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The structural design in terms of radon of the ground-supported base floor solutions currently used in Finland are largely based on the extensive measurements conducted in the 1980s and the 1990s. In addition to ventilating/underpressurising the base of the building, the instructions in Finland and other European countries i.e. Denmark [1] focus on the adequate sealing of the base floor structure in order to prevent the air containing radon from the soil from entering the rooms at harmful levels with air flows. According to the Finnish radon guidelines, the transmission of radon by means of diffusion is prevented by using a sufficiently thick concrete slab. The structural solutions used in base floors have, however, developed significantly over the decades. The thickness of thermal insulation of a ground-supported base floor slab has been increased significantly especially due to reasons related to the conservation of energy. In the 1980s, the polystyrene layer in the base floor was 50...100 mm, whereas it currently is 200 mm. This change reduces the ground temperature under the thermal insulation, thereby also reducing the moisture diffusion flow from the ground to the concrete slab of the base floor. As a result, the current concrete slabs used in the base floor will be significantly drier than those of the 1980s after the construction humidity has dried. Underfloor heating, which is nowadays used in Finland very commonly, also impacts the moisture of the concrete slab by increasing the concrete slab’s temperature and drying it further. The measurements now conducted on concrete have shown that the radon diffusion permeability of dry concrete (Rh approximately 50%) is so high that stopping the radon diffusion flow solely by means of an 80-mm concrete slab may not be an adequate solution in some cases.

Changes to structural solutions and their effect on radon functionality on the structures base on the ground