Abstract: A one-year monitoring study was conducted in a pilot house with extremely high radon levels to investigate the ability and efficiency of radon mitigation by soil depressurisation (SD) both active and passive. The study included monitoring of radon concentration, pressure field extension (PFE) under the slab and some atmospheric parameters for different testing phases. Periods in which the house remained closed to foster radon accumulation were alternated with phases of active and passive soil depressurisation under different conditions. The behaviour of the radon concentration in the pilot house was analysed along with the influence of atmospheric variables, significant correlations were found for the radon concentration with atmospheric pressure, outdoor temperature and wind. From the PFE analysis it was proven that the pressure drop with distance from the suction point of the SD system is proportional to the depressurisation generated. A behaviour law was found for the permeability characterisation of the house based on the active SD performance and also, the relationship between wind velocity and extraction airflow during passive SD operation by means of a rotating cowl was obtained. Radon reductions in excess of 85% were achieved for the different testing phases in all cases. Finally, from the results it was postulated that a fan power of 20 W is sufficient to ensure radon reductions over 85% for dwellings with similar aggregate layer and soil permeability.

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