Radon levels in your home are not constant — they fluctuate throughout the year based on temperature, barometric pressure, soil conditions, and how you use your home. This raises a natural question: does the timing of your test matter? And is there a best time of year to test?
The short answer: winter is the most informative time to test, but any time is better than not testing at all.
Why Radon Levels Fluctuate
Indoor radon concentrations are driven by two competing forces: how much radon enters your home from the soil below, and how much escapes through ventilation. Several seasonal factors tip this balance:
- Temperature differential (stack effect). In winter, warm indoor air rises and escapes through upper floors, creating negative pressure at lower levels. This draws more radon-bearing soil gas in through foundation cracks and openings. The colder the outside temperature relative to inside, the stronger this stack effect.
- Windows and ventilation. In summer, open windows dilute indoor radon continuously. In winter, homes are sealed — radon accumulates rather than escaping.
- Barometric pressure. Low-pressure weather systems reduce pressure on the soil surface, allowing more radon to migrate upward. High pressure suppresses this movement. These effects are short-term but can cause significant day-to-day variation.
- Frozen ground. In prairie and northern climates, frozen soil can sometimes reduce radon entry by sealing surface pathways. This is highly localized and unpredictable.
- Soil moisture. Saturated soil after heavy rain can temporarily block radon pathways, causing a dip in indoor levels. Dry conditions often see the reverse.
When Are Radon Levels Highest?
For most Canadian homes, radon levels are highest in winter (November through March) and lowest in summer (June through August). The difference between winter and summer peaks can be a factor of 2–3× in some homes.
This matters for testing because a short-term test conducted in summer may significantly underestimate your home's true average annual exposure.
The Best Time to Test: Winter
The ideal window for a long-term alpha track test is November through March. Testing during this period captures:
- The season of highest radon accumulation
- Typical closed-window, high-occupancy conditions
- The greatest stack effect (most conservative, protective estimate)
A test started in November and completed in February captures the worst-case period. Even if your annual average is somewhat lower, knowing your winter peak gives you the most actionable safety information.
What About Long-Term Tests? Does Season Matter Less?
Yes — the longer your test runs, the less sensitive it is to seasonal variation. A 12-month test captures a full seasonal cycle and produces the most accurate annual average. A 90-day winter test is nearly as reliable and is the standard used by C-NRPP certified professionals for post-mitigation confirmation testing.
Labs typically apply a seasonal correction factor to shorter tests, adjusting the result to estimate what the annual average would be. This correction is based on regional averages and may not perfectly reflect your specific home — which is another reason to test for as long as practical.
When Not to Test
Avoid testing during periods of unusual home conditions that don't represent normal occupancy:
- While doing major renovations that affect the foundation or basement
- During extended periods with windows and doors left open
- When the home is unoccupied for weeks at a time
- Immediately after sealing foundation cracks (wait 30+ days for conditions to stabilize)
Real estate tests are sometimes conducted under "closed house conditions" — all windows and exterior doors kept closed for 12 hours before and during the test. This artificially elevates the reading and is designed to capture worst-case exposure, not everyday conditions. Understand what conditions your test was conducted under before interpreting the result.
Can I Test in Summer?
Yes — a summer test is far better than no test. If your summer result comes back above 200 Bq/m³, your winter levels are almost certainly significantly higher, and immediate action is warranted. If your summer result is below 100 Bq/m³, consider following up with a winter long-term test to confirm your annual average.
How Often Should You Retest?
- Every 2–5 years in a home that has previously tested below 100 Bq/m³
- After any significant renovations, particularly basement work or HVAC changes
- After moving into a new home, regardless of prior test results
- After radon mitigation, to confirm system effectiveness (typically 30–90 days post-installation)
Ready to test this winter? Our Radtrak³ Long Term Alpha Track Test Kit is the same independently lab-analyzed detector used by home inspectors and mitigation professionals across Canada. Place it today and return it to the lab at the end of your test period using the included pre-addressed mailer.
