Passive Purple as a Radon and Methane Barrier: One Continuous Coating
One coating, airtight layer and ground-gas barrier 💨
On many UK and Irish sites, the air you're trying to keep out isn't just draughts. It's radon and other ground gases rising from the soil beneath the building. The good news: Passive Purple, our liquid-applied airtight and vapour-control membrane, has been independently tested as a barrier to both radon and methane — so a single continuous coating can do two jobs at once.
Why ground gas matters in UK and Irish construction
Radon is a naturally occurring radioactive gas that seeps up from the ground and can accumulate inside buildings. Large parts of the UK and Ireland fall within radon-affected areas, where building regulations expect protective measures at the ground floor. It's a genuine health concern — prolonged exposure is a recognised cause of lung cancer.
Radon isn't the only ground gas to plan for. On brownfield and former landfill sites, developments frequently have to contend with:
- Methane — flammable landfill gas generated by decomposing waste.
- Carbon dioxide (CO₂) — often present alongside methane in ground conditions.
Where a site assessment flags these risks, a reliable gas-resistant barrier becomes part of the ground-floor specification — not an optional extra.
Radon: tested to a very low diffusion coefficient
The key measure for a radon barrier is its radon diffusion coefficient (D) — how readily radon can pass 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, giving a radon diffusion coefficient of:
- D = (3.3 ± 0.3) × 10⁻¹² m²/s (report 124044/2019)
For context, a benchmark commonly cited for radon-proof membranes sits around 10⁻¹¹ m²/s — and Passive Purple's result comfortably beats it, by roughly an order of magnitude. (We present this as context rather than a formal pass/fail; the specifier should always confirm the requirement for their particular project and design.)
Methane: tested to ISO 15105
For methane, Passive Purple was tested for gas permeation to ISO 15105 at Mecadi GmbH in Germany. Across three samples, methane permeation was measured at:
- 62, 72 and 75 cm³(STP)·mm·m⁻²·day⁻¹·atm⁻¹
These figures give designers and their gas-protection consultants real, independent data to work from when assessing a barrier for a methane-affected site.
The continuous-membrane advantage 🛡️
Here is where a liquid-applied membrane has a structural advantage over traditional sheet systems. Sheet barriers are only as good as their weakest joint — every lap, seam, tape and penetration detail is a potential path for gas to bypass the membrane, and on a real site those details are easy to compromise.
Passive Purple is applied wet and cures into a single continuous film with no laps or joints. It bonds directly to the substrate and self-seals around service penetrations, pipes and awkward junctions — exactly the places where sheet systems tend to fail. For a gas barrier, continuity is everything, and a liquid coating delivers it by design.
Airtight White does the same job
Airtight White — the same formulation as Passive Purple — is described on our site as also protecting against radon and methane. So whichever finish suits your build-up, the ground-gas performance comes with it.
Why this matters
Specifying separate products for airtightness and gas protection means more layers, more interfaces, and more opportunities for something to go wrong on site. With Passive Purple, one continuous coating can serve as both the airtight layer and the ground-gas barrier — simplifying the detail, reducing the number of trades, and removing the joints that gases exploit.
Working on a radon-affected or brownfield site? See the full test data & certifications for the radon and methane reports referenced here.