Why Is My Radon Higher in Winter?
Maine winter readings often run 30–50% higher than summer. It's not the test that's wrong — it's three real physical effects working against you all season.
Cold weather creates negative pressure inside heated homes, which actively pulls radon out of the soil and into your basement. Combine that with frozen ground (which seals normal escape routes) and closed windows (which prevents dilution), and a 4 pCi/L summer reading can easily become 7–10 pCi/L in January. Mitigation is the fix — not seasonal testing.
The Three Reasons Your Numbers Spike
1. Stack Effect
Heated air rises through your home and exits through the roof, attic bypasses, and upper-floor leaks. That creates a mild vacuum at the basement level, which actively sucks soil gas — including radon — through every crack, sump pit, and pipe penetration. The colder it gets outside, the stronger the stack effect. A January reading is essentially radon being pulled in, not just leaking in.
2. Frozen Ground
In summer, radon escapes upward through the soil into open air. When the ground freezes solid (and especially when it's snow-covered), that escape route is sealed. The radon has nowhere to go but laterally — and the warm air column under your home is the path of least resistance.
3. Closed-House Conditions
You're not opening windows in February. With the house sealed and the HRV either off or running at low speed, there's almost no fresh air coming in to dilute whatever radon does enter. Concentrations build up overnight and persist all day.
What the Seasonal Difference Looks Like
Real-world example from a typical central Maine home (poured basement, oil heat, no mitigation):
| Month | Outdoor temp (avg) | Typical reading | Above EPA action level? |
|---|---|---|---|
| July | 72°F | 3.8 pCi/L | No |
| October | 50°F | 5.2 pCi/L | Yes |
| January | 18°F | 7.4 pCi/L | Yes (significantly) |
| April | 42°F | 4.8 pCi/L | Yes |
A short-term test in July would have told this homeowner they were fine. The truth: their annual average is around 5.3 pCi/L, well above the 4 pCi/L action level.
What to Do About It
Re-test in winter
A short-term winter test (3–7 days, closed-house) gives you the realistic worst-case number. If summer was borderline (2–4 pCi/L), winter could put you well into action range.
Run an alpha-track 90+ days
An alpha-track or year-long continuous monitor averages out seasonal swings and gives you the true annual exposure — which is what actually matters for health risk.
Mitigate — don't wait for summer
Winter readings are real readings. A properly designed mitigation system reverses the stack effect and brings levels below 2 pCi/L year-round, even on the coldest days.
Got a High Winter Reading?
It's not noise — it's signal. Call us for a no-pressure quote, or use our cost calculator to get an instant estimate.
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