The effect of indoor gamma dose rate, permeability of soil, substructure, and ventilation habits of inhabitants were studied using data of 84 low rise residential houses collected in an area of enhanced indoor radon concentration. The radon concentrations varied from 30 to more than 5000 Bqm-3. Cross-tabulation, comparisons of means and multiplicative models were used to test the significance of the effects. In this study a quite high percentage of explained variation R2 - 68% - was found. It was found that the most important factors were the substructure and the permeability of the soil.