How Is a Radon Barrier Installed?

Radon is a naturally occurring radioactive gas that seeps up from the ground in certain geological areas. A radon barrier is a gas-resistant membrane built into the floor construction to stop it entering a building, usually combined with a well-sealed floor and, where risk is higher, a sub-floor void or sump that can be fitted with mechanical extraction if needed.

It is not a single product but a system: the membrane itself, the way it is lapped and sealed, how it meets the walls, and how every penetration through it is detailed. Any one of those elements failing undermines the rest.

Most radon membranes on the market perform adequately when installed correctly. The failures that turn up on inspection are almost always installation failures: laps that were not sealed, tears from foot traffic or reinforcement that were not repaired, or service penetrations patched with general-purpose tape instead of a proper collar.

Because the barrier is buried under a concrete slab for the life of the building, there is no opportunity to correct a defect later without breaking out flooring. That makes the pre-pour inspection stage disproportionately important compared with most other trades.

Whether radon protection is required, and to what standard, depends on the radon potential mapped for the site's location and the type of construction. Some areas require basic protection, some require full protection including a sub-floor void or sump, and some require none at all.

This is a Building Control matter, not a judgement call made on site. The radon risk assessment or the Building Control officer's requirement should specify what level of protection applies before groundworks design is finalised — retrofitting full protection after a slab is down is expensive and disruptive.

The sub-base is compacted and blinded with sand or a fine material to give a smooth surface free of sharp stone that could puncture the membrane. The barrier is then rolled out with generous laps — typically a set minimum overlap specified by the manufacturer — and those laps are sealed using the specified tape or a welded joint, not just overlapped and left.

The membrane is dressed up the inner face of the wall construction at the perimeter and linked into the damp proof course, so the DPC and the radon barrier form one continuous barrier around the building rather than two separate systems that happen to meet.

Every pipe, duct and cable passing through the floor breaks the continuity of the barrier. Each one needs a proprietary top-hat or collar detail bonded and sealed to the membrane, not a wrap of building tape that will not hold once concrete is placed over it.

Internal wall junctions, changes in floor level, and cavities where they meet the membrane all need forming deliberately. Cavities should be closed at the point the barrier crosses them, so there is no route for gas to bypass the membrane through the wall construction itself.

Where full protection is specified, a sump — a small chamber or perforated pipe arrangement — is formed beneath the slab, with a duct taken to an accessible position outside or into a service void. This allows a fan to be fitted later if radon testing after occupation shows levels are still too high, without having to break into the floor.

Not every site needs a sump. Whether one is required depends on the radon risk assessment for the site and the type of construction being used.

A radon barrier is vulnerable for as long as it is exposed. Reinforcement being dragged across it, insulation boards with sharp edges, and general foot traffic from other trades are the most common causes of damage found at inspection.

Good practice is to inspect the completed membrane before it is covered by insulation or concrete, photograph it, and repair any damage found with a proper patch rather than tape. Once concrete is poured, the opportunity to fix a defect has gone.

The radon barrier sits at a point in the build where several trades overlap: groundworks, plumbing and electrical first fix, insulation and the concreting gang. Coordinating who is on site when, and in what order, matters — a plumber running a new pipe through the slab after the barrier is signed off is a common way for the detail to be compromised.

SHS treats the membrane installation as a discrete, inspected stage within the groundworks package rather than something squeezed between other trades' work.

Radon testing after a building is occupied — typically using detectors placed for a set period — confirms whether the installed protection is actually keeping indoor radon levels within acceptable limits. This is worth doing on any property in a higher radon potential area, even where full protection was installed, simply to confirm the system is performing as intended.

If a test after occupation shows levels are still too high, and a sump and duct were formed during construction, a fan can usually be fitted relatively straightforwardly to add mechanical extraction. Where no sump exists, remediation is considerably more disruptive, which is why including a sump at build stage is worth considering even where it is not strictly mandated.

Every site is different. Figures discussed here are general indications of what drives cost, not a quotation — the only reliable way to price groundworks is a site visit and a written price against a defined scope.

The membrane and tape themselves are a modest material cost. What drives price is the level of protection required (basic versus full protection with a sump), the complexity of the slab layout and number of penetrations, and how much extra time the installation and its inspection add to the groundworks programme. A straightforward rectangular slab with basic protection is a quick, low-cost addition; a complex plan with multiple penetrations and full protection takes considerably longer to detail properly.