Radon Protection Calculator — Membrane, Sump & Ventilation

Check the UK Radon Map at ukradon.org or request a report from the British Geological Survey. If 3-10% of homes in the area exceed the action level (200 Bq/m³), basic protection (membrane only) is required. If more than 10% exceed it, full protection (membrane + sump) is needed. Building Control will specify the requirement based on the postcode.

Basic protection uses a gas-resistant membrane across the ground floor with sealed joints and upstands — sufficient for lower risk areas. Full protection adds a radon sump (a void beneath the floor slab connected to an external vent pipe) which can be activated with an electric fan if radon levels remain high after occupation. Full protection is required in higher-risk areas.

The membrane must comply with BS 8485 and BRE Report 211. It should be a minimum 1200 gauge (300 micron) polyethylene or purpose-made radon barrier. Visqueen Radon Membrane and Proofex Engage are popular UK choices. The membrane must be continuous, with all joints lapped by at least 150mm and sealed with proprietary tape. Penetrations must be sealed with top-hat details.

A radon sump is a void formed in the permeable fill beneath the ground floor slab — a proprietary unit (e.g. Visqueen Radon Sump, roughly 700 × 700 × 250mm) or a hand-built gravel-filled void. It connects to a 110mm pipe that rises through the building and exits above the roof. In passive mode the stack effect draws radon from beneath the slab and vents it safely above roof level; one sump typically protects about 150–250 m² depending on fill permeability. If levels are still high after testing, an in-line fan is added to the pipe to actively extract the radon.

Test radon levels 6-12 months after occupation, once the building is sealed and heated. Use a pair of validated passive detectors (one in the living area, one in a bedroom) for the full 3-month measurement period — UKHSA's UKradon Home Measurement Pack costs £52.80 inc VAT and includes both detectors and a pre-paid return envelope. The UK action level is 200 Bq/m³ (target level 100 Bq/m³). If results exceed the action level, fit or activate the in-line fan on the sump and re-test.