Radon forms continuously in the soil around your home. Outdoors, it disperses harmlessly. The problem begins when it moves into an enclosed building. Homes typically sit at slightly lower air pressure than the surrounding soil, especially in winter when heating draws warm air upward. That pressure difference pulls soil gas in through any available gap. Common entry points include:
Because the basement is in direct contact with the soil, it is almost always the highest-concentration area in the home. A finished basement used as a bedroom, home office or legal suite, where people spend hours sleeping and breathing, is where exposure risk is greatest.
The main danger comes from radon’s radioactive decay products. These short-lived particles can be inhaled, settle in lung tissue and damage DNA over years of exposure. The risk depends on dose: the higher the concentration and the longer the exposure, the greater the risk. Someone who sleeps in a basement bedroom with elevated radon for ten years accumulates far more exposure than someone who passes through occasionally. The International Agency for Research on Cancer, part of the World Health Organization, classifies radon as a Group 1 carcinogen, the same category as asbestos and tobacco smoke. The link to lung cancer is well established. What varies is awareness among homeowners and, in many cases, among contractors.
Health Canada’s guideline for radon in homes is 200 Becquerels per cubic metre (200 Bq/m³). Above this level, Health Canada recommends reducing radon, and the higher the result, the sooner you should act. For comparison, the World Health Organization recommends a reference level of 100 Bq/m³, half the Canadian guideline. Ontario does not currently require radon testing in existing homes. However, several developments are changing expectations:
Even where testing is not legally required, it is quickly becoming a due diligence baseline, and ignoring it carries consequences for both occupant health and resale.
Radon testing is inexpensive and simple. There are three main approaches:
Long-term test (recommended). Health Canada recommends a test of at least three months, ideally during fall and winter when homes are closed up and radon levels tend to be highest. A passive detector, typically an alpha-track or electret device, is placed in the lowest lived-in level of the home and then mailed to a lab for analysis. DIY kits typically cost $30 to $75. Long-term tests are the most reliable because radon levels fluctuate with weather, air pressure and season.
Short-term test. A test lasting a few days under closed-house conditions gives faster results and is sometimes used in real estate transactions. Because radon varies so much, short-term results are less reliable and should be confirmed with a long-term test.
Professional continuous monitoring. Certified radon measurement professionals use calibrated electronic monitors that record levels hourly, providing an average result and a detailed fluctuation profile. For a basement renovation, especially one creating a legal suite, professional testing is strongly recommended. Expect to pay $150 to $350, depending on the monitoring period and report.
| Reading your results: Above 200 Bq/m³, mitigation is warranted. Between 100 and 200 Bq/m³, mitigation is worth serious consideration given the WHO reference level, especially while the walls are still open. |
This is the most effective radon reduction method and the one used in most successful residential mitigation projects in Ontario. A pipe is installed through the basement floor slab into the gravel layer beneath. A continuously running fan draws soil gas from under the slab and vents it outdoors, typically above the roofline, before it can enter the home. The system is quiet and needs little more than periodic checks.
Effectiveness: Sub-slab depressurization commonly reduces indoor radon by 80% or more, and often over 90%.
An HRV or ERV increases fresh air exchange, diluting radon by bringing in outdoor air and exhausting stale indoor air. It also improves basement air quality generally by helping control humidity and condensation. In higher-radon homes, an HRV works best as a complement to sub-slab depressurization. On its own, it suits homes with mild to moderate levels.
Sealing slab cracks, fitting an airtight sump pit lid and sealing pipe penetrations reduces how much soil gas enters the home. On its own, sealing usually delivers only a modest reduction and is rarely enough in homes well above 200 Bq/m³. It is, however, a low-cost step that belongs in any basement renovation regardless of test results.
| Mitigation Option | Typical GTA Cost |
| Sub-slab depressurization, during a renovation | $1,500 to $3,000 |
| Sub-slab depressurization, retrofit in a finished basement | $2,500 to $4,500 |
| HRV or ERV, supply and install | $2,000 to $5,000 |
| Sealing foundation entry points | $500 to $1,500 |
| Sub-slab depressurization rough-in during construction | $300 to $600 |
The gap between installing a system during a renovation and retrofitting one into a finished basement is the strongest argument for dealing with radon now rather than later.
The most cost-effective radon decision a GTA homeowner can make is to plan for mitigation during a basement renovation, before drywall and flooring are finished. At minimum, ask your contractor to install a sub-slab depressurization rough-in: a 4-inch pipe through the floor slab into the gravel beneath, capped flush with the finished floor. It costs $300 to $600 during construction and is easy to conceal. If testing later confirms elevated radon, the fan, exhaust piping and electrical connection can be added at a fraction of the cost of cutting through a finished floor and routing pipe through finished walls. This is standard practice among informed contractors and part of the baseline conversation we have with every client planning a basement renovation.
An unfinished basement where no one spends much time carries a very different exposure risk than a legal basement suite where a tenant sleeps eight hours a night, most nights of the week. When you accept rent from someone who relies on that space as their home, radon testing and mitigation become more than best practice. They are an ethical responsibility. Expectations around landlord responsibility for environmental hazards continue to evolve. Testing, mitigating where needed and documenting both is the responsible approach, and the one that best protects you as a property owner.
At Maple Leaf Quality Renos, radon awareness is part of every basement renovation consultation. We coordinate with certified radon measurement professionals, build sub-slab depressurization rough-ins into our scope wherever appropriate, and can include full active mitigation systems in any basement renovation project.
Contact us for a free, no-obligation consultation.
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