The effectiveness of various insulating materials for limiting radon entry into houses has beeninvestigated experimentally in 90 existing houses and in laboratory conditions. Each material hasbeen evaluated according to several aspects - placeability, durability, tear resistance and diffusionproperties. The results of the radon diffusion coefficients measurement in more than 80 insulatingmaterials are summarized. We have found out that great differences exist in diffusion properties,because the diffusion coefficient varries within four orders from 10-13 m2/s to 10-10 m2/s. Amethodological approach is proposed in order to identify the minimal thickness of radon-proofmembranes in dependence on building and soil characteristics. General guidelines for selection ofradon-proof insulation and the principles of application are presented.