If you've ever had your home tested for radon and gotten a higher result in winter than in summer, you're not imagining things. Maine's cold climate creates conditions that consistently drive indoor radon levels higher during the heating season — sometimes significantly higher than warm-weather readings from the same home.
Understanding why this happens is useful, because it affects when you should test, how to interpret your results, and why a summer test that comes back "fine" can give you false confidence about your actual year-round exposure.
The Science: Why Cold Weather Increases Radon Entry
Radon is a naturally occurring radioactive gas that forms continuously in Maine's granite and schist bedrock as uranium decays. It migrates upward through soil and enters homes through cracks in foundations, gaps around pipe penetrations, sump pits, and other openings. The rate at which it enters your home — and how much accumulates — depends on several factors that are directly affected by temperature.
The Stack Effect
The most significant winter radon driver is something called the stack effect (or chimney effect). When your home is heated, warm indoor air rises and escapes through upper levels — around window frames, through exhaust vents, at the roofline. This rising warm air creates a slight negative pressure at the lower levels of your home, particularly in the basement and at the foundation.
That negative pressure acts like a pump. It draws soil gas — including radon — upward from beneath the foundation and into the living space. The greater the temperature difference between inside and outside, the stronger the stack effect. In Maine, where winter temperatures regularly drop into the teens and single digits, the temperature differential is substantial. The stack effect is most pronounced on the coldest days of the year — exactly when you're spending the most time indoors.
Closed-House Conditions
In summer, Maine homeowners typically open windows, run whole-house fans, and ventilate naturally. This dilutes radon accumulation significantly. In winter, homes are sealed — storm windows in, weatherstripping in place, doors kept closed. The same amount of radon entering the foundation has nowhere to go. It accumulates in basements and lower levels and migrates upward into living spaces.
This is also why the EPA requires radon testing to be conducted under closed-house conditions — to simulate the actual conditions under which radon builds up. A test conducted with windows open would produce artificially low results that don't reflect your real exposure during the months you're most at risk.
Frozen Ground and Soil Pressure
When the ground freezes in a Maine winter, the top layer of soil becomes less permeable. Radon that would normally escape through the surface instead gets redirected laterally — and often toward building foundations, which represent a path of least resistance. Frozen ground effectively increases the pressure of radon-laden soil gas seeking an exit point, and your foundation is often that exit point.
Heating Systems and Air Movement
Forced-air heating systems can contribute to radon concentration in ways homeowners don't always consider. Furnaces and air handlers that draw return air from basement areas can pull radon-laden air from the lowest parts of the home and distribute it throughout the living space. Combustion appliances — furnaces, water heaters, fireplaces — also consume household air and create additional negative pressure at lower levels.
Homes with wood stoves or fireplaces that draw combustion air from inside the house are particularly susceptible to this effect. Every time the fire draws air, it creates a slight depressurization that encourages radon entry.
How Much Higher Are Winter Radon Levels?
The EPA and radon researchers have documented that winter radon levels in the same home are typically 25–50% higher than summer levels, and sometimes more. A home that tests at 3 pCi/L in August might test at 4.5–5 pCi/L in January under the same measurement conditions.
This matters because the EPA action level is 4 pCi/L. A home that appears borderline safe in summer may be meaningfully above the action level during the months when occupants are home the most, the windows are sealed, and children are spending hours in lower-level play spaces.
When Is the Best Time to Test for Radon in Maine?
The EPA recommends testing during closed-house conditions for at least 12 hours before and during the test, which can be achieved in any season. However, for the most conservative and representative results, testing during the heating season — roughly October through April in Maine — captures conditions closer to your highest-exposure period.
Here's how to think about testing timing:
- Best time for a definitive test: Mid-winter (December through February). This captures peak stack effect, maximum closed-house conditions, and frozen-ground soil pressure simultaneously.
- Acceptable for real estate transactions: Any time of year under proper closed-house conditions, per EPA protocol.
