House problems · Radon · Test and sale
You cannot see radon and you cannot guess it from the neighbourhood: you measure it for three months. Here is the guideline, the test, the cost of the system, and what seller and buyer do with it.
Radon is a radioactive, odourless, colourless gas produced by the natural decay of uranium in the soil; it enters the house through slab cracks, the floor drain, joints and the sump, and builds up in the basement. Health Canada sets the guideline at 200 becquerels per cubic metre (Bq/m³) as an annual average and recommends corrective measures above it; it is the second cause of lung cancer after tobacco. You cannot guess it from the age of the house or the neighbourhood: two neighbouring houses can differ tenfold. The only way to know is a long-term test, a detector placed at the lowest occupied level for at least 91 days in heating season, windows closed; the kit costs under $60, analysis included, from the Quebec Lung Association. Above 200 Bq/m³, mitigation by active sub-slab depressurization (a pipe sealed into the slab connected to a fan that vents the gas outside) costs $800 to $3,500 on the 2026 market and brings the concentration under the guideline in the vast majority of cases. At the sale, a known test result is disclosed; an installed and tested mitigation system is shown.
Radon comes from the soil, everywhere in Quebec, with higher concentrations more frequent in some regions (Outaouais, Laurentians, Appalachians, Eastern Townships) but present in the St. Lawrence valley as well. It is unrelated to construction quality: an airtight new house can concentrate more than an old bungalow that breathes. The health risk depends on concentration and duration of exposure, hence the annual average and the long-term test. In Canada, the 200 Bq/m³ guideline is what sellers, buyers, inspectors and courts use as the reference.
A passive detector (alpha track) placed at the lowest level occupied at least four hours a day, at breathing height, away from walls, windows and heat sources, for at least 91 days, preferably in heating season when the house is closed; then sent back to the lab. Kit under $60 (Quebec Lung Association, hardware stores), result within a few weeks. A short-term test (2 to 7 days) gives a useful indication in a transaction, but a mitigation decision is made on a long test. Continuous electronic detectors, around $200 to $300, let you track the effect of the work.
The standard method, active soil depressurization, consists of drilling the slab, sealing a pipe into it connected to a continuously running fan that draws air from under the slab and vents it above the roof, and sealing cracks, the sump and joints. A certified contractor (Canadian C-NRPP program) installs it in a day; count $800 to $3,500 depending on the house and the 2026 market, plus a few tens of dollars of electricity a year for the fan, and a new test after the work. The Construction Code has required since 2010 a membrane and a rough-in pipe under the slab of new houses, which makes adding the fan easier; older houses are treated the same way with one more step.
The Declarations by the seller form asks whether a radon test was done and its result: a seller who knows a result above 200 Bq/m³ discloses it, and an installed system is disclosed with its control test. Nothing requires testing before selling, but a recent test under the guideline, or an installed and verified system, is a listing argument at almost no cost, especially in regions where buyers think about it. The buyer can request a short-term test during the inspection period, or negotiate a clause: long test after purchase and cost-sharing of mitigation above 200 Bq/m³. A high result is not grounds for refusal by a lender; it is a $800 to $3,500 cost to build into the price. A seller who hides a known result faces a latent defect claim.
| Long test result | What Health Canada recommends | What to do in a sale |
|---|---|---|
| Under 200 Bq/m³ | No measure required; retest after major renovations | Put the report in the file |
| 200 to 600 Bq/m³ | Remediate within two years | Disclose; negotiate mitigation or a credit |
| Over 600 Bq/m³ | Remediate within one year | Disclose; mitigate before the sale preferably |
Health Canada's guideline is 200 Bq/m³ as an annual average; above it, it recommends corrective measures, within two years between 200 and 600, within a year above 600.
At least 91 days for a long-term test, preferably in heating season; a short test of a few days is only for screening.
$800 to $3,500 depending on the house and region (2026 market), installed in a day by a certified contractor, plus a control test.
A high result known to the seller and not disclosed can be; radon never measured by anyone is a latent defect for no one, hence the buyer's interest in testing or providing a clause.
These problems are easier to settle before listing than after the buyer's inspection: see the pre-purchase inspection in Quebec, latent defects in Quebec and renovating before selling.
The Loaa & Manseur team, led by Lotfi Manseur and Loaa Al Daow, is the number 1 team at RE/MAX Platine in 2024 and 2025, 6th RE/MAX team in Quebec, 15th in Canada and in the global top 50 of large residential teams according to the official RE/MAX ranking, computed on actual transactions. Its Google profile counts 221 reviews with a 4.9 rating. Its offices are at 55 avenue de l'Équinoxe in Brossard, one bridge from the island, and every broker's licence can be checked in the OACIQ public register. Three things to verify yourself before calling us, and to demand of any other broker.
Also available in French: version française de ce guide.
Published September 7, 2026 by the Loaa & Manseur team, the number 1 team at RE/MAX Platine in 2024 and 2025, 6th RE/MAX team in Quebec, 15th in Canada and in the global top 50 according to the official RE/MAX ranking. Tell us about your project.
Équipe no 1 de RE/MAX Platine en 2024 et en 2025, 6e au Québec, 15e au Canada et dans le top 50 mondial au classement officiel RE/MAX. Réponse en moins de 24 heures, sept jours sur sept.