A note before you read. This is general consumer information for Canadian homeowners, buyers, sellers, and renters, drawn from publicly available Health Canada and Take Action on Radon sources. It is not medical, legal, or warranty advice. Radon results are reported against the Health Canada guideline of 200 Bq/m³; RadonTest.ca coordinates testing logistics and does not interpret individual results or provide health assessments. The key idea below is that the trigger to retest an unmitigated home is change to the building, not a fixed clock — for how long a test should run, see when is radon testing season in Canada, and for where to put the detector, see where to place a radon test kit.
Key facts at a glance
- Retest after any renovation that changes the structure or ventilation of your home — finishing a basement, a new furnace or HVAC system, adding a bathroom. Changing how air and soil gas move can change your radon level.
- Retest after an energy retrofit that tightens the home — new windows, added insulation, air-sealing. These improvements can actually raise indoor radon by reducing the natural air exchange that used to dilute it.
- Retest after excavation or foundation work near the home, or a new addition — anything that disturbs the ground or the foundation can open new paths for radon.
- If you have a mitigation system, Health Canada's cadence is a check at activation, a long-term test the following heating season, an initial long-term re-check within two years of activation, then a long-term test every five years to confirm it's still working.
- New homes — test after you move in, ideally over the first full heating season; testing once at occupancy is the meaningful measurement, regardless of any radon rough-in.
- There is no blanket "every five years" rule for an unmitigated home. Health Canada's trigger to retest is change to the building — but because radon levels drift over time, periodic retesting is still a sensible habit (more below).
- If a retest comes back at or above 200 Bq/m³, Health Canada recommends acting within one year — sooner the higher the level.
The short answer
If you have never tested, test once — a long-term test of at least 91 days, ideally over the heating season. That single measurement tells you where your home stands today.
After that, the question of how often has two clean answers depending on your situation:
- If your home is unmitigated, you don't need to retest on a fixed schedule just because time has passed. The thing that should prompt a retest is a change to your home — a renovation, an energy retrofit, excavation, or an addition that could alter how radon enters or how air moves. That's Health Canada's trigger: change, not the calendar.
- If you have a radon mitigation system, you do test on a set schedule — a check when it's switched on, a long-term test the next heating season, a re-check within two years, and then a long-term test every five years to verify it's still holding the level down.
Everything else in this article is detail on those two answers — plus one piece of good-practice advice for low, unmitigated homes that we're careful to attribute correctly.
Retest after these changes
For an unmitigated home, the reason to test again is that something about the house has changed. Radon enters from the soil and accumulates based on how your foundation, heating, and ventilation interact — so when you alter any of those, the old result may no longer describe your home. Health Canada specifically points to renovations and changes that affect the structure or ventilation as a reason to retest. The common triggers:
- Renovations that change the structure or ventilation. Finishing a basement, installing a new furnace or HVAC system, adding a bathroom, or otherwise reworking how air moves through the home can shift your radon level — up or down. A finished basement, in particular, turns a low-occupancy space into a lived-in one, which is exactly where you'd now want a representative reading.
- Energy retrofits that tighten the home. New windows, added insulation, weather-stripping, and air-sealing all reduce the drafts and natural air exchange that used to dilute indoor air. That's great for your heating bill — but it can raise radon, because once the gas is inside there's less fresh air coming in to carry it back out. This is one of the most overlooked retest triggers: people assume an upgrade can only help, when a tighter envelope can concentrate radon indoors.
- Excavation or foundation work near the home. Digging near the foundation, underpinning, adding a sump, repairing the slab, or other ground-disturbing work can open new entry points for soil gas and change the pressure relationship between the house and the ground.
- A new addition. Building out new living space sits on its own patch of soil and changes the home's footprint and airflow — reason enough to measure again once it's complete.
After any of these, retest in the lowest lived-in level, and in the area that was renovated or is least like the rest of the home, using a long-term test. The point is to capture how the changed home behaves over a full season, not to re-confirm the old number. For where exactly to put the detector, see where to place a radon test kit.
If you have a mitigation system
A mitigation system is not "install it and forget it." A fan can lose performance, a seal can fail, or the home around it can change — so Health Canada lays out a clear retesting cadence to confirm the system keeps doing its job. The sequence:
- A short-term test at activation. Right after the system is switched on, a short-term measurement is the normal way to confirm the fan is pulling the level down quickly — before anyone has lived with it for a season. (This is one of the few legitimate uses of a short-term test; for the difference, see short-term vs long-term radon test in Canada.)