- Summer testing caveat: A summer test that comes back at 2–3 pCi/L should be interpreted with the understanding that winter levels in the same home may be meaningfully higher. If you have any reason for concern — granite bedrock area, older slab or block foundation, Zone 1 county — consider re-testing in winter or opting for a long-term test that spans seasonal variation.
Long-Term vs. Short-Term Testing in Maine
Short-term tests (48–96 hours) are practical for real estate transactions and initial screening. But they capture conditions at a single point in time. A long-term test (90 days to 1 year, using an alpha track detector) gives you the most accurate picture of your actual annual average exposure by spanning multiple seasons and weather patterns.
For Maine homeowners who want the most meaningful number — rather than a snapshot — a long-term test deployed in fall and retrieved in spring captures both the high-radon heating season and the seasonal transition. This is the most defensible measurement if you're deciding whether mitigation is warranted in a borderline case.
Maine-Specific Radon Risk Factors
The winter amplification effect is relevant everywhere, but several factors make it particularly significant in Maine:
- Geology: Maine's granitic bedrock is high in uranium. Much of the state — particularly Oxford, Somerset, Kennebec, Androscoggin, and Franklin counties — is EPA Radon Zone 1, meaning average indoor levels are estimated above 4 pCi/L. The radon source is abundant.
- Cold winters: Maine's heating season is long and the temperature differentials that drive the stack effect are among the highest in the contiguous U.S.
- Housing stock: Maine has a high proportion of older homes with block foundations, stone foundations, and slab-on-grade construction — all of which provide more radon entry points than poured concrete slabs.
- Private wells: A significant portion of Maine homes use private well water. Radon in well water — particularly from granite aquifers — contributes to indoor radon levels through aeration during showering, cooking, and other water use. This is an additional source that is independent of soil entry and active year-round.
What to Do if Your Winter Radon Test Comes Back Elevated
If you test your Maine home this winter and get a result at or above 4 pCi/L, the appropriate response is straightforward:
- Don't panic. Elevated radon is common in Maine and represents a cumulative risk over years of exposure, not an immediate emergency. You have time to address it properly.
- Confirm with a second test if the result is borderline. If your result is between 3.5 and 5 pCi/L and you want to be sure before investing in mitigation, a second short-term test or a long-term test can provide additional certainty.
- Hire an NRPP-certified contractor. The National Radon Proficiency Program (NRPP) certifies radon mitigators. Certification ensures the contractor has demonstrated knowledge of proper system design and installation. Ask to see their NRPP credential number.
- Get post-mitigation testing. A professional installation should be followed by a post-mitigation test to confirm the system is working. Results should drop well below 4 pCi/L — in many Maine homes, properly designed systems achieve levels under 1 pCi/L.
Seasonal Testing: A Practical Recommendation for Maine Homeowners
Given everything above, here's a practical approach for Maine homeowners:
If you've never tested: Test now, regardless of season, using a professional short-term test or a long-term alpha track detector. Any result above 4 pCi/L warrants mitigation. Any result between 2 and 4 pCi/L warrants a follow-up test during winter conditions.
If you tested in summer and got a borderline result: Re-test during the heating season. Winter conditions in Maine can push a 2.5 pCi/L summer reading to 4+ pCi/L when the stack effect and closed-house conditions are both present.
If you have a mitigation system: Re-test every two years, or after any significant renovation, foundation work, or changes to your HVAC system. Systems are durable, but fan failures and changes to building pressure dynamics can affect performance.
If you're on a private well: Test both air and water radon. Well water radon testing is a separate test from air radon — the two sources are independent and both can be significant in Maine's granite geology regions. Learn more about radon in well water.
The Bottom Line
Winter is when radon matters most in Maine — and it's the best time to get a clear picture of your actual exposure. The combination of cold outdoor temperatures, sealed homes, frozen ground, and heating system dynamics consistently produces the highest indoor radon levels of the year. If you've been meaning to test your home, the heating season is the time to do it.
Central Maine Radon & Water Mitigation provides professional radon testing and mitigation throughout Maine. If you'd like to schedule testing or get a quote for a mitigation system, contact us here or call us at (207) 483-5637.