Why Radon Levels Spike in Winter (and What It Means for Testing)
The same home can test 80 Bq/m³ in July and 250 Bq/m³ in January. Same house. Same soil. Same family. The number nearly triples, and nothing about the house changed.
The season changed. In winter, your home pulls more radon up from the ground. That’s the winter radon spike, and it’s the single biggest reason a summer test can lie to you.
Here’s why it happens, how big the swing gets in Ontario, and the best months to test so you get a number you can trust.
The short version:
- Radon levels rise in winter, often 2 to 3 times higher than in summer.
- The cause is the stack effect. Warm indoor air rises, and that suction pulls soil gas in.
- Closed windows and a running furnace trap the radon inside.
- A summer test can read low and miss a real winter problem.
- The best months to test in Ontario are October through March.
- A 91-day long-term test spans the seasons and gives the truest number.
What is the winter radon spike?
Radon almost always reads higher in winter than in summer. Health Canada and Take Action on Radon both point to the colder months as the right time to test, for this exact reason.
Radon is a gas that seeps up from the soil under your home. It comes in through cracks, sumps, and gaps around pipes. How much gets in depends a lot on the pressure difference between your basement and the ground.
In summer, that pressure gap is small. Windows open. Air moves. Radon that comes in tends to leave.
In winter, the gap grows. Your house starts acting like a chimney. And a chimney pulls hard.
The stack effect, in plain English
Warm air rises. You knew that already. Your whole house works on it in winter.
Your furnace heats the air inside. That warm air floats up and leaks out the top of the house, through the attic, gaps, and vents. When air leaves the top, new air has to come in somewhere lower to replace it.
Guess where that make-up air comes from? The bottom. The basement. The soil.
This is the stack effect. The rising warm air creates suction at the base of your home. That suction reaches right into the ground and draws soil gas, including radon, straight through your foundation.
The colder it is outside, the harder your house pulls. A deep January cold snap widens the temperature gap between your warm basement and the frozen soil, and that’s when radon readings climb the highest.
Add sealed windows and a furnace running around the clock, and the radon has nowhere to go. It builds up in the air you breathe.
How big is the swing in Ontario?
Big enough to change your decision.
A home can read comfortably below the 200 Bq/m³ Canadian guideline in August and sit well above it in January.
That’s why the timing of your test matters as much as the test itself. Two facts drive the Ontario swing:
- Cold winters. Ontario gets a real freeze. The bigger the indoor-outdoor temperature gap, the stronger the stack effect.
- Tight modern homes. Newer builds are sealed for energy savings. Less natural air exchange means trapped radon stacks up faster.
Older drafty homes swing too, just for a different reason. They leak more air out the top, so they pull more make-up air in from the soil.
Either way, the January number is the one that reflects your real risk. You spend more time indoors in winter anyway, with the windows shut, so it’s also the air you actually breathe most.
Winter vs summer testing: the honest comparison
Not every test season is equal. Here’s how they stack up.
| Factor | Winter test (Oct to Mar) | Summer test (Apr to Sep) |
|---|---|---|
| Stack effect | Strong, pulls radon in | Weak, radon escapes |
| Windows | Closed, radon trapped | Often open, air exchanges |
| Reading vs true yearly average | Runs high (conservative) | Runs low (can mislead) |
| Risk of a false “all clear” | Low | High |
| Health Canada preference | Preferred season | Valid, but less reliable |
A winter reading errs on the safe side. If your home passes in January, it very likely passes all year. A summer pass is weaker proof, because the season was hiding the spike.
That doesn’t mean a summer test is useless. It just means a low summer number isn’t the final word. If you test in July and get a borderline result, retest in winter before you relax.
The best months to test in Ontario
Aim for October through March. That window catches the stack effect at full strength, so your reading reflects the worst case, which is exactly what you want to base a decision on.
November even has a name for it. It’s Radon Action Month in Canada, and the timing isn’t an accident. It lines up with the start of the high-radon season.
