Radon Barrier Membrane (1600 Gauge): The Gas-Tight Sheet That Replaces Your Floor DPM

A groundworker rolls out a red membrane across the oversite, laps the joints the way he always laps a floor DPM, tucks it up the wall, and gets ready for the pour. The building control officer arrives for the pre-pour inspection, crouches at a lap, and lifts the top sheet with one finger. It is not taped. There are no collars on the two pipes coming through. He stops the pour. On a good day that costs an afternoon and a couple of rolls of tape. On a bad day nobody checks, the slab goes down over an open membrane, and the only sign anything is wrong arrives two years later as a failed radon test on a finished floor that now has to be opened up. A radon barrier laid like an ordinary DPM gives no radon protection, whatever colour it is.

This page is about the product itself: what makes a membrane a certified radon barrier, the standards on the datasheet decoded, and the small system of tape and collars that turns a sheet of polythene into a gas-tight envelope. If you are still working out whether your extension needs any of this, that decision lives on the radon protection task page: postcode check, the three tiers, the 30m² rule for small extensions, and the standby sump. This page assumes you already know you need it.

What it is, and how it differs from an ordinary DPM

A radon barrier membrane is a heavy polythene sheet, the same family of product as the damp proof membrane you would lay under any extension floor. It does two jobs at once. It stops ground moisture rising into the slab, exactly like a standard DPM, and it stops radon gas seeping up through the floor. Because it covers the whole footprint and ties into the wall DPC, it replaces the ordinary 1200 gauge DPM rather than sitting on top of it. You lay one membrane, not two.

The difference between the two products is not really the sheet. It is what you do at the joints. An ordinary DPM only has to be watertight: liquid water cannot track through a generous overlap, so lapping the joints is enough. A radon barrier has to be airtight, because radon is a gas carried up in soil air, and a gas walks straight through a loose overlap that water would never cross. So every lap is taped or welded shut, and every pipe that breaks through the floor gets a sealed collar. The industry spent years fitting "a red DPM" with no thought to airtightness and wondering why test results stayed high. Watertight is not airtight, and that single distinction is the whole product.

The standards on the datasheet, decoded

A radon barrier datasheet lists three or four standards, and they are routinely misunderstood, including by people selling the stuff. Each one governs a different thing, and knowing which is which lets you read a datasheet and tell a genuine certified barrier from a hopeful red sheet.

BR 211 is the one that matters most. It is BRE's guidance on radon protective measures, and Approved Document C of the building regulations points to it for the technical detail. The current edition is BR 211:2023, the sixth edition, which for the first time covers all four UK nations and explicitly includes extensions, conversions and refurbishments. The 2015 edition is withdrawn. A product that "conforms to BR 211:2023" is telling you it is fit for the radon job. This is the line to look for.

BS EN 13967 is the product standard for waterproofing and damp-proof sheets. It tests watertightness, thickness, mass, impact resistance and water vapour transmission. Here is the catch almost nobody spells out: EN 13967 does not include a radon gas test at all. CE or UKCA marking to EN 13967 confirms the moisture function of the sheet. It says nothing about gas. Gas permeability is measured separately (under the ISO 15105-1 method), and that result feeds into the BBA certificate, not the EN 13967 declaration. So if a listing waves EN 13967 at you as proof of radon performance, it is quoting the wrong standard.

BS 8485 is the code of practice for protecting buildings against methane and carbon dioxide ground gases, not radon. It appears on radon datasheets for one specific reason: its drafting committee set the minimum membrane thickness, and the BBA adopted that figure as the certification minimum for every polyolefin gas membrane, radon included. So "compliant with BS 8485:2015+A1:2019" on a radon roll means it meets that thickness threshold, not that BS 8485 governs radon. That standard is BR 211.

1600 gauge / 400 micron

Every BR 211-compliant radon barrier is 1600 gauge (400 micron). That number is a durability floor, not a gas rating, so a heavier sheet is not a higher grade of protection. Paying for thicker is wasted money.

Why 400 micron, and why heavier is not better

The most counter-intuitive thing about this product: the 400 micron thickness has almost nothing to do with how well the membrane stops gas. It is about surviving the build.

Gas resistance is a function of the polymer and the airtightness of the joints, not the thickness of the sheet. A correctly sealed 400 micron membrane stops radon. What the thickness buys is durability: a sheet thin enough to tear under a wheelbarrow or a dropped scaffold board is no use once it is buried. The BS 8485 committee judged that a reinforced polyethylene membrane below 400 micron is unlikely to survive installation and the following trades intact, and the BBA made 400 micron the certification minimum on that basis. The trade phrasing is that a gas membrane has to be strong enough to come through the build still in one piece.

So 1600 gauge is a floor, not a target. Buying a 2000 gauge sheet because it sounds safer adds cost and gives no extra radon protection, since the membrane was never gas-permeable at 400 micron in the first place. The one real-world nuance: a membrane stretched thin over a slab edge or a sharp arris loses thickness locally, so tensile strength and careful laying matter more than chasing a bigger headline gauge. Specify a BBA-certified 1600 gauge barrier and stop there.

