Radon-resistant building materials: basement details Canada's code misses

What to take away

Two numbers that decide the work

Radon advice written for US readers starts from a different number. The Environmental Protection Agency sets an action level of 4 pCi/L, roughly 148 Bq/m³. Health Canada's radon guidance sets the Canadian guideline at 200 Bq/m³ and recommends long-term testing before any decision.

A reading of 150 Bq/m³ sits below the Canadian guideline and still tells you soil gas is moving into the building.

What Article 9.13.4 asks for

The National Building Code of Canada requires a rough-in for subfloor depressurization in buildings with a floor on ground or a crawlspace. The National Research Council publishes the model code through its codes centre, so read the article yourself.

Provinces adopt that model with their own amendments, so the local edition governs your permit. The clause is short, and it stops well before the parts that matter most.

A rough-in contains these parts:

The article stops there. Membranes, joints and lids are left to the builder. Insulation choices in the same basement follow a similar pattern, as natural insulation upgrades sets out.

The materials the code does not name

Article 9.13.4 names no product. The sheets, panels, lids, boots and sealants below are what builders specify instead. Prices are typical 2026 ranges, and they move with sheet thickness, roll width and order size.

Stego Wrap 15-mil. Stego Industries sells this cross-laminated polyethylene sheet for under-slab use. It lists a permeance below 0.01 perms when tested to ASTM E96. Canadian distributors stock it for sealed sub-slab layers. Typical price is $0.30 to $0.60 per square foot as of 2026. Ask for the permeance figure, not the thickness.

Viper VaporCheck 15-mil. Viper, part of Insulation Solutions, makes VaporCheck for underslab vapor control. It is a 15-mil polyethylene sheet with a stated permeance of 0.01 perms or less. It suits builders who want one roll across a large slab. Typical price is $0.25 to $0.55 per square foot.

W.R. Meadows Perminator 15-mil. W.R. Meadows sells Perminator as a 15-mil vapor barrier for slabs on grade. It carries a low permeance rating and is often specified with taped laps. Typical price is $0.28 to $0.58 per square foot.

Raven Tu-Tuff 15-mil. Raven Industries makes Tu-Tuff, a reinforced polyethylene underslab barrier. It resists puncture on a rough base. Typical price is $0.30 to $0.65 per square foot.

Mar-flex Radon Guard. Mar-flex makes Radon Guard as a dimpled HDPE panel for sub-slab depressurization. It creates a drainage layer under the slab. Panels are commonly 4 feet by 8 feet. Typical price is $2.50 to $4.00 per square foot.

GSE HDKE and Solmax LLDPE geomembranes. GSE Environmental makes HDKE geomembrane and Solmax makes LLDPE liners. Both arrive as factory-welded sheets for large slabs or wet soil. A welded seam beats a taped lap because the joint is fused, not glued. Typical installed price is $1.50 to $4.00 per square foot for a 40-mil liner.

Carlisle CCW-705 and W.R. Meadows Air-Shield LMP. Both go onto foundation walls and footing joints before backfill. Carlisle CCW-705 is spray-applied, and W.R. Meadows Air-Shield LMP is a liquid membrane. They bridge the cold joint where the wall meets the footing, a common entry point for soil gas. Typical price is $2.00 to $4.50 per square foot installed.

RadonAway Sump Lid and Jackel Sump Basin Cover. These are bolted covers with rubber gaskets and grommeted pipe entries. RadonAway sells a gasketed lid for sump pits, and Jackel makes covers for 18-inch and 24-inch basins. A plywood disc over an open pit is a direct path from soil to basement air. Typical price is $40 to $150.

RadonAway Pipe Boot and PSI Link-Seal. These rubber or EPDM seals clamp around a pipe where it crosses the membrane. RadonAway's boot fits 3-inch and 4-inch pipes. PSI Link-Seal uses interlocking rubber links for larger penetrations. They hold through slab movement in a way spray foam does not. Typical price is $15 to $80 per penetration.

RadonSeal Plus and Tremco Vulkem 116. Both are specified for concrete-to-membrane joints. RadonSeal Plus is a concrete sealer, and Tremco Vulkem 116 is a polyurethane sealant. They stay flexible, so a hairline crack does not open into a leak. Typical price is $25 to $70 per cartridge.

None of these are named in Article 9.13.4. A name on a package proves little, and material selection mistakes shows how often labels outrun performance.

Details that look finished

A membrane lapped but not sealed. Sheets overlap by 100 to 150 mm with no tape. Soil gas travels sideways beneath the slab and rises through the cold joint at the wall. Seal every lap with the maker's tape and carry the sheet into the wall barrier.

A cap nobody can reach. The riser is cut below the slab or buried behind insulation. Label the cap, keep it visible, and photograph the rough-in before drywall goes on.

An open sump. A loose disc sits over the pit. A bolted, gasketed lid with grommeted entries closes it. RadonAway and Jackel both sell gasketed covers.

Penetrations filled with foam alone. Floor drains, water lines and conduits get packed with spray foam. Slab movement opens those joints within a few years. Mechanical seals do the work, with sealant as a backup.

The mistake that waits for winter

Situation: a new basement passes inspection, the slab looks clean, and the project moves on. Consequence: a February test reads 240 Bq/m³, and the repair means opening a joint or adding a fan. Prevention: treat the sub-slab layer as unproven until a long-term test runs through a heating season.

Radon has no color, smell or taste, so a small breach stays hidden for months. The first honest feedback on that work is a detector that sat through cold weather. Tests you cannot skip also drive the calendar of related jobs, which is the argument in natural insulation timelines.

What these failures share

Each detail above looks acceptable on a walkthrough. The shared thread is that the under-slab layer gets judged on appearance, and nobody owns the joints between trades. A plumber's penetration becomes the finisher's problem, then the homeowner's.

Labels make this harder. A product sold as a radon barrier can be a sheet of polyethylene. The name on the package proves little, and test data rarely appears there. material selection lessons records the same gap between claims and performance on other jobs, from insulation to flooring.

Common questions