Pre-fabricated waterproofing membrane for specific use as a total barrier to the passage of Radon gas. The waterproofing mass is made of distilled bitumen and elasto-plastomeric polymers (APP), reinforced with a rot proof fibre glass reinforcement and aluminium film which allows to obtain a barrier to the transmission of Radon gas. Due to the characteristics, the membranes of the SKILLFLEX RN range are used with success in the waterproofing of both civil and industrial works where an absolute barrier to the transmission of Radon gas is required.