A soil gas measurement method developed earlier  was applied to boreholes drilled to belowfoundation depth. Radon concentration and permeability were measured at 50 cm intervals. Inradon prone areas permeability showed to increase with depth over several orders of magnitude,indicating a low permeability top layer with a thickness of 0.5 m and more. A radon availabilityindex (RAI) was empirically defined and the maximum RAI of each boring proved to be a reliableindicator for radon problems in nearby houses. The permeability of the top layer also proved to bean important factor for a better understanding of soil gas transport and the influence of rain.Implications for radon mitigation are derived.