In situ observation and theoretical study of temporal variations in radon exhalation rates from the gypsum board of a cavity wall: a comparison with a solid concrete wall

Building materials are one of the most important indoor radon sources, prompting research into their radon exhalation rate (J_Rn). Most previous studies have relied on laboratory tests using pieces or blocks of materials such as concrete and brick. However, creating samples that mimic real cavity walls, defined as walls with a cavity between the two panels of the associated building material, has been challenging owing to structural complexities. In this study, we conducted the first long-term in situ measurement of J_Rn from the interior board of a common Japanese cavity wall comprising a thin gypsum board (interior wall), air/insulation, and concrete (exterior wall). Results indicated clear diurnal and seasonal variations in the observed J_Rn data. In general, the highest and lowest J_Rn values were observed in summer and winter, respectively, exhibiting the same pattern as that observed for thick solid concrete walls analyzed in our previous in situ study. Interestingly, J_Rn values increased during the day in summer and at night in winter, contrasting with the stable J_Rn value observed for the thick solid concrete walls over several days. Theoretical calculations indicated that in this case study, J_Rn was predominantly driven by diffusion, not by advection. These results could be explained by considering the wall thickness, diffusivity, and boundary conditions of radon activity concentrations. Our findings can help the selection and refinement of input parameters for radon sources when modeling the spatiotemporal dynamics of indoor radon in buildings. More in situ tests in diverse buildings are essential in the future.