The deposition-based direct indoor 222Rn and 220Rn progeny measurement techniques are mostly affected by the indoor environmental conditions, such as the ventilation, concentration of condensation nuclei, and reactions with the structure and its furnishings. In this study, a theoretical model of a direct 222Rn and 220Rn progeny monitor based on allyl diglycol carbonate (ADC or CR-39) was established to analyse the factors that influence the detection process by using the parameter sensitivity analysis. The aerosol parameters contributed the highest to the variance, followed by the aerodynamic parameters. With respect to the result of the Spearman’s correlation analysis, the aerosol-related and the room-related parameters are positive, whereas the aerodynamic parameters – which affect the turbulence of indoor deposition – are negative. It means that both the attachment process and the deposition process of 222Rn and 220Rn progenies are important to the performance of the progeny monitor.