Best Time of Year to Test for Radon in Canada
If you want the short answer first, here it is: the best time of year to test for radon in Canada is during the heating season, usually from October to April, with late fall and winter often being the most practical time to start.
That is the answer most Canadian homeowners are looking for. But like most good radon answers, it needs a little explanation. The reason the heating season is best is not just because it is colder outside. It is because radon behaves differently indoors during colder months, and Health Canada wants homeowners to use a long enough test during the part of the year when indoor levels are typically more conservative for estimating real annual exposure.
This matters because radon is not constant. It rises and falls from hour to hour, day to day, and season to season. A reading taken during one random weekend does not tell the same story as a proper long-term test during the part of the year when homes are more closed up and radon is more likely to accumulate.
So if you are wondering when to test in Canada, the answer is not just “winter.” The more complete answer is this: start a long-term test during the heating season, make sure at least 91 days of the test occur during that season, and place the detector in the lowest lived-in level of the home.
This article explains why that timing matters, what “heating season” actually means, whether you should wait if it is summer, and how Canadian homeowners should think about timing in the real world.
Table of Contents
- The short answer
- Why the time of year matters for radon testing
- The best months to test in Canada
- Why fall and winter are preferred
- What Health Canada actually recommends
- Should you wait if it is spring or summer?
- Why long-term testing matters more than a quick test
- Where to place the test in your home
- Practical timing advice for real homeowners
- Common mistakes people make about test timing
- Bottom line for homeowners
- Sources
The short answer
The best time of year to test for radon in Canada is during the heating season, which Health Canada says is typically October to April, though it can vary by location. In practical terms, that means fall and winter are usually the best times to start a test.
Health Canada’s current technical guide says radon testing in homes should take place over no less than 91 days, and that the test should be conducted during the heating season so that those 91 days occur during that period. It also says that testing during the heating season is preferable because indoor radon levels are typically higher during colder months and because testing outside that season may underestimate the average annual radon level.
So if you want the easiest rule to remember, it is this: start your radon test in fall or winter and let it run for at least three months.
Why the time of year matters for radon testing
Many homeowners assume radon is a fixed number, almost like the square footage of a house or the height of a ceiling. It is not. Radon changes over time. That is one of the most important things to understand before talking about the best time of year to test.
Health Canada says radon levels in homes vary over time because of residents’ habits, ventilation systems, building construction, entry pathways, weather conditions, and daily and seasonal fluctuations. That means the same house can show different radon readings depending on when you measure it and how long you measure it.
This is exactly why timing matters. If you test during a period when the home is more open, more ventilated, or less prone to drawing soil gas in from below, you may get a lower reading than you would during a colder period when the house is more sealed up. That lower reading might still reflect the test period accurately, but it may not be the best conservative estimate of the annual average level you actually care about.
For homeowners, the key idea is simple. The goal is not just to get a radon number. The goal is to get a number that tells you something useful about your long-term exposure in the home.
The best months to test in Canada
In most parts of Canada, the best testing window is October through April. That is the period Health Canada generally describes as the heating season, though the exact timing can vary by region and climate.
If you want an even more practical answer, many homeowners will find that the most convenient time to begin a long-term test is sometime in October, November, December, or January. Starting during those months usually gives you an easy path to getting a full 91-day test completed during the colder season without having to worry about the test drifting too far into late spring.
For example, a test started in early November can usually run through January. A test started in December can run through February or March. A test started in January can still work well in much of Canada, depending on the local heating season. All of those are usually solid windows for homeowner testing.
That does not mean February, March, or even early April are wrong. It just means the earlier part of the heating season is often easier from a planning standpoint, especially if you want your full long-term test to stay well inside the preferred seasonal window.
Why fall and winter are preferred
Health Canada gives a very practical reason for preferring colder months. During the heating season, indoor radon levels are typically higher. That happens for two main reasons.
First, less radon escapes the home because people tend to keep windows and doors shut during colder weather. Second, more radon can enter the home from the soil because of what is known as the stack effect. As warmer indoor air rises and escapes from upper parts of the house, the lower levels of the home become depressurized. That lower pressure can pull more soil gas, including radon, into the home through cracks, joints, drains, gaps around pipes, and other openings near the foundation.
This is why Health Canada says testing during the heating season provides a conservative annual estimate. That phrase is important. It does not mean your home is only a radon problem in winter. It means testing during colder months is less likely to give you a falsely reassuring result that understates your typical annual exposure.
In plain language, winter testing is preferred because it gives homeowners a safer and more cautious estimate of what they are living with over time.
What Health Canada actually recommends
Canadian homeowners often hear simplified radon advice like “test in winter” or “do a three-month test,” but the official guidance is more specific than that.
Health Canada’s homeowner-facing materials say people should do a long-term test for 3 months, ideally during the fall or winter. Its more technical residential testing guide goes into even more detail. It says radon testing should take place over no less than 91 days and recommends conducting radon tests over a period of 3 to 12 months. It also says the test should be conducted during the heating season so that 91 days of the test occur during that season.
That means the real Canadian recommendation is not just “winter is best.” It is more accurately this: use a long-term test, make sure it lasts at least 91 days, and try to ensure that full period occurs during the heating season.
Health Canada also says homeowners should not alter their normal lifestyle during a long-term test. That is another point that sometimes gets missed. You do not need to live unnaturally or keep the house sealed in a strange way for three months. The point of the test is to measure radon under normal living conditions in the season that most conservatively reflects annual exposure.
Should you wait if it is spring or summer?
This is one of the most useful homeowner questions, and the honest answer is: it depends on why you are testing and how urgently you need an answer.
