Test for radon before you finish a basement

Finishing a basement changes how a house is used: a storage area becomes a bedroom, an office or a family room where people spend hours every day. That matters for radon, a radioactive gas that seeps from the ground into homes and is a known cause of lung cancer. You cannot see or smell it, and the only way to know the level in a house is to test.

The practical mistake is ordering the test after the renovation. Testing first costs little, and if the level is high, the fix is easier to plan before finished walls and ceilings are in the way.

The Canadian guideline

Health Canada's guideline for radon is 200 becquerels per cubic metre (Bq/m3) in the normal occupancy area of a home, meaning any part of the building where a person is likely to spend more than 4 hours a day. When the level is above the guideline, Health Canada recommends taking corrective action to lower it within 1 year, and sooner the higher the level is.

How to test

Health Canada does not recommend tests shorter than 1 month and recommends a minimum of 3 months; its measurement guide recommends testing over 3 to 12 months. Place the detector in the lowest level that is or will be lived in. If you are about to finish a basement, test the basement itself. Starting a long-term test in the heating season and reading the result before construction drawings are final gives you time to plan.

If the result is high

Health Canada describes active soil depressurization as the most trusted and effective solution, and says it can reduce radon by 90% or more. By comparison, ventilation systems such as HRVs and ERVs generally reduce levels by 20% to 50%. Sealing cracks and gaps around pipes helps a system work, but Health Canada does not present sealing on its own as the fix. Health Canada recommends using a professional certified through the Canadian National Radon Proficiency Program (C-NRPP).

Why before the drywall (practice guidance)

An active soil depressurization system needs a pipe from below the slab to the outside, plus a fan. Planning its route while walls and ceilings are open is usually simpler than cutting into a finished room later. This is practical renovation guidance, not a Health Canada requirement: ask your radon professional and your contractor to agree on the pipe route, the fan location and any fire-separation or permit issues before framing starts.

After the fix

Health Canada recommends a short-term test by the certified professional once the system is running, then a long-term test in the same location during the first heating season after installation. After that, retest every five years and keep the system inspected and maintained. Health Canada also recommends that new buildings use construction techniques that minimize radon entry and make it easier to remove radon later if needed.

Prevention Checklist

• Order a long-term radon test (at least 3 months) for the basement before you finalize the renovation plan.
• If the result is above 200 Bq/m3, plan mitigation within a year, sooner for higher results.
• Hire a C-NRPP certified professional for mitigation.
• Agree the pipe route and fan location with the contractor before framing and drywall.
• Retest after the system is installed, during the first heating season, and every five years after that.

References

1. Health Canada. Radon guideline, Web page, modified 2025-09-24, Guideline; normal occupancy area; remedial action; new construction. Accessed 2026-11-01.
2. Health Canada. Reduce radon in your home (factsheet), Web page, modified 2026-09-22, Active soil depressurization; C-NRPP; testing after mitigation. Accessed 2026-11-01.
3. Health Canada. Guide for radon measurements in homes, Web page, Test duration. Accessed 2026-11-01.

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