RADON MITIGATION IN BLOCKS OF FLATS

The average indoor radon concentration in Finnish flats is 80 Bq/m3. Typically walls have been made using concrete elements. Building materials are the dominant source of indoor radon. However, in the flats of the lowest floor, with a floor slab in direct ground contact, the main source of radon is often the inflow of radon bearing soil air. The number of these ground contact flats is less than 10 % of the total number of flats. The average indoor radon concentration of these ground contact flats is 150 Bq/m3. Consequently in more than 10 % of these dwellings the concentration limit of 400 Bq/m3 is exceeded. The data presented is based on a nation-wide radon survey in 900 flats. Highest concentrations observed in flats have exceeded 10.000 Bq/m3. Best mitigation results were achieved using radon wells and sub-slab-depressurisation, the typical radon concentration reductions being 50-80 %. High depressures in flats decrease the efficiency of both radon wells and sub-slab-suction installations. In comparison, the radon concentration reductions for radon wells, obtained in low rise residential houses where depressures are lower, were typically remarkably higher, 80-95 %. The exhaust ventilation, in combination with the air tightness of the concrete elements and windows, causes remarkable underpressures over the floor slab, increasing the inflow of radon-bearing air fro