Theoretically, a gastight barrier could be placed between the soil and foundation to eliminate radon entry from the soil. Like many other building details, it is much easier to draw such a detail than to actually install it. Many materials form effective retarders to gas transport. The problem is effectively sealing cracks, joints, and penetrations. As anyone who has tried to build an airtight house can tell you, it is not as easy as it seems.
The types of mechanical barriers that have been tried or suggested for radon control fit into one of the following categories9,27:
1. Foundation materials themselves
4. Possibility of a "site" barrier.
Ongoing U.S. EPA research on radon-resistant new construction has encountered numerous difficulties in making a gastight mechanical barrier effective enough to confidently keep indoor radon levels below 4 pCi/L. The types of problems encountered included
1. Quality control on the job.
2. Incomplete communication between researchers, contractors, and subcontractors.
3. Reluctance of builders to change drainage detailing.
The first problems on the list are not specific to radon control but are encountered on nearly every construction job. In spite of quality control and communication problems and the understandable wariness builders show when asked to build something in a different way, the residential construction industry has responded to new techniques, materials, and public demands. The average house being built today is very different from a home built 20 years ago. If a product or a method can be demonstrated to reliably keep radon out without presenting significant problems with cost, scheduling, or installation, many builders would learn to use it. The major difficulty faced by mechanical barrier approaches is the thoroughness that seems to be required to ensure that no radon problem will occur.
In 1988 and 1989, U.S. EPA projects studied newly constructed houses, which incorporated mechanical barriers and provisions for active and passive SSD to determine the effectiveness of each approach. Preliminary results from these five studies found that, when there was a source of radon beneath the houses, the mechanical barriers were not adequate to ensure basement levels below 4 pCi/L. However, there is no way to judge how high the radon concentrations in these buildings would have been had the mechanical barriers not been employed. These data should not be used as evidence that the barriers used (or that mechanical barriers in general) do not reduce radon levels indoors. In fact there are good reasons to employ barriers to enhance the performance and reduce the energy penalty of soil depressurization techniques.
When trying to make a barrier to soil gas entry, the routes of concern in new construction are the same as those that have previously been identified for existing houses. Houses that are combinations of the above substructures often provide additional entry routes at the interface between the two substructures. The following subsections address the types of mechanical barriers (foundation materials, coatings, and membranes), the potential radon entry routes associated with common foundation detailing, and suggestions for details that reduce the risk of elevated indoor radon. When possible, these alternatives include barriers that can be used to block radon entry while continuing to use traditional construction methods. Depending on current local or regional building practices, some of the suggestions may require significantly different construction methods.
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