How does radon actually get into a building?

Radon is a naturally occurring radioactive gas formed when uranium in soil, rock and groundwater decays. It is invisible, odourless and tasteless, so it can be present without anyone noticing, and is identified only through specialist testing. Outdoors it disperses harmlessly; indoors it can accumulate. Long-term exposure is recognised as the UK's second-largest cause of lung cancer after smoking.

It enters from the ground beneath the building, through cracks in concrete floors, gaps around service penetrations, joints between slabs and walls, and construction joints and cavities. The mechanism is the ordinary physics of a heated building: as warm air rises inside, it creates a slight negative pressure at low level, and that pressure difference draws gases out of the ground. This is the stack effect, and it is why ground gas is an air-movement problem before it is a chemistry problem.

So the entry points are exactly the air leakage paths a blower door test would find: floor-to-wall junctions, service penetrations and poorly sealed construction details. The radon level in a given building depends on the ground conditions, how the building is constructed, its airtightness and its ventilation strategy — which is why two houses on the same street can return completely different readings. We have written that up in Why Two Houses on the Same Street Can Have Completely Different Radon Readings, and the fundamentals in What Is Radon?

Does this site need ground gas protection?

For radon, the starting point is the UK radon map published at ukradon.org, checked by postcode. In designated radon-affected areas, building regulations require protective measures in new buildings and extensions: basic protection, meaning a radon barrier across the footprint, and in higher-risk areas full protection, meaning the barrier plus a ventilated sump or void beneath the floor. Existing buildings can be tested inexpensively with a passive detector first.

Radon is not the only ground gas to plan for. On brownfield and former landfill sites, developments frequently have to contend with methane, the flammable landfill gas generated by decomposing waste, and carbon dioxide often present alongside it. Where a site assessment flags those risks, a gas-resistant barrier becomes part of the ground-floor specification — specified off the back of a site investigation and a gas risk assessment, not a product datasheet.

Modern construction sharpens the question rather than softening it. Building more airtight reduces uncontrolled air movement, which is the point — but it also makes it more important to manage deliberately how air and gases enter and leave. A building that used to leak its way out of a ground gas problem no longer does.

What do the standards a gas consultant will name actually cover?

Ground gas protection has its own literature, and a specifier will hear the same references repeatedly. What each one is, in plain terms:

Two points worth being blunt about. First, every one of those documents places heavy emphasis on verification — the installed barrier being inspected and signed off, not merely specified. That is a strong argument for a membrane whose continuity can be seen across the whole surface rather than inferred from a set of laps. Second, the descriptions above state what each standard is, not that any product complies with it: design compliance for a specific building is a matter for the project's gas protection consultant.

Why do sheet gas barriers fail where liquid ones do not?

Traditional radon protection is a polyethylene sheet, and its weakness is that of any sheet membrane: every joint, lap, tear and pipe penetration is a potential leak path. A sheet barrier is only as good as its weakest seam, and those seams are formed by hand, on a substrate that may be dusty, damp or cold, under time pressure, then covered by insulation and screed before anyone can check them.

The places that fail are predictable: pipe and cable penetrations through the floor, the perimeter where slab meets wall, cavity trays and closers, column bases and pile caps, and any junction where the barrier changes plane. Each needs a top hat, a collar, a preformed corner or a taped detail — each an operation whose success depends on a specific person on a specific day, and gas exploits whichever one was rushed.

A liquid membrane removes the category. Passive Purple is applied wet, by spray or brush, across floors, walls and junctions, curing into a single continuous film with no laps or joints. It bonds to brick, block, concrete, timber and steel, follows complex details, and self-seals around service penetrations — precisely where sheet systems fail. For a gas barrier, continuity is everything, and a liquid coating delivers it by design rather than by workmanship. Detail joints and larger penetrations with IM Fleece Tape and airtight sealant to keep the envelope continuous through the awkward parts.

It is also the only realistic option on many retrofit jobs. Fitting a sheet gas barrier under an existing floor, around existing services and against existing walls is somewhere between miserable and impossible. A coating applied to the surfaces already there is not. See Radon Retrofit: The Ultimate Extra Line of Defence.

What independent test data sits behind the claim?

The key measure for a radon barrier is its radon diffusion coefficient (D) — how readily radon passes through the material. The lower the number, the better the barrier.

Passive Purple was tested to ISO/TS 11665-13 (method C) at the Czech Technical University in Prague, returning D = (3.3 ± 0.3) × 10⁻¹² m²/s, report 124044/2019. A benchmark commonly cited for radon-proof membranes sits around 10⁻¹¹ m²/s, so this result is roughly an order of magnitude below it. We present that as context rather than a formal pass or fail: the specifier should confirm the requirement for their particular project and design.

For methane, Passive Purple was tested for gas permeation to ISO 15105 at Mecadi GmbH in Germany, returning 62, 72 and 75 cm³(STP)·mm·m⁻²·day⁻¹·atm⁻¹ across three samples. Those are independent figures a gas protection consultant can work from, rather than a claim that a product "is a methane barrier".

Airtight White is the same formulation as Passive Purple, so the ground-gas performance comes with whichever finish suits the build-up. Waterproof Blue liquid DPM separately carries radon barrier certification, which is what makes it useful where the ground floor build-up has to serve as damp proof membrane and gas barrier at once.

Because the same coating is also the airtight and vapour control layer, the ground-gas specification stops being a separate layer with its own interfaces. Passive Purple is certified by the Passivhaus Institut in Class A (phA) at an air permeability of 0.02 (±0.002) m³/(h·m²) at 50 Pa to EN 12114, tested as a system including its connections — one fifth of the phA limit of ≤0.10. Ghent confirmed 0.019; in the US the air permeance is 0.02 L/s·m² at 75 Pa under ICC-ES ESR-4715. Emissions are classified M1. An ACH figure is a property of a finished building, never of a product: no membrane has an ACH.

The full write-up, with the reports referenced, is in Passive Purple as a Radon and Methane Barrier, with the wider picture in Radon Barriers Explained. We are members of the Radon Council — the announcement is here.

Which products go on a ground gas job?

View radon barrier membranes

How much material should we allow?

Full figures are on the application and coverage rates page. Ground floor slabs and blockwork sit at the top of the range; allow for that in the take-off.

Has it been proved on real buildings?

The gas performance is evidenced by the laboratory reports above; the continuity of the installed membrane is evidenced by whole-building air tests, because a barrier that seals against air at those levels has no meaningful gaps in it. Our best result on a school is 0.078 ACH at Wester Hailes High School, Edinburgh — a whole-building pre-test after our works on McLaughlin & Harvey's £44m Passivhaus school, measured by EDP Building Sciences to ATTMA TSL4 in November 2025 against a 0.6 design target.

Reuben College is the most directly relevant: a below-ground environment sealed with Airtight White, the same formulation carrying the radon and methane test data. On projects we install, we guarantee 0.6 ACH or lower — for scale, the limiting air permeability for a new dwelling under Approved Document L (2021 edition incorporating the 2023 amendments) is 8.0 m³/(h·m²) at 50 Pa, with the notional dwelling at 5.0.

Can we see it on site before we specify it?

Verification is central to every ground gas guidance document, and the same instinct applies before specification. The 360° tours let a client-side team walk completed applications and look at floor perimeters, service penetrations and junctions as they were left rather than as drawn; the video channel shows the spray and brush-detailing sequence in real site conditions.

360° site tours Application videos Technical details and drawings

Frequently asked questions