concentrations of at least 37,000 Bq m-3 (which is close to the US average soil-gas radon concentration) are needed to provide an adequate test of radon resistance in houses. This concentration was used as the selection criterion for many of the houses evaluated as part of the FRRP because it was felt that lower soil-gas radon concentrations would not test the system (Fowler and others 1994).

Of the 16 homes, four had basement radon concentrations greater than 150 Bq m-3, and one of these also had radon concentrations measured on the first floor greater than 150 Bq m-3 (all these values were measured with the passive system fully operational, that is, with the passive stack open). In each case, these were single sets of measurements, conducted with the passive stack first closed, then open. In four of the 16 cases, the stack-open measurements were conducted in the summertime, when driving forces for both radon entry and passive-stack operation are minimal.

Other studies of radon-resistant construction have had similar results and limitations (Saum 1991; Brennan and others 1990; Saum and Osborne 1990). None of these studies had a non-radon-resistant baseline against which to evaluate the overall effect of the techniques. In many cases, soil-gas concentrations were not measured, so it is difficult to determine the radon-entry potential for the specific houses. Most of the studies conducted stack-open versus stack-closed evaluations.

Overall, the inclusion of the passive stack had the largest effect in houses with basements. In some cases, the short-term measurements suggest reductions as great as 90%, compared with stack-closed basement radon concentrations. On the average, reductions of about 40% are more typical. For slab-on-grade houses, the effect of the passive stack is considerably reduced. Comparing the stack-open and stack-closed measurements from the few houses for which there are data reveals no discernible effect. In part, that is due to the low soil-gas radon concentrations in one study. Thus, with one exception, all the initial stack-closed indoor radon concentrations are low. In the FRRP studies, soil-gas radon concentrations were often high (this was one criterion in the choice of study houses), but, as noted earlier, the passive stack was thought to be a less integral part of the radon-resistance system.

Three key questions remain unanswered:

  • How well do houses built to be radon-resistant perform when compared with those built without radon-resistance features?

With one exception, no side-by-side comparisons address this question. A study of 89 homes in two areas near Colorado Springs was conducted in which 54 new homes were built within two subdivisions; 35 homes built without any radon-resistance features served as "control" homes. Of the 54 homes, 12 were tested as radon-resistant (the remaining 42 used active, mechanical systems for radon control). Radon was measured in the basements with 2-d open-face charcoal canisters, all on the same 2 days in December to remove weather effects



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