- A long-term test the following heating season. Because the quick check can't capture seasonal swings, a long-term (about three-month) test during the next heating season confirms the system is holding the level down over time, not just on the day it was turned on.
- An initial long-term re-check within two years of activation. Health Canada recommends a further long-term test within two years of the system being switched on, to verify it's still performing as it settles in.
- A long-term test every five years after that. From then on, a long-term test every five years confirms the system is still keeping radon down — fans wear, and homes change, so periodic verification matters.
If a retest on a mitigated home comes back at or above the guideline, treat it like any high result: the system likely needs attention from a C-NRPP-certified professional, and Health Canada's "within one year, sooner the higher" timeline applies. For how to tell whether your system is actually working — including what the fan gauge does and doesn't tell you — see is my radon mitigation system working in Canada.
New homes and recent buyers
New build? Test after you move in — ideally a long-term test over the first full heating season after you take possession. A radon rough-in (the capped pipe required by the building code) is a head start, not a measurement: it does not prove your home is below 200 Bq/m³, so the meaningful step is to test once you're living in the home. For the full picture, see radon in new construction homes in Canada.
Just bought a home? Test even if the seller says the home was tested. You generally don't know the conditions of an earlier test, how long ago it was, whether the home has changed since, or whether it was a long-term measurement at all — and the result follows the home's current state, which is now your responsibility. Buying is itself a natural moment to get your own long-term reading. For how testing fits into a transaction, see radon in real estate: buying and selling in Canada.
What about an unmitigated home that tested low?
This is the most common follow-up question: "My test came back low and I haven't changed anything — do I have to test again?"
There is no Health Canada rule that says you must retest a stable, unrenovated home on a fixed clock. Health Canada's trigger to test again is change — the renovations, retrofits, excavation, and additions described above. If none of those has happened, you are not breaking any guideline by not retesting.
That said — and this is the nuance — radon is not a fixed number for the life of a home. It varies over time with weather patterns, how you use the space, and gradual changes you might not think of as "renovations." Because of that natural drift, Take Action on Radon suggests testing again roughly every five years as a good-practice habit for homes that previously tested low.
To be precise about where this comes from: the periodic five-year retest for a low, unmitigated home is Take Action on Radon's good-practice recommendation — it is not a Health Canada requirement. (The firm five-year cadence in Health Canada's guidance applies to mitigated homes, alongside the within-two-years first re-check.) So the honest summary is: you're not required to retest a stable home on a clock, but checking again every few years is a reasonable habit, and you should retest whenever your home changes. And remember no home is ever "risk-free" — a low result means no action is recommended, not that radon is absent.
How to retest
Retesting uses the same method as a first test — there's no special "retest" device:
- Run a long-term test of at least 91 days (about three months). This is what the 200 Bq/m³ guideline is written against, because radon swings hour to hour and season to season; a short-term snapshot can't tell you the changed home's true exposure. (The one exception is the quick check right after a mitigation system is switched on.)
- Test over the heating season where you can. Roughly October through April, when homes are closed up and radon tends to be highest, gives the most conservative, representative reading — though you can start a long-term test any time of year. See when is radon testing season in Canada.
- Place the detector in the lowest lived-in level, in a room used more than four hours a day, about 30 cm off the floor and away from drafts, vents, exterior walls, and direct sun — not the kitchen, bathroom, or laundry room. After a renovation, test the newly changed or least-tested area. See where to place a radon test kit.
- Use a long-term test kit with lab analysis. You can order a long-term radon test kit and the lab reports your result against the Health Canada guideline.
If a retest comes back at or above 200 Bq/m³, Health Canada recommends taking corrective action within one year — and sooner the higher the level. For a step-by-step on a high result, see what to do if your radon level is above 200 Bq/m³.
Frequently asked questions
How often should I test for radon? Test once if you never have. After that, for an unmitigated home there's no fixed Health Canada schedule — the trigger to retest is a change to your home (renovation, energy retrofit, excavation, or addition). If you have a mitigation system, Health Canada's cadence is a check at activation, a long-term test the next heating season, a re-check within two years, then a long-term test every five years. As a good-practice habit, Take Action on Radon suggests retesting a low home about every five years too.
