Every year, radon inquiries follow a predictable pattern: they climb through October, peak across the winter, and quiet down by early summer. It’s tempting to read that purely as behavioural — people simply think about indoor air quality more once the windows are shut. But there’s a real physical mechanism behind the pattern too, and it’s worth understanding before this year’s heating season starts: the stack effect.
What the Stack Effect Actually Is
Warm air rises. Every home behaves a little like a chimney — some more than others, depending on height and construction. As furnace-heated air rises and escapes through upper-level gaps, attic penetrations, and exhaust systems, it draws replacement air in from somewhere lower in the structure. In summer, with windows open and only a small difference between indoor and outdoor temperature, this effect is minor and easy to ignore.
Once Calgary’s outdoor temperature drops well below indoor temperature — which starts well before the first snowfall — the pressure differential driving the stack effect increases substantially, and it stays elevated for months rather than days.
How That Pressure Differential Pulls Radon Indoors
The replacement air drawn in during the stack effect doesn’t only enter through obvious gaps like doors and windows. Basements and foundations sit at the bottom of that pressure gradient, and it’s precisely there — through sump pits, slab cracks, cold joints, and utility penetrations — that soil gas, including radon, gets pulled in fastest.
A home doesn’t need a reputation for being “leaky” for this to matter. Even well-built, well-sealed homes rely on replacement air coming from somewhere, and a depressurized basement is very often exactly where that path leads.
Why Summer Test Numbers Can Be Misleading
This is precisely why Health Canada recommends long-term testing that spans fall and winter rather than relying on a summer reading. A test conducted with windows open, doors propped, and minimal furnace operation can understate a home’s actual year-round average considerably.
A homeowner who tests in July and gets a comfortable, reassuring number may be looking at a very different reality by January — not because anything in the home changed, but because the conditions producing the reading changed completely. A single summer reading, used on its own, tells you about summer. It doesn’t tell you about your actual annual exposure.
What Changes Specifically in a Calgary Home
Calgary’s climate adds a few local wrinkles worth knowing about directly:
- Furnace run-time increases sharply from October through March, increasing the volume of air continuously being drawn through the stack-effect cycle.
- Bathroom fans, range hoods, and clothes dryers all exhaust air out of the home — each one adds to the negative pressure a basement experiences.
- Chinook winds can cause rapid, dramatic swings in outdoor temperature and barometric pressure within a single day, occasionally spiking radon entry beyond what a steady cold snap alone would produce.
- Homes sealed up further for winter — weatherstripping, storm windows, plastic sheeting over older windows — reduce the make-up air available from upper floors, which can shift more of that replacement-air draw toward the basement.
What This Means for Timing a Test
If the plan is to test this fall or winter — which it should be, per Health Canada guidance — earlier is better than later. A long-term test needs a minimum of three months to produce a reliable average, and results are most useful when they’re already in hand before major heating-season decisions come up: holiday guests staying in a basement bedroom, a planned basement development project, or a home sale process already underway.
Starting a test in September or October puts a homeowner in a position to have real, usable numbers by the time winter is in full swing — rather than waiting until January to start the clock and not having answers until spring.
What This Means for Homes With an Existing Mitigation System
The stack effect doesn’t only matter for untested homes. A previously installed mitigation system that was verified as effective under summer test conditions is working against a smaller pressure differential than it will face in January.
This is one of the reasons post-mitigation verification testing matters on an ongoing basis, not only once at installation. A system that performed well on a June test should be expected to perform well on a January test too — but “expected” and “confirmed” are two different things, and only re-testing closes that gap.
The Short Version
Radon doesn’t rise in the winter by coincidence, and it isn’t just a matter of people paying more attention. The stack effect is a real, physical, and highly predictable mechanism — and understanding it is the difference between testing on a schedule that reflects how your home actually behaves, versus testing on a schedule that happens to be convenient.
FAQ's
A: The stack effect increases as the temperature difference between indoors and outdoors grows. This creates a stronger pressure differential that pulls more soil gas, including radon, into a home through the foundation.
A: No. Every home experiences some degree of stack effect — it’s a function of physics (warm air rising), not construction quality. Even new, well-sealed homes are subject to it.
A: Health Canada recommends a long-term test, a minimum of three months, conducted during fall and winter, when the stack effect and closed-house conditions produce the most representative year-round reading.
A: Yes. A system verified as effective under summer conditions faces a larger pressure differential in winter. Periodic re-testing confirms the system continues performing as conditions shift seasonally.
Want a Calm Second Opinion?
If you haven’t tested yet this year, now is the right time to start — before the stack effect kicks into high gear.
More information is available online, or call or text (403) 614-4094. No guessing, no pressure, no surprises once the work starts.