Here’s a simple way to think about timing:
- Best: December, January, February. Peak cold, peak stack effect.
- Good: October, November, March. Still cold enough for a solid reading.
- Okay for a first screen: April and September, shoulder months.
- Weakest: June, July, August. A low result here needs a winter retest to confirm.
If you can only test once, test in the cold. You’ll get the number that matters.
Why a long-term test beats the seasonal guessing game
There’s a way to skip the timing question entirely. Test for longer.
A short-term test is a snapshot of a few days. Its result swings with the weather that week. A long-term test runs for 91 days or more, so it spans different conditions and averages them out.
Health Canada calls the long-term test the best basis for deciding whether to fix your home. It reflects how you actually live across a season, not one lucky or unlucky week. If you start a 91-day test in the fall, it runs straight through the winter peak and captures it.
We break down both clocks in our guide to how long a radon test takes. And if you’re unsure what your final number even means, our post on what radon levels are safe walks through the ranges.
DIY vs hiring a pro: getting the season right
You can absolutely test for the winter spike yourself. The trick is doing it at the right time, the right way. Here’s the honest split.
When DIY makes sense
You own your home. You have no deadline. You just want to know.
Buy a test kit and run it in winter. A short-term charcoal kit works for a quick screen. A long-term alpha track kit, left out for 91 days starting in the fall, gives you a far better yearly picture for about $40.
The lab process is the same one a pro uses. The chemistry doesn’t care who set the kit down. What matters is that you test in the cold months and follow the closed-house rules: windows and outside doors shut for 12 hours before you start and during a short-term run.
When to book a pro instead
Some situations call for a certified test with a signed report.
- Buying or selling a home. Your closing date won’t wait for winter. A pro real estate test runs 48 to 72 hours under strict closed-house rules any time of year.
- Filing a Tarion warranty claim. Tarion needs a certified measurement, not a mail-in slip.
- Confirming a fix worked. The check-up test after a mitigation system goes in should be independent.
- A borderline DIY result. If your kit read close to the line, a professional retest removes the doubt.
Our residential test is $99 flat and follows Health Canada protocol on every job. You reach a co-founder, not a call centre. See our FAQ for how scheduling and reports work.
What to do with a high winter reading
A high number in January is not an emergency. It’s information. You have time.
Radon causes lung cancer only through years of exposure, not days.
The usual path is a mitigation system that vents soil gas out before it enters your home. It knocks levels down fast, often by 80% or more. Until then, small steps help a little: run an HRV if you have one, and don’t seal the basement up any tighter.
The real move is to test, then act on the number. Start with a quick quote and we’ll walk you through it.
Frequently asked questions
Why is radon higher in winter?
Your home heats up and the warm air rises and escapes near the top. That creates suction at the bottom, which pulls soil gas, including radon, in through the foundation. Sealed windows and a running furnace then trap it inside. This is the stack effect, and it’s strongest when it’s coldest outside.
How much does radon change between summer and winter?
It varies by home, but doubling from summer to winter is common, and tripling happens. A house that reads safely below 200 Bq/m³ in July can climb above the guideline in January without anything about the house changing.
When is the best time to test for radon in Ontario?
October through March, with the deep-cold months of December to February being ideal. That window catches the stack effect at full strength, so your reading reflects the worst case. November is Radon Action Month in Canada for this reason.
Can I trust a summer radon test?
A low summer result isn’t proof your home is safe, because the season hides the spike. A high summer result is worth acting on. If you test in summer and land near the line, retest in the winter before you decide.
Does a long-term test solve the seasonal problem?
Yes. A 91-day test spans changing conditions and averages them, so timing matters less. Start one in the fall and it runs right through the winter peak. Health Canada considers the long-term test the best basis for a mitigate-or-not decision.
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Authored by the Breathe Radon Free Team. Every claim is backed by a primary source. Full bibliography at /sources.
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