Do I need to retest if my first test was low? Not on a fixed clock, per Health Canada — the requirement to retest is tied to change, not the calendar, so a stable, unrenovated home that tested low isn't obligated to retest. But because radon drifts over time, Take Action on Radon suggests testing again roughly every five years as good practice, and you should always retest after a renovation, retrofit, or excavation. Remember a low result means no action is recommended, not that the home is risk-free.
Should I retest after finishing my basement or installing new windows? Yes. Finishing a basement changes the structure and ventilation (and turns a low-occupancy space into a lived-in one), and new windows are an energy retrofit that tightens the home — which can actually raise radon by reducing air exchange. Both are Health Canada retest triggers. Run a long-term test in the changed area afterward.
How often should I retest after mitigation? Health Canada's sequence is: a short-term test when the system is activated, a long-term test the following heating season, an initial long-term re-check within two years of activation, and then a long-term test every five years to confirm the system is still keeping radon down. If a retest is at or above the guideline, have a C-NRPP-certified professional check the system.
Do I retest when I buy a house? Yes — test even if the seller tested. You usually can't verify when or how a previous test was done, whether it was long-term, or what's changed since, and the result reflects the home's current condition, which is now yours. Buying is a natural time to get your own long-term reading. See our real-estate guide.
Does radon change over time? Yes. Radon levels rise and fall with weather and air pressure, swing between seasons (winter is typically higher), and shift when you renovate, retrofit, or change how you use your space. That variability is exactly why the trigger to retest is change — and why periodic retesting of even a low home is a reasonable habit.
Is there an "every five years" radon rule? For mitigated homes, yes — Health Canada's cadence includes a long-term test every five years (plus a first re-check within two years). For unmitigated homes there is no Health Canada five-year rule; the trigger is change. The "every five years" suggestion for a low, unmitigated home comes from Take Action on Radon as good practice, not from a Health Canada requirement.
Do I need a special kit to retest? No. A retest uses the same long-term test kit as a first test — at least 91 days, ideally over the heating season, placed in the lowest lived-in level (or the newly renovated area). The only short-term test in the whole process is the quick check right after a mitigation system is switched on.
Retest your home the way Health Canada recommends
The rule of thumb is simple: test once, retest whenever your home changes, and follow the set schedule if you have a mitigation system. A long-term test over the heating season gives you a number you can act on.
Order your RadonTest.ca kit → — lab analysis by a C-NRPP-certified laboratory (with an all-in-Canada analysis option), tracked shipping both ways, and your result delivered with clear Health Canada context.
Sources
- Health Canada — Guide for radon measurements in residential dwellings (retest after renovations and changes affecting the structure or ventilation; long-term test of at least 91 days; heating-season guidance; detector placement), modified 2025-12-22. https://radontest.ca/links/hc-measurements-guide
- Health Canada — Radon: Reduction Guide for Canadians (after mitigation: short-term test at activation, long-term test the following heating season, initial long-term re-check within two years of activation, then a long-term test every five years; corrective action at/above 200 Bq/m³ within one year, sooner the higher; C-NRPP), published Feb 2023, modified 2025-09-24. https://www.canada.ca/en/health-canada/services/environmental-workplace-health/reports-publications/radiation/radon-reduction-guide-canadians-health-canada.html
- Take Action on Radon — Protect yourself and your family from radon (good-practice guidance on testing and reducing radon; periodic retesting of previously tested homes). https://takeactiononradon.ca/protect/
Lab analysis is performed independently by a C-NRPP-certified laboratory; results are reported against the Health Canada guideline of 200 Bq/m³. RadonTest.ca coordinates kit logistics and sample submission only — it does not interpret or modify lab results and does not provide medical, legal, or warranty advice. The fixed five-year retest cadence applies to mitigated homes (alongside the within-two-years first re-check); for unmitigated homes Health Canada's trigger to retest is a change to the building, and the suggestion to retest a low, unmitigated home about every five years is Take Action on Radon's good-practice recommendation, not a Health Canada requirement. No radon result means a home is "risk-free" or risk-free; a result below the guideline means no action is recommended. Mitigation should be carried out by a C-NRPP-certified professional (directory at c-nrpp.ca/find-a-professional). Information attributed to Health Canada and Take Action on Radon is summarized from the public sources listed above; confirm time-sensitive details with the responsible body.