferred for radon reduction is crushed aggregate with a minimum of 80% of the aggregate at least 3/4 in. in diameter. Sometimes drainage mats designed for soil stabilization should be used.
To summarize, any foundation
opening or connection that allows the depressurization system to draw air
from anywhere except beneath the slab is detrimental and must be avoided.