Rules
Radon-resistant building materials: basement details Canada's code misses
Radon resistant building materials for Canadian basements: Health Canada's 200 Bq/m³ guideline, NBC 9.13.4 rough-in rules and where US advice misleads.
What to take away
- Health Canada's guideline is 200 Bq/m³, measured over a test of at least three months. The US EPA action level is 4 pCi/L, about 148 Bq/m³.
- Article 9.13.4 of the National Building Code of Canada requires a capped radon rough-in for new floors on ground and crawlspaces.
- A rough-in is a pipe, not a barrier. Slab laps, wall joints, sump lids and pipe entries are separate work the code leaves to the builder.
- The worst failures stay invisible until a long-term test runs through a heating season.
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, which converts to 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.
| Measure | Canada | United States |
|---|---|---|
| Reference level | 200 Bq/m³ | 4 pCi/L, about 148 Bq/m³ |
| Guidance body | Health Canada | US EPA |
| Recommended test length | At least 3 months | 2 days or more |
| New-build requirement | NBC Article 9.13.4 rough-in | Varies by state and county |
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 you can read the article before you accept a builder's summary.
A rough-in contains these parts:
- A clean granular layer under the slab, with a perforated collection pipe inside it.
- A riser, commonly 100 mm across, that passes through the slab and ends in a labelled cap.
- A sump pit with a gasketed lid and sealed entries where pipes cross it.
- A continuous route to the exterior or the attic, ready for a fan later.
- A power provision near the point where that fan would sit.
The article stops there, leaving membranes, joints and lids to the builder. Insulation choices in the same basement follow a similar pattern, as natural insulation upgrades sets out.
Details that look finished
A membrane that is lapped but not sealed. The sheet under the slab overlaps by 100 to 150 mm with no tape or adhesive. 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. A mitigation contractor opens the floor later to find it. Label the cap, keep it visible, and photograph the rough-in before drywall goes on.
A sheet chosen on thickness alone. Under-slab membranes often arrive as moisture products, listed by thickness and water vapour permeance. Soil gas asks a different question. Ask for permeance and puncture data in writing. A name on a package proves little, and material selection mistakes shows how often labels outrun performance.
An open sump. A plywood lid or a loose disc sits over the pit. That pit becomes a path for soil gas into the basement. A bolted, gasketed lid with grommeted entries closes it.
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 or pipe boots 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 colour, smell or taste, so a small breach in the barrier 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 with a good name, and test data rarely appears on the package. material selection lessons records the same gap between claims and performance on other jobs, from insulation to flooring.
Common questions
Does a passive rough-in pipe lower radon on its own? Yes. A pipe that runs to the exterior can vent some soil gas by stack effect, and that is why the rough-in exists. Many homes still need a fan after testing.
What should I ask a supplier for? Permeance and puncture resistance in writing, plus the installation instructions your installer must follow. Those numbers tell you more than the word radon on a label.
Is 200 Bq/m³ a safe level? Health Canada treats the guideline as the point where remediation is recommended. Lower is better, so test for three months and decide from the result.
