Reducing indoor radon concentrations by passive subslab ventilation

The primary objective of our study was to establish whether passive soil ventilation systems installed under existing houses have an effect on indoor radon concentrations. Experiments were conducted in two single-family houses. The soil ventilation under each house consists of the network of flexible perforated pipes laid into the layer of coarse gravel of the minimal thickness 150 mm. Soil air from the perforated pipes is ventilated by means of the vertical exhaust pipe that runs through the heated part of the house and ends above the roof of the house. At the top of the vertical exhaust a wind turbine is mounted in order to improve the stack effect during the windy weather .In addition to the soil ventilation both houses were provided with new floors composed of concrete slab and radon proof insulation made of LDPE membrane. The efficiency of passive soil ventilation systems varies within the year in dependence on the temperature gradient and wind speed. Preliminary results indicate that temperature gradient predominates. However the maximum under-pressure at the base of the vertical exhaust pipe caused by temperature differences is not so high. During one-year observation period the maximum temperature related under-pressure was only -8 Pa. The wind effect starts to be noticeable for speeds higher than 5 m/s and more apparent becomes for speeds above 10 m/s. The maximum values of under-pressure due to wind forces were measured within the range - 20 Pa and -30 Pa for wind speeds from 20 m/s to 25 m/s. Quite significant variations of the subslab under-pressure within one day were observed. The maximum under pressure was measured at late night or early morning when the outdoor temperature was the lowest. Annual variations were also confirmed. During the winter the temperature gradient is higher than in the summer time and thus the subslab under-pressure is consistently higher in the winter. Preliminary results indicate that passive soil ventilation systems with perforated pipes laid into the subslab layer of coarse gravel and vertical exhaust pipe running through the heated part of the house and terminating above the roof can be surprisingly effective. Discovered effectiveness for two houses varied between 75 % and 89 %, which is a better result than 50 % observed for passive mini sump systems. The difference can be explained by the fact that perforated pipes ensure better pressure distribution and soil ventilation compared to simple sumps. This could lead to the conclusion,that passive soil ventilation based on perforated pipes can be convenient for houses with indoor radon concentration below approximately 1200 Bq/m3. However there is insufficient data to draw any firm conclusions. Further research in this field is needed. In new houses passive systems should be preferable to a fan driven ones because they are usually cheaper to install, run and maintain. In addition it is a simple task to add an electric fan later if the passive system does not adequately reduce indoor radon concentrations. (authors)