requires a high-permeability zone (such as gravel) below the floor and, in some cases, around the foundation footings so that the pressure field extends up the basement walls. Sealing major openings, such as at the joint of the basement floor and the wall usually is also necessary to ensure that there are few flow ''short-circuits" that will degrade the pressure field. Achieving these in existing buildings can sometimes be problematic because the extent of the high permeability zone might not be known, although flow and pressure can be measured to provide a coarse assessment. In addition, it can be difficult to identify all the major soil-gas flow pathways through the building shell.

In new construction, both those problems are more readily addressed. The extent and quality of a gravel bed, for example, can be specified as part of the building design and as part of the construction-inspection process. Many of the leakage paths can be eliminated through design (for example, minimizing utility penetrations of ground-contact floors or walls), materials use (for example, use of low-shrinkage concrete for floors), and construction practices (for example, adequate sealing of utility penetrations).

One of the important benefits of these methods is that passive-stack methods might become more applicable. In some cases where wind or other effects might increase the depressurization of a house, thus potentially overriding the reverse pressure gradient established by the passive stack, the use of low-power fans for mechanical stack depressurization is attractive. Such systems have been tested in a limited number of homes and show promise (Fisk and others 1995; Saum 1991).

Radon-Resistant Buildings

Most of the elements required for making a building radon-resistant have already been described. In principle, if all entry routes through which soil gas can flow are eliminated or the pressure gradient that drives air flow through such openings is reversed, advective transport will not contribute to indoor radon concentrations. If successfully implemented, this approach can be achieved without the use of mechanical systems—it will constitute a so-called passive radon-resistance system. Such an approach has two important advantages over active radon-control systems: there are no mechanical or electric components to fail (for which the building occupants must maintain an awareness), and there is no concomitant energy use. On the other hand, the operation of mechanical systems can be easily monitored, for example, with a pressure gauge. Failure of a fan would, in principle, be easily detected by a change in pressure in the radon-mitigation pipe. The potential system-failure modes in a passive system are likely to be more subtle, such as those induced by the differential settling, cracking, and aging of building components, particularly foundation walls, footings, and floors.

Radon-resistant features, including those designed to reduce or eliminate radon-transport pathways and those in some cases, designed to reverse or decrease the differential driving pressure, have been proposed or incorporated into



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