Radon moves through soil and enters anywhere the below-grade envelope is not sealed:
It concentrates below grade for two reasons: basements are less ventilated than upper floors, and the pressure difference between a warm house and the soil actively draws soil gas in. Your house is, in a modest way, a vacuum pulling on the ground beneath it.
The Canadian guideline is 200 becquerels per cubic metre (Bq/m³) as an average in the normal occupancy area, and the wording matters as much as the number:
| Long-Term Result | Health Canada Recommendation |
| Below 200 Bq/m³ | No action required, though lower is better |
| 200 to 600 Bq/m³ | Reduce levels within two years |
| Above 600 Bq/m³ | Reduce levels within one year |
Health Canada also advises reducing levels as low as reasonably achievable, not just under the line. No level is risk-free, so 180 is not a clean bill of health, and the World Health Organization’s reference level is 100 Bq/m³. Between 100 and 200, mitigation is a reasonable choice, particularly with children or a full-time basement occupant. Public Health Ontario estimates about 4.6% of Ontarians live in homes above the Canadian guideline.
Radon is intensely local. Two houses on the same street, built by the same builder in the same year, can test an order of magnitude apart. Foundation construction, slab condition, sump configuration, HVAC balance and the soil under each footprint all matter. Regional maps are useful for public health planning, but they tell you almost nothing about your house. The only way to know is to test your own basement.
The 2024 Ontario Building Code, which took effect in 2025, strengthened soil gas requirements for new Part 9 homes:
Two honest caveats. The rough-in does not reduce radon on its own; it makes a future fix cheap instead of expensive. And the rules apply to new homes, not existing basements. But they show where the Code is heading, and why the next section matters.
Installing a rough-in during a basement renovation is a minor line item. Retrofitting a system into a finished basement means coring the slab, routing pipe through finished rooms, finding an exhaust path and repairing what you cut. If the slab is already open, for example for a backwater valve or new plumbing, the added cost of a radon stub is small. A sensible sequence:
Air-sealing a basement, adding a suite, rebalancing HVAC or adding powerful exhaust fans all change how the house breathes. A tighter envelope holds soil gas in, and a high-CFM range hood, dryer or bath fan running without make-up air increases depressurization, pulling harder on the soil. A low result before renovation does not guarantee a low result after it. Retest after any significant basement work.
Active soil depressurization (ASD) is the standard fix and the one Health Canada points to. A pipe through the slab into the granular layer below connects to a continuously running fan that draws soil gas out and vents it outdoors, away from windows and doors, before it can enter the house. Properly installed, it typically cuts levels by 80% to 90% or more.
Supporting measures help but do not replace ASD at elevated levels: sealing cracks and penetrations, a gasketed sump cover where the sump is the main entry path, and improved ventilation such as an HRV serving the basement. Use a C-NRPP certified mitigation professional and retest afterward. Documented before-and-after results are the point.
A basement tenant spends more hours at the measured concentration than anyone else in the house. That alone is a reason to test. On the legal side, accuracy matters more than alarm. Ontario does not currently have a regulation requiring radon testing or mitigation in rental units. Landlords do have a duty under the Residential Tenancies Act to keep a unit fit for habitation and compliant with health and safety standards, and Health Canada’s guideline is a recognized national benchmark. Whether that creates liability for a landlord who knew of elevated levels and did nothing is not settled, and it is not a question you want to be the test case for. Test before construction, mitigate above the guideline, retest to confirm, and keep both results on file.
| Item | Typical Cost |
| Long-term alpha track test kit | $40 to $80 |
| Professional long-term measurement | $200 to $500 |
| Radon rough-in during an open-slab renovation | Often a few hundred dollars |
| Active soil depressurization, retrofit | $2,000 to $3,500 |
| Complex or multi-point systems | Higher |
| Fan replacement, roughly every decade | $300 to $600 |
The test costs less than a nice dinner, and a mitigation system less than a mid-range appliance package. Both are far cheaper before the drywall goes up.
Maple Leaf Quality Renos raises radon testing at the design stage on every basement and legal suite project, so a result is in hand before the scope is set and a rough-in can go in while the slab is open, not after the drywall is up.
Contact us for a free site assessment.
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