Testing and design of radon resisting membranes based on the experience from the Czech Republic

Testing of barrier properties of insulating materials against radon is usually based on the measurement of the radon diffusion coefficient. Presented report summarizes results of radon diffusion coefficients measurements in more than 120 insulating materials obtained throughout Europe. All measurements were performed by the Czech Technical University, Faculty of Civil Engineering in cooperation with the Radiation Protection Institute. We have found out that great differences exist in diffusion properties, because the diffusion coefficients vary within eight orders from 10-15 m2/s to 10-8 m2/s. For each material category of different chemical composition statistical evaluation of results is presented. Possibilities of usage of the radon diffusion coefficient for the design of radon resisting membranes are discussed. Based on the experience from the Czech Republic the paper is trying to show that controlling applicability of membranes by setting of the upper limit for the radon diffusion coefficient is not a convenient approach. The main reason is that it is almost impossible to choose correctly one limit value for the whole Europe. The second reason is that a great number of common waterproofing materials have their radon diffusion coefficients above the probable limit value. As a consequence of this the protection against radon will be solved preferably by materials with Al foils, which is from the technical point of view meaningless, because membranes with Al foils have very low elongation and therefore they can very easily loose their barrier properties by destroying of the Al foil. The paper will show that it seems to be reasonable to replace strict limits by the real design of the insulation in dependence on particular building and soil characteristics. The design is proposed to be based on the calculation of the insulation thickness according to the formula: d ≥ l.arc sinh ((α1.l.λ.CS.(Af + Aw))/Cdif.n.V)) (in m), where CS is the third quartile of radon concentration in the soil gas on the building site (Bq/m3), λ is the radon decay constant (0,00756 h-1), d is the thickness of the radon-proof insulation (m), l is the radon diffusion length in the insulation l = (D/λ)1/2 (m), D is the radon diffusion coefficient in the insulation (m2/h), α1 is the safety factor (-), V is the interior air volume (m3), n is the air exchange rate (h-1), Af and Aw are the floor and the basement walls areas in direct contact with the soil (m2) and Cdif is a fraction of indoor concentration caused by diffusion, i.e. Cdif is estimated to be 10% of the highest permissible radon concentration indoors (in the Czech Republic Cdif is equalled to 20 Bq/m3 for new buildings and 40 Bq/m3 for existing buildings). Based on this approach we can predict interval of optimal values of the radon diffusion coefficient. If the insulation is supposed to protect a typical family house, the measured value of the radon diffusion coefficient in the insulation should be within the range 5.10-12 m2/s and 3.10-11 m2/s, because for this interval the calculated thickness will correspond to the production thickness of the insulating material. This indicates that the proposed method of the design helps to ensure higher effectiveness of the radon-proof insulation. Results presented in the paper were thoroughly verified in practice since the above-mentioned method of the radon-proof insulation design has been used in the Czech Republic since 1996. (author)