Why winter is the best time to test for radon

Why does Health Canada tie a radon test to the heating season?

Because that is when indoor radon tends to peak, and a test should not miss the peak. Health Canada's measurement guide wants no fewer than 91 days of a test to sit inside the heating season, the part of the year when windows stay closed and indoor radon tends to be highestsource. Its advice, as quoted in the Canadian mitigation standard, is a test of at least three months, ideally run between October and Aprilsource.

The reasoning is cautious on purpose. A decision about a house ought to rest on the months in which the level runs high, not on the months that flatter it. A winter average tells an owner more than a summer one does, whichever side of the guideline it lands on.

Calgary makes the rule easy to follow. The furnace here works from fall well into spring, so the window is wide and there is no narrow date to hit. What matters is that the detector is out while the heat is on and the windows are mostly shut.

What does cold weather do to the air inside a house?

It makes the building behave like a slow chimney. Health Canada explains that indoor air pressure is usually lower than the pressure in the soil, which draws radon in through foundation cracks, construction joints, gaps around service pipes, floor drains and sumps, and that the stack effect increases this entry in cold monthssource.

The stack effect is easy to picture once it has a name. Heated air is light, so it climbs through the storeys and leaks out high up. Something has to take its place, and part of that replacement air arrives through the openings at the very bottom of the building, where the floor meets the ground. Closed windows complete the picture, because less outdoor air passes through the rooms to mix with whatever has come in below.

None of this points to a fault in the house. Every heated building does it to some extent, whether it was built last year or decades ago. It is simply the reason the measurement is scheduled for the cold months, and it is not something a visual inspection can find or rule out.

When should a 91-day test start in Calgary?

Start once the furnace is running regularly, early enough that three full months of heating still lie ahead. Health Canada's recommended test length is 3 to 12 months, and 91 days is the shortest it acceptssource. Count forward from the day the detector is set out, and write the earliest end date somewhere you will see it.

Dates and temperatures are missing from that table on purpose. The rule is written in terms of the heating season, and in any given year that is settled by when your own furnace is working, not by a fixed day on the calendar. If you are unsure whether a late start still fits, the safe reading of the rule is to leave the detector out longer.

The mechanics of setting a detector out, from choosing the room to recording the serial number, are covered in our walk-through of a do-it-yourself radon test.

Is it worth running the test longer than 91 days?

Yes, when you can spare the time, because the upper end of the recommended range is a full year. A longer test takes in more of the year and so sits closer to the annual average that the guideline is written around. The minimum exists so that a household can have an answer within one season. It is a floor, not a target.

Going the other way is not an option Health Canada supports: it does not recommend testing for any period under 91 dayssource. Ending a test early to have the number before a trip, a renovation or a family decision gives up the one property that makes the number usable. If the timing is awkward, extend the test instead of trimming it.

There is a practical ceiling as well. A detector left out and forgotten is of no use to anyone, and each kit comes with its own instructions on how long it may stay open. Pick an end date at the start, inside the range Health Canada gives and inside what the kit allows, and keep to it.

What does a short reading in summer tell you?

It tells you what the level was in that room on those days, with the house as it was then. That is real information, and it is narrower than it looks.

• What it does tell you: the instrument found that much radon while it ran. A summer reading far above the guideline is worth a conversation with a professional without waiting for winter.
• What it does not tell you: the annual average. A low figure from a warm week with the windows open says little about the same rooms closed up and heated.
• What it cannot stand in for: the long-term test, which remains the measurement the guideline refers to in every season.

The Canadian mitigation standard describes short-term tests as typically lasting between 2 and 7 dayssource, and Health Canada's guide does not recommend any test shorter than the 91-day minimumsource. Set the two lengths side by side and the gap explains itself. If a summer reading is all you have, keep it on file and start the long test when the heating comes back on. Our guide to reading a radon report shows how to weigh a short number against a long one.

Does the weather during the test change the result?

Not in a way you need to manage. A three-month test spans the ordinary swings in the weather that any winter brings, and that is the whole purpose of making it long. Cold weeks, mild weeks, windy days and still ones all land inside the same average.

We have no sourced figure for what a particular kind of weather does to indoor radon in Calgary, so this page does not offer one. What the guidance does provide is a method that works without knowing: measure for long enough that no single week decides the answer.

Two habits follow from that. There is no reason to wait for a "typical" stretch of winter before starting, because the test will be over before anyone can say whether the season was typical. And there is no reason to throw out a finished test because the winter felt unusual. An odd winter is still a winter the house was lived in.

What if the house is empty over the holidays?

Leave the detector where it is and let the test carry on. Time away is part of how a household uses its home over a winter, and a long-term test is meant to describe the home as it is really lived in.

Two practical points apply. Do not tuck the detector into a drawer or a box while you travel, because moving it breaks the record. And add the dates of the absence to your notes, along with anything else that was different, such as the thermostat being turned down. If the trip covers a large share of the test, the simplest fix is to let the detector stay out after you return, since the recommended length extends far beyond the minimum.

The same thinking covers the opposite case. A full house over the holidays, with guests in the basement bedroom and the door opening more often, is an ordinary winter too. Tell visitors what the small device on the shelf is, and ask them to leave it alone.

Does a fireplace or wood stove affect a winter test?

Use it as you normally would, and keep the detector well away from it. Health Canada's placement rules put the detector in a regularly occupied room on the lowest lived-in level, about 30 cm or more above the floor, and away from heat sources, vents and direct sunsource. A hearth or a stove is a heat source, so the detector belongs elsewhere in that room, or in another occupied room on the same level.

Beyond placement, we have no sourced statement on how much a fire changes a reading, and we will not guess. The principle from the rest of this page still applies: the test should capture your usual winter. If the fireplace is lit most evenings, that is your usual winter. Do not begin lighting it, or stop, on account of the test.

What if a renovation is planned for this winter?

Test the house as it stands when the work is still a season away, and plan a second test for afterwards. Health Canada recommends testing again after any renovation that affects the structure or ventilation of a home; the same guidance calls for a test once a mitigation system is installed and a retest every 5 years while an active system is in servicesource.

A basement development, new windows, an addition or a replacement furnace can all fit that description. When the work will begin partway through the measuring period, the cleaner choice is usually to wait and run the whole test in the first heating season after the trades have left, in the room where people will then spend their time. A result from a layout that no longer exists answers a question nobody is asking any more.

When the work is further off, do not hold the test back for it. A result this winter tells you whether radon reduction should be part of the renovation plan, which is far easier to arrange before the walls are closed than after.

What should you do when the winter result arrives?

Compare the average on the report with the Canadian guideline. That guideline is 200 Bq/m³, and Health Canada recommends corrective action when a home's annual average is above itsource. Above that level its advice is to mitigate within one year, and sooner the higher the resultsource.

Under 200 Bq/m³ the recommendation to act does not reach your result, and the decision passes to you as the owner. No level is risk-free and lower is better, so choosing to bring down a moderate winter average is a reasonable course as well.

The radon result checker places your figure on the scale in either unit. If you have not started yet, our overview of radon testing options in Calgary compares the routes, and a long-term winter test placed and reported for you is the service to ask about if you would prefer not to manage the detector yourself.