was completed but before occupancy—usually within a period of a week or two. As a result, house ventilation rates might not be similar to those during occupancy, nor will the short-term measurements of radon concentration be representative of a longer-term average indoor radon concentration.

A more generic evaluation of the effects of the different elements of the passive mitigation system proposed by the FRRP was conducted through the use of a radon-transport model with some "calibration" against data obtained in several house-evaluation projects (Nielson and others 1994). The model could not, of course, simulate failures, only the presence or absence of specified resistive features, openings through the floor, and so on. However, the results do provide insight into the relative importance of some features as applied to typical Florida residential construction. Most important was use of a vapor barrier below the floor slab, including particular attention to the details of treatment at all slab penetrations and at the slab edge, avoidance of floating slab construction, limiting concrete slump, and sealing all slab penetrations, openings, and large cracks. Interestingly, in this analysis, the presence of a passive stack to depressurize the subslab region was rated less effective than the other features of the radon-resistance system (Nielson and others 1994; Rogers and Nielson 1994). That is due in part to the assumed effectiveness of the features thought to block radon entry by reducing or eliminating air pathways between the subslab region and the house interior. In addition, the differential pressures generated across the slab by passive stacks in the houses are limited by the relatively small driving forces established by the passive stack. These occur for two main reasons: there is a narrow range of temperature differences between indoors and outdoors for most of the year, and many of the buildings are single-story (with no basement). Both limit the influence of the thermal-stack effect.

Data on the effectiveness of radon-resistance systems in other parts of the country are very sparse, and decidedly mixed. In several studies, totaling about 80 houses, measurements were made in radon-resistant houses with the passive stack closed and then after the stack was open (uncapped). Such studies appear to be based on the premise that radon-entry rates when the passive stack is closed would be similar to those observed in houses not built with radon-resistance features. However, because the passive stack is only one element in the radon-resistance system, negating its effect with a cap should not substantially affect the behavior of other parts of the radon-resistance system. More importantly, it is not clear how the radon-entry potential is affected by the use of high-permeability materials.

In one study, 46 homes were investigated in eight states (Dewey and others 1994; NAHB 1994); 41 in counties designated by EPA as having high radon potential. However, soil-gas concentrations were measured at 38 homesites (33 of them in counties designated as having high radon potential), and only 16 had soil-gas concentrations above 37,000 Bq m-3 (in one case the measurement was made at an adjacent site). Several researchers have suggested that soil-gas radon

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