Radon Barrier Membranes: Where They Fit in Radon Control

Radon is an invisible, odorless gas that comes from the ground a building sits on, which puts it squarely in the conversation about foundations, slabs and airtightness. This guide covers what radon is, how it gets in, how US codes and standards handle it, and where a sealed, continuous radon barrier membrane fits: as a supporting measure alongside testing and mitigation, never a substitute for them.

What radon is and why it matters

Radon is a naturally occurring radioactive gas produced by the breakdown of uranium in soil and rock. It moves up through the ground as soil gas and can collect indoors, especially in basements and lower floors.

According to EPA estimates, radon is the number one cause of lung cancer among non-smokers and the second leading cause of lung cancer overall. EPA attributes about 21,000 lung cancer deaths a year in the US to radon, about 2,900 of them among people who have never smoked.

The EPA action level and testing

Radon is measured in picocuries per liter of air (pCi/L). EPA recommends fixing a home if the radon level is 4 pCi/L or higher (about 150 Bq/m³), and recommends that people consider fixing homes with levels between 2 and 4 pCi/L, because no level of radon exposure is known to be safe.

The only way to know a building's radon level is to test it. You can use a do-it-yourself test kit or hire a certified radon measurement professional.

How radon gets into buildings

Indoor air pressure is often slightly lower than the pressure in the soil around the foundation, so soil gas gets drawn in through any opening in the ground-contact envelope. Typical entry routes include:

• Cracks in concrete slabs and floors
• Control, isolation and construction joints, including the slab-to-wall joint
• Gaps around service penetrations: plumbing, electrical conduit, floor drains
• Sump pits and drain tile openings
• Hollow CMU (concrete block) foundation walls, whose cores can carry soil gas up and into the building
• Exposed soil and gaps in crawlspaces and suspended floors

The same openings that leak soil gas also leak air and moisture, which is why radon control and air-barrier detailing tend to overlap. For the energy-code side of that story, see our guide to continuous air barrier requirements in US energy codes.

Radon-resistant new construction: IRC Appendix F and EPA RRNC

For new homes, the International Residential Code includes Appendix F, Radon Control Methods. Like other IRC appendices, it is not mandatory unless the jurisdiction adopts it, so check your local code. EPA promotes the same approach under the name radon-resistant new construction (RRNC). The basic system is a combination of features working together:

1. Gas-permeable layer. Typically at least 4 inches (102 mm) of clean aggregate under the slab, so soil gas can move freely toward a collection point.
2. Soil-gas retarder. Polyethylene sheeting of at least 6 mil over the gas-permeable layer, with laps of at least 12 inches (305 mm). In crawlspaces, a similar sheet covers the soil and extends to the foundation walls.
3. Sealing entry routes. Slab penetrations, joints and cracks are sealed, sumps get a gasketed or sealed lid, and hollow block walls are capped with a solid or grouted course.
4. Vent pipe. A gas-tight pipe of at least 3 inches in diameter runs from the gas-permeable layer up through the building and out above the roof.
5. Electrical junction box. Installed in the attic or another likely fan location so the passive system can be made active with a fan if testing shows it is needed.

The ANSI/AARST consensus standards go further. ANSI/AARST CCAH covers reducing radon in new one- and two-family dwellings and townhouses, including rough-in, verification testing and system activation where needed. ANSI/AARST SGM-SF covers soil gas mitigation in existing single-family homes.

Active mitigation: sub-slab depressurization

In existing buildings with basements or slabs on grade, EPA describes sub-slab suction (sub-slab depressurization) as the most common and usually the most reliable radon reduction method. A fan pulls soil gas from beneath the slab through a sealed pipe and exhausts it above the roofline, so the area under the slab stays at lower pressure than the rooms above. In crawlspace homes, the equivalent is submembrane depressurization: a sheet over the crawlspace soil with a fan drawing from beneath it.

Sealing matters in both cases because it improves how well the system works, but EPA's guidance is clear that sealing alone has not been shown to lower radon levels significantly or consistently. Mitigation systems should be designed and installed by a certified radon mitigation professional. In the US, national certification is offered through the National Radon Proficiency Program (NRPP, operated by AARST) and the National Radon Safety Board (NRSB). Some states have their own licensing as well.

Where a radon barrier membrane fits

A sealed, continuous fluid-applied membrane cannot replace testing or a mitigation system. What it can do is support them by closing the many small, scattered leakage paths that are hard to seal with caulk alone:

• Basement and crawlspace slab surfaces, especially older slabs with hairline cracking
• Foundation walls, including porous concrete and CMU
• Slab-to-wall joints and the areas around penetrations
• Crawlspace walls and the transitions where sheeting meets the foundation

Reducing leakage through the slab and walls can help a sub-slab depressurization system maintain the pressure difference it relies on, and it limits the places where soil gas can move straight into living space. The same work also adds airtightness. Our guide on fluid-applied air barriers vs. housewrap and sheet membranes explains why a seamless, fully adhered coating handles complex surfaces differently from sheet goods.

Passive Purple's radon and ground-gas data

Passive Purple is a water-based, low-VOC fluid-applied airtight and vapor control membrane. It has been tested for radon resistance to ISO/TS 11665-13, with a radon diffusion coefficient of 3.3 × 10⁻¹² m²/s.

In plain terms, the diffusion coefficient describes how easily radon atoms work their way through a material. The lower the number, the slower radon moves through it and the more resistant the material is. It is a laboratory material property: it shows the cured film resists radon diffusion, not what radon level a particular building will reach, because that depends on soil, pressures, cracks, unsealed details and every other entry route.

Passive Purple has also been tested as a methane and ground-gas barrier, showing low gas permeability to ISO 15105. As with radon, treat this as one layer in a system designed by a qualified professional, not as a stand-alone solution.

Practical detailing tips

• Test first. Get a baseline radon measurement before any work so you can judge what changed.
• Prepare the substrate. Surfaces should be sound, clean and free of dust, efflorescence, oil and loose material. Prime where the substrate calls for it; porous concrete and block often need a primer to get good adhesion and an even film.
• Treat cracks, joints and penetrations first. Use Passive Purple Brush, the fiber-reinforced brush grade, on slab-to-wall joints, around pipes and conduit, and across cracks of about 1/16 to 3/16 in (2 to 5 mm). Larger or moving cracks may need repair before coating.
• Keep it continuous. A membrane only works where it is unbroken. Carry it through corners and transitions, and tie it into sheeting, sump lids and pipe collars rather than stopping short.
• Coordinate with the mitigation contractor. Do not bury or block suction points, monitoring ports, labels or manometers, and agree on sequencing so sealing and system installation support each other.
• Build up the right film. Apply by roller or airless spray to the specified coverage, check for pinholes and let it cure fully.
• Retest after the work. Measure again once sealing and any mitigation system are complete, and keep testing periodically. EPA suggests retesting at least every two years.

Bottom line

Radon control starts with a test and, at or above the EPA action level, a mitigation system installed by a certified professional. A continuous membrane on slabs, foundation walls and crawlspace surfaces is a useful supporting measure that closes leakage paths and helps systems perform. It is never a replacement for testing and mitigation.

Talk to us about your project

Explore Passive Purple and Passive Purple Brush for slab, wall and joint sealing, and find CSI MasterFormat-format specifications on our US specifications page. Our technical team can help with section editing, substitution requests and condensation risk analysis for below-grade walls. Contact us or call +1 (347) 220 8403.