Two houses with radon-resistant monolithic-slab foundations and a crawlspace house were built on a reclaimed phosphate mining area with soil radium concentrations of 5 to 12 pCi/g near Bartow. The houses also included a number of additional features suggested as methods for increasing the radon-resistance of foundations. These were a site membrane, an improved subslab vapor-barrier membrane, and passive subslab ventilation.
Construction of the foundations was monitored to identify field problems, and the costs of the individual features were identified. A six month measurement program found that the average radon concentrations in all three houses were less than 2 pCi/l. Conventional housing on reclaimed land of that radium content would have been expected to have average radon concentrations greater than 10 pCi/l.
As this program demonstrated that radon-resistant foundations could be produced, it is expected that HUD will resume FHA insured loan approval for housing on reclaimed phosphate lands, conditional on the use of radon-resistant foundations. Radon-resistant slab-on-grade foundations can be produced cost effectively by the use of monolithic concrete foundations with sealed plumbing openings in the slab and subslab barrier membrane, and improved concrete practice. The total cost of the radon-resistant monolithic foundation is estimated to be less than that of a conventional concrete block foundation.
The ventilated crawlspace effectively isolated a house from the ground, but at a cost of several thousand dollars higher than that of a house on a standard foundation. The additional radon-resistant features did not appear to have any value in this study. The site membrane was found difficult to use, expensive, and not very effective. The improved vapor-barrier membrane was found to be as easily damaged as the standard polyethylene sheet. Radon levels in the houses were too low to detect any effect of the passive subslab ventilation systems.
A.G. Scott and W.O. Findlay, American Atcon, Inc. September 1987.