The paper discusses causes of poor sealant performance in soil-gas-resistant foundations. Sealants for radon-resistant foundation construction must seal the gap between concrete sections. The surface permeability of concrete walls and floors was measured by a specially designed permeameter, which measures the airflow induced by a pressure difference across a temporary test seal applied to the surface. The permeability of bulk concrete is about 10 to the minus 16th power sq m. Areas free of surface defects had surface permeabilities ranging from 10 to the minus 14th to the minus 16th power sq m. However, surface defects are common on concrete wall surfaces, which increase the permeability to less than 10 to the minus 12th power sq m, too high for standard seal designs to be adequate as the only method of soil gas and radon exclusion. Radon-resistant seals require either extended contact widths or mechanical removal of the surface layer and defects.