Subslab Preparation

Figure 31.12 illustrates that a low-pressure area being developed beneath the slab will draw the radon out of the soil, up the pipe, and exhaust the gas outdoors. If the subslab material consists of tightly packed soil or contains large rocks, the pressure field may not extend to all areas of the soil surrounding the foundation, and allow radon to enter the home where the pressure field does not exist. One way of ensuring the proper extension of the pressure field is to install media beneath the slab prior to the pour that will allow the easy movement of the air, thus helping to extend the pressure field.

In areas where it is available, crushed gravel is an inexpensive material to use. Subslab gravel 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 being at least 3/4 in. in diameter. This stone should have a free void space above 40%. One standard specification of this type of gravel is D.O.T.No.2 gravel. A minimum of 4 in. of aggregate should be placed under the entire slab. Care must be taken to avoid introducing fine dirt particles during and after placement of the aggregate.

In areas where gravel is not readily available, drainage mats designed for soil stabilization may be used. The use of these drainage mats may not be cost-effective in areas where gravel is available, but where gravel must be shipped in from long distances, drainage mats can be cost-effective.

Some builders prefer laying perforated PVC piping in the gravel before the slab is poured and connecting the perforated pipe to the exhaust pipe of the system. The use of perforated pipe may not be necessary in active systems but probably will assist a passive system. Membranes beneath the slab help us to keep a continuous radon barrier in the event of slab cracking.

The use of footing drains for water control can affect the distribution of the pressure field. Interior footing drains sometimes terminate in a sump hole. If this is the case and the sump hole is not sealed airtight, the possibility exists for air to be drawn into the sump by the subslab system and weaken the pressure field. Make sure that all sumps are sealed air tight. Sometimes interior footing drains extend out beneath the footing and run to daylight, as shown in the section on mechanical barriers. If this is the case, provision must be made to make the ends of the drain airtight while still allowing water to drain. Reverse-flow valves are ideal for this application.

To summarize, any opening or connection that allows the depressurization system to draw air from anywhere but beneath the slab is detrimental to its effectiveness and must be avoided.

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