Does Radon Change With the Seasons? Why Winter Is the Best Time to Test
A radon test is a snapshot. It tells you what the level was during the days the detector was sitting there — not what it is every day of the year. And radon is not a steady number. It moves with the weather, the seasons, and how you're running your house. That's worth understanding before you read too much into a single result.
The stack effect is the whole story
Radon is a gas that seeps up out of the soil and finds its way in through cracks in the foundation, gaps around pipes, sump pits, and the pores of the concrete itself. What pulls it in is pressure. Warm air inside your house rises and escapes out the upper floors, and that creates a slight vacuum down low that draws replacement air — and soil gas — in through the basement. Building scientists call it the stack effect, and it's the same principle that makes a chimney draw.
In winter that effect goes into overdrive. The bigger the temperature difference between the warm inside and the cold outside, the harder the house pulls from below. Add a running furnace and a house sealed up tight against the cold, and you've built a machine for drawing radon up out of the ground and holding it inside. That's why levels tend to run highest in the coldest months.
What makes levels swing day to day
- Temperature swings — the colder it is outside relative to inside, the stronger the pull.
- Barometric pressure — a falling barometer ahead of a storm lets more gas rise out of the soil.
- Wind — strong wind can pressurize one side of a house and pull gas in on the other.
- Frozen or snow-covered ground — it caps the soil like a lid, so more gas takes the path of least resistance: up into your basement.
- How you run the house — closed windows, running heat, and exhaust fans all shift the pressure balance.
So when should you test?
Because winter is usually the worst case, a short-term test during the heating season gives you a conservative, realistic look at your exposure — if the number is fine in January, it's likely fine most of the year. That's why many real-estate and radon programs favor cold-weather testing. Keep the house closed up as usual ("closed-house conditions") for at least 12 hours before and during a short-term test so you're measuring a real lived-in scenario, not an aired-out one.
If you test in summer with the windows open and get a low number, don't assume you're in the clear — you may just have caught radon at its seasonal low. The most trustworthy answer of all comes from a long-term test (more than 90 days) that spans both warm and cold months and averages out the swings. If a quick winter test lands anywhere near the EPA's 4.0 pCi/L action level, a long-term test is the smart follow-up.
| Test type | Duration | Best for |
|---|---|---|
| Short-term | 2–7 days | A fast first look — do it in winter for a conservative read |
| Long-term | 90+ days | Your true year-round average across seasons |
Joe tested his basement one July with the windows cracked and the whole-house fan humming, got a tidy 2.1, and filed it under "handled." Come February he was spending evenings in that same finished basement watching TV with the furnace roaring and every window sealed. He never retested. Had he, the winter number would have told a very different story. Joe — the septic-ignoring, well-neglecting legend himself — proved that a low summer reading and closed-house winter living are two different worlds.
The short version
- Radon isn't a fixed number — it rises and falls with weather and season.
- Winter usually peaks: cold outside plus a sealed, heated house pulls the most soil gas in.
- A short-term winter test under closed-house conditions gives a conservative read.
- A long-term (90+ day) test is the most reliable, because it averages the whole year.
The EPA's radon FAQ covers the test types, the 4.0 pCi/L action level, and how often to test and retest: EPA: Radon Frequently Asked Questions. Test once in the cold months, and you'll know whether the number you got is the good news it looks like.