Formats: which one you actually buy

Three formats exist. For a domestic extension floor, one of them is the answer and the other two are specialist.

Loose-laid sheet is the standard product and the one you want. A roll of polythene unrolled across the oversite, lapped, taped and collared in place. Visqueen Radon R400 (red), Novia Radon Barrier (green) and equivalent BBA-certified rolls are all this format, sold as 4m x 20m / 80m² rolls. This is what "radon barrier membrane" means in nearly every domestic context.

Self-adhesive gas membrane is a peel-and-stick HDPE sheet with a bituminous backing, used on vertical and below-ground surfaces, slab-edge details and contaminated land where a loose sheet cannot be held in place. It costs roughly ten times as much per square metre as loose-laid, so it is not a floor product. You would only meet it on an awkward perimeter or a basement.

Liquid-applied gas membranes are two-part coatings poured or trowelled over a deck, used mainly on contaminated sites and for sealing irregular clusters of penetrations that no off-the-shelf collar fits. For a standard floor it is overkill, though a two-part radon sealant is genuinely useful for sealing, say, a bank of six small conduits where a single top-hat will not work.

The accessory system: your buy-list

The membrane is half the order. A radon barrier only works as a sealed system, and the sealing parts are cheap, easy to forget, and the reason inspections fail. Order them with the roll.

The two tapes work together: a double-sided butyl tape goes between the sheets inside the lap to bond them, then a single-sided foil or BOPP tape runs over the top edge to seal the join. Use the membrane manufacturer's own tapes where you can, because compatibility of adhesive and sheet is the point. The collars are sized to the external pipe diameter, so a standard 110mm soil pipe takes a 110mm top-hat; bond it to the membrane and clip the upstand to the pipe. The radon DPC strip is the gas-resistant equivalent of an ordinary DPC, used in the wall course so the floor barrier and the wall barrier form one continuous envelope rather than meeting at an unsealed junction.

How to confirm a roll is genuinely certified

Anyone can dye a sheet of polythene red. Before you buy, check the datasheet or product page for two things. First, a BBA certificate number (for example, the Visqueen R400 carries BBA cert 13/5069). The BBA certificate is what backs the gas-barrier claim, because the BBA process requires the gas-permeability test that EN 13967 does not. Second, an explicit statement that the product conforms to BR 211:2023. A 1600 gauge sheet that lists only EN 13967 and no BBA number is being sold as a DPM, whatever the colour, and has not been certified for radon.

External resource

BRE BR 211:2023 (radon protective measures)

The definitive UK guidance behind Approved Document C, and the standard a compliant radon barrier datasheet should cite. Available from the BRE bookshop. Manufacturer datasheets (Visqueen, Novia) restate the relevant performance free of charge.

bregroup.com

What it costs

A 1600 gauge radon barrier roll (4m x 20m, covering 80m²) runs £120 – £175, depending on supplier and brand. One roll covers most single-storey rear extensions with margin for upstands and laps. The headline to take away: this is only a modest uplift over the standard 1200 gauge DPM you would have laid anyway, roughly 30 to 50 pounds more per roll, not a different order of cost. The radon premium is real but small.

Across a whole extension, the build-time radon materials (the upgraded membrane, the tapes, the collars, and a sump kit where full protection applies) come to £100 – £250. The labour folds into the groundworker's oversite work, since laying and sealing the barrier is part of preparing the floor. Watch out for outlier pricing: some own-label rolls list well above the typical range for no extra performance, while a few headline prices online are quoted ex VAT and look cheaper than they are. The certification matters more than the brand, and a BBA-certified 1600 gauge roll from any reputable supplier does the same job.

Install-quality acceptance checks

You do not lay this yourself, but you can check it, and the membrane is invisible the moment the slab goes down, so the time to check is before the pour. Walk the floor and confirm:

• The roll is 1600 gauge / 400 micron and carries a BBA number (look at the packaging or the delivery note).
• Every lap is at least 150mm and is taped, double-sided inside the lap and single-sided over the top edge. A loose overlap is a fail.
• Every pipe, duct and cable through the floor has a collar bonded to the membrane. A taped-over hole is not a collar.
• The membrane turns up at least 150mm at the perimeter and links to the radon DPC in the wall course, with the cavity bridged where the detail calls for it rather than stopping at the inner leaf face.
• There are no tears or punctures, and a little deliberate slack is left at the wall upstand so minor slab settlement cannot tear the membrane later.

Where you'll need this

• Radon protection for your extension - whether your postcode needs a barrier at all, basic versus full protection, the 30m² rule, and the standby sump
• Ground floor oversite and floor build-up - where the membrane sits in the floor layers, and the pre-pour building control hold point it is checked at