Winter vs. Summer Radon Levels: Does the Season Affect Your Results?
By Air Sense Environmental 29-07-2026 6
I test the same handful of homes multiple times a year sometimes, usually for research curiosity or a client who wants to track things closely, and the seasonal difference is genuinely one of the more interesting patterns I see on the job. Two tests, same house, same equipment, sometimes wildly different numbers depending on whether it's July or January. This comes up constantly across my radon testing St. Louis appointments, so let's dig into why that happens and how much it actually swings.
The Short Answer
Yes, season affects radon results, sometimes significantly. Winter readings tend to run higher than summer readings in the same home, often by a meaningful margin, because of how heating season changes airflow and pressure dynamics inside a house. This isn't a testing error or a fluke. It's a real physical phenomenon tied to how your home breathes differently depending on the weather outside.
The Mechanics Behind Seasonal Swings
The Stack Effect in Winter
Picture your house as a tall chimney. Warm air rises and escapes through the upper levels and roofline, and as it leaves, it creates a slight vacuum pulling air in from lower down, including through foundation cracks and gaps where soil gas sits waiting to get in. This effect, known as the stack effect, gets stronger the bigger the temperature difference between inside and outside, which is exactly what winter delivers. Your furnace runs harder, the temperature gap widens, and the vacuum effect pulling soil gas upward intensifies right along with it.
Why Summer Behaves Differently
In summer, that temperature gap between indoors and outdoors is smaller, so the stack effect weakens. Homes are also more likely to have windows open occasionally, even with central air running, and general ventilation patterns work less aggressively at pulling air upward from the foundation level. None of this means radon disappears in summer. It's still there, just moving through your home under gentler pressure dynamics.
How Big Is the Actual Difference?
Typical Percentage Swings I See in the Field
In my own testing across St. Louis area homes, I've seen winter readings run anywhere from twenty to eighty percent higher than summer readings in the same house, though the exact swing varies a lot depending on the home's specific construction and how tightly sealed it is. This pattern shows up consistently enough across radon testing St. Louis appointments that I mention it to nearly every client comparing results from different times of year. A loosely sealed older home might show less dramatic seasonal swing since air moves through it more freely year-round anyway. A newer, tightly sealed home built for energy efficiency can actually show a bigger seasonal gap, since winter sealing traps soil gas more effectively than a leakier structure would.
Why the Swing Isn't Identical From House to House
Foundation type plays a role too. Basement homes with a single open level tend to show more consistent seasonal patterns than homes with crawl spaces or mixed foundations, where airflow gets more complicated. This is part of why any radon mitigation St Louis technician worth hiring should ask about your home's construction before making assumptions about what your specific seasonal swing might look like. It's also why a radon mitigation St Louis quote should never be based purely on a generic seasonal average without seeing your actual home first.
What This Means for Interpreting Your Results
A Winter Reading Near the Action Level
If your winter test comes back right around 4.0 pCi/L, there's a decent chance your summer average sits somewhat lower, though not necessarily low enough to dismiss the winter number as irrelevant. Winter conditions represent real months you're actually living through, not some artificial worst-case scenario that doesn't matter. I generally tell clients in this position that the winter number still deserves a response, since you're not skipping winter every year. That's usually the point where scheduling radon mitigation St Louis installation makes the most sense, rather than waiting to see if a summer retest happens to come back lower.
A Summer Reading Near the Action Level
This one actually concerns me more in some ways. If a summer test already comes back close to 4.0 pCi/L, your winter number is likely higher, sometimes considerably so. A borderline summer reading is a signal to take the situation seriously rather than assume you're probably fine, since the season working in your favor during testing might be masking a bigger problem waiting for colder months.
Side-by-Side Data From Two Houses I Tested
I have two long-term clients whose homes I've tested across multiple seasons over the years, which gives a nice real-world comparison. One home in Kirkwood consistently shows around 2.3 pCi/L in summer and climbs to about 4.8 pCi/L in winter, roughly a hundred percent swing. Another home in Maplewood, older with a leakier foundation, shows less dramatic movement, 3.1 pCi/L in summer versus 3.9 pCi/L in winter, a smaller but still noticeable seasonal difference. Same general region, same testing methodology, meaningfully different seasonal behavior based entirely on how each house is built and sealed. It's a good illustration of why generic seasonal rules of thumb only get you so far, your specific home's construction matters as much as the calendar.
How to Account for Seasonality When You Test
If you want the most complete picture, a long-term test spanning ninety days to a year smooths out seasonal swings into a genuine average, rather than relying on any single snapshot. If you only have time for a short-term test, testing during winter gives you a more conservative, protective number to work from, since it captures closer to worst-case conditions. Either way, radon testing St. Louis homeowners run should be interpreted with season in mind rather than treated as a fixed, unchanging fact about the house. This is a distinction I raise with nearly every client scheduling a radon testing St. Louis appointment for the first time.
If you want help figuring out which testing approach fits your timeline and goals, our radon testing page covers short-term, long-term, and professional monitor options. And if any seasonal result, winter or summer, comes back elevated, our radon mitigation page explains what a properly installed system does to address the underlying source regardless of season.
Conclusion
Season genuinely affects radon results, sometimes dramatically, due to the stack effect and how differently your home behaves in winter versus summer heating and cooling patterns. The exact swing depends heavily on your home's specific construction, so there's no single universal percentage that applies to every house. What matters most is interpreting whatever result you get with the season in mind, a summer reading near the action level deserves real attention, not a pass just because it's warm outside. Radon mitigation St Louis installation addresses the underlying soil gas source regardless of which season prompted the test that found it, so once you know you have a problem, the season stops mattering for the fix itself. If you want to understand how your specific home behaves across seasons, reach out to Air Sense Environmental and I'll help you build an accurate picture rather than relying on a single snapshot.
FAQs
1. If my winter test comes back low, can I skip summer testing entirely?
Generally yes, a low winter result is a good sign since winter usually represents your home's higher end, though periodic retesting every couple of years is still a reasonable habit regardless of season.
2. Does radon mitigation St Louis installation perform differently depending on the season it was installed in?
No, once installed, a system works the same way year-round. The seasonal factor only affects testing and interpretation, not how the fan and suction system function mechanically. Any radon mitigation St Louis contractor should be able to install and service a system regardless of the time of year.
3. Is it worth paying for two separate tests, one winter and one summer, instead of one long-term test?
A long-term test spanning both seasons is usually more cost-effective and gives you a single reliable average, rather than paying for two separate short-term tests and having to interpret the gap yourself.
4. Do newer, energy-efficient homes always show bigger seasonal swings?
Often, but not always. Tighter sealing generally increases the stack effect's impact in winter, though other factors like foundation type and soil conditions also play a significant role.
5. Should real estate radon testing St. Louis transactions specifically request winter testing for the most conservative number?
It's not typically required or requested specifically, since transactions happen year-round on their own timelines, but buyers concerned about seasonal variation can reasonably ask about it or request a long-term follow-up radon testing St. Louis appointment after closing.
