Do I Need Radon Protection?

Radon is a naturally occurring radioactive gas produced by the decay of uranium in rocks and soils. It disperses harmlessly outdoors but can accumulate inside buildings, where prolonged exposure at elevated levels carries a recognised health risk.

Radon potential is mapped nationally, and parts of Cumbria fall within radon affected areas. For new construction, the requirement for basic or full protective measures is determined from that mapping and, where needed, a site-specific radon report. Building control confirms what is required for your specific project.

Neighbouring properties can fall into different categories even a short distance apart, so the check is done for the specific site rather than assumed from the general area or a neighbour's experience.

Basic protection typically means a radon-resistant barrier laid across the whole floor area, carefully lapped, sealed and detailed at every junction and service penetration so there is no gap for gas to enter through.

Full protection adds a means of extraction — usually a sump formed in the sub-base with pipework taken to an accessible external position — that can be connected to a fan and activated later if testing after completion shows radon levels are still elevated despite the barrier.

Building control is the authority that confirms the radon requirement for a specific project, working from the national mapping and, where the site falls into a higher-risk category, a more detailed site assessment. This is not something a contractor decides independently — it should be established and documented as part of the building regulations process before floor construction begins.

For existing homes, the starting point is a radon test rather than construction work. If levels are found to be elevated, remedial measures such as retrofitting a sump with an extraction fan, improving underfloor ventilation, or positive pressure ventilation can be installed without the scale of work a new floor requires.

An extension can trigger a radon requirement even where the existing house has none, because it is treated as new construction with its own assessment against the mapping. Where a radon barrier is required in an extension, it also needs to be linked properly to the existing building's damp proofing at the junction, which needs careful detailing rather than a simple butt joint.

A radon barrier is laid as a continuous membrane across the full floor area, typically above the insulation and below the slab or screed, though the exact position within the floor build-up depends on the system specified. Every joint is lapped and taped or welded according to the manufacturer's method, because a gap in the barrier is a gap for gas to enter through, and the finished barrier is only as good as its weakest junction.

Service penetrations — pipes, ducts, cable entries — are the points most likely to be got wrong, since each one has to be sealed individually around the barrier rather than assumed to be covered by the general membrane run. Internal walls that rise from the slab also need the barrier carried up or otherwise detailed at that junction, not simply stopped at the wall line.

Where full protection is specified, a sump is formed within the sub-base beneath the barrier — typically a void or perforated chamber connected to pipework that runs to an accessible point outside the building. During construction this pipework is usually left capped rather than connected to a fan, because the barrier alone is often sufficient and there is no need to run an active extraction system unless testing later shows it is required.

If post-completion radon testing inside the finished building shows levels are still elevated despite the barrier, a fan can be connected to the existing pipework to actively extract gas from beneath the floor, without needing to break into the finished slab to do so. This is the practical reason full protection is specified in higher-risk areas even where basic protection might turn out to be enough — it keeps a proven remedy available without future disruption.

Radon protection has to be coordinated with the rest of the floor build-up and with any services entering through the slab, because the barrier is normally installed as one continuous operation across the full floor area rather than fitted around other trades afterwards. Service runs that penetrate the floor should be identified and positioned before the barrier goes down, so each penetration can be sealed properly as the membrane is laid, rather than cut into and patched after the fact.

Inspection before the barrier is covered is worth building into the programme, since it is straightforward to check laps, seals and penetrations while the membrane is exposed and effectively impossible once the slab is poured over it.

A radon barrier is one layer within a floor build-up that usually also has to satisfy insulation, damp proofing and structural requirements at the same time, and these should be designed together rather than treated as separate specifications applied one after another. A damp proof membrane and a radon barrier can sometimes be combined in a single product, but this needs to be confirmed against the specific product's certification rather than assumed, since not every damp proof membrane is tested or rated as a radon barrier.

Radon potential mapping is generated from geological data at a fine enough resolution that it can genuinely vary within a small area, so two plots a short distance apart, or even different corners of the same larger site, can sit in different radon categories. Relying on what protection a neighbouring property installed, rather than getting the requirement confirmed for the specific address, is a common source of confusion and occasionally of under-protection.