If it is late spring or summer and there is no urgent reason to test right now, waiting until fall is often the cleanest approach. That lets you run the kind of test Health Canada most clearly prefers.
But real life is not always that tidy. Sometimes homeowners are about to finish a basement. Sometimes they are buying a house. Sometimes they have just learned about radon and do not want to wait six months to do anything. In those cases, testing outside the heating season may still be better than doing nothing, but it comes with an important limitation: Health Canada says tests outside the heating season may underestimate the average annual radon level.
That does not make a spring or summer test useless. It means you should interpret it more cautiously. If the result is high, that is still meaningful and should not be ignored. If the result is comfortably low, you may still want to re-test during the next heating season to get the more conservative annual estimate that Canadian guidance is built around.
So if it is summer, the best practical advice is this: do not use the wrong season as an excuse to never test, but understand that fall or winter remains the better window for a definitive home decision.
Why long-term testing matters more than a quick test
When homeowners ask about the best time of year to test, what they often really mean is, “Can I get a trustworthy answer quickly?” In Canada, the official answer is mostly no.
Health Canada’s technical guide says testing shorter than 91 days is not recommended for informing mitigation decisions. It specifically states that short-term measurements are not appropriate for deciding whether a home exceeds the Canadian Radon Guideline or whether corrective action is necessary.
That is a major difference from the way some homeowners think about radon. They assume a two-day or one-week test can give them a final answer. In Canada, the preferred system is much more focused on long-term average exposure. That makes sense because radon is a long-term health issue, not a short-term emergency metric.
Longer tests smooth out the daily noise. A three-month or longer test is less likely to be thrown off by one cold snap, one stretch of open windows, one storm system, or one temporary change in how the home is used. That is why Health Canada says longer tests give a more accurate estimate.
So yes, the best season matters. But even more important than season is that the test is long enough to reflect real living conditions over time.
Where to place the test in your home
Timing and placement go together. Even if you test during the perfect month, you can still get a less useful result if the detector is in the wrong area.
Health Canada says all measurements should be made in a normal occupancy area of the lowest lived-in level of the home. A normal occupancy area is any area occupied by an individual for more than 4 hours per day. That could include a basement bedroom, family room, office, playroom, den, or main-floor bedroom depending on how the home is used.
This point matters because many homeowners think “lowest level” automatically means unfinished basement. That is not always correct in the Canadian system. If nobody regularly spends time in the unfinished basement, it may not be the right place for the detector. On the other hand, if the basement includes a home office, guest suite, playroom, or bedroom that gets regular use, then that lower level becomes the important one to test.
So the best seasonal test still needs to happen in the right space. Good timing cannot fix bad placement.
Practical timing advice for real homeowners
Sometimes broad advice becomes easier to follow when it is translated into common situations.
If it is September and you are thinking about testing:
Wait a few weeks and start in October if you can. That lines up well with the beginning of the typical Canadian heating season and makes it easier to complete a full long-term test under preferred conditions.
If it is November or December:
This is an excellent time to start. You are right in the sweet spot for the kind of long-term test Health Canada prefers.
If it is January:
Still a strong time to begin in many parts of Canada. Just make sure your full 91-day period still aligns with heating-season conditions in your area.
If it is March or early April:
This may still work depending on your location and how long heating-season conditions usually last. But it becomes more important to think about whether a full 91 days will truly occur during the relevant colder period.
If it is May through August:
This is generally not the preferred window for a definitive Canadian home test. If there is no urgency, waiting until fall is often better. If there is urgency, testing now may still be worthwhile, but it may be smart to plan a follow-up test during the next heating season.
If you just bought a home and want answers fast:
This is where radon timing becomes frustrating for some people. The best Canadian answer is still a long-term test during heating season. But if you need information sooner, you may end up doing an initial measurement now and then a more definitive long-term follow-up later.
Common mistakes people make about test timing
“Any weekend test should tell me enough.”
Not in the Canadian framework. Health Canada does not recommend short-term testing as the basis for mitigation decisions in homes.
“If I test in summer and get a low result, I am done forever.”
That is too simplistic. Health Canada says tests outside the heating season may underestimate the annual average radon level.
“The basement always has to be tested.”
Only if it is the lowest lived-in level or a normal occupancy area. An unfinished basement that is rarely used is not automatically the correct test location.
“Winter testing only matters in very cold provinces.”
Not really. Health Canada’s guidance is national, though it does note that exact heating-season timing can vary by location.
“I should keep windows shut in an unnatural way for three months.”
No. Health Canada says residents should not alter their lifestyle during a long-term radon test. The idea is to measure normal living conditions, not create artificial ones.
“If it is not the ideal month, I may as well do nothing.”
That is a mistake too. If timing is not ideal, it may still be worth testing, especially if there is urgency. Just understand the limitations and consider re-testing during the next heating season if needed.
Bottom line for homeowners
The best time of year to test for radon in Canada is during the heating season, usually October through April, with late fall and winter often being the best overall starting window.
The reason is straightforward. Health Canada wants homeowners to measure radon for at least 91 days during the part of the year when indoor levels are usually higher and more representative of a conservative annual estimate. Colder months create conditions that tend to trap more radon indoors and increase the pressure effect that pulls radon in from the soil.
So if you are planning ahead, the best move is simple. Start a long-term radon test in fall or winter, place it in the lowest lived-in area where people spend more than 4 hours per day, and let it run for at least three months. That is the Canadian gold standard for homeowner testing.
If it is not the ideal season right now, do not let that become an excuse to ignore radon altogether. But if you want the clearest and most Health Canada-aligned answer, the best time to test is still the heating season.