Yes, indoor radon levels typically increase during winter months, often significantly. Research across multiple countries consistently shows that winter produces the highest indoor radon concentrations of any season, while summer produces the lowest. In some studies, average winter radon levels were nearly three times higher than summer levels in the same buildings.
Why Radon Levels Rise in Cold Weather
The primary driver is something called the stack effect. Air pressure inside your home is usually lower than the pressure in the soil surrounding your foundation. When you heat your home in winter, warm air rises and escapes through the upper levels, creating a slight vacuum at the lower levels. Your house essentially pulls air from the path of least resistance: the soil beneath and around your foundation. If that soil contains radon, the gas gets drawn in through cracks, gaps around pipes, sump pits, and other openings in the foundation.
The bigger the temperature difference between indoors and outdoors, the stronger this suction effect becomes. On a bitter January night with your furnace running, the pressure differential is far greater than on a mild October evening. That’s why the coldest months tend to produce the highest readings.
Frozen Ground Funnels Radon Toward Your Home
Cold weather also changes how radon moves through soil. Frozen ground acts like a cap, preventing radon from escaping naturally through the soil surface into the outdoor air. Uranium in the soil continues breaking down and producing radon regardless of temperature, but that gas has fewer paths to the surface when the ground is frozen or covered in snow. The warmth of a heated foundation keeps the soil directly beneath your home unfrozen, creating a preferential pathway. Radon migrates laterally through the soil toward the warm, unfrozen zone under your house, concentrating right where the stack effect can pull it inside.
Sealed Homes Trap Radon Indoors
Winter habits compound the problem. You keep windows and doors shut. You may have weatherstripped, caulked, or otherwise sealed your home to save on heating costs. All of this reduces the air exchange rate, meaning less fresh outdoor air dilutes the radon that enters. The same volume of radon gas accumulates in a smaller effective air volume, pushing concentrations higher.
This effect is becoming more pronounced as homes get more energy efficient. Research on renovated buildings found that energy-efficiency upgrades increased indoor radon levels by 22 to 120% on average. One set of repeat measurements taken roughly 20 years apart in energy-efficiently renovated buildings showed radon concentrations two to five times higher than before the renovations. In Romania, newly built homes meeting modern energy-saving standards had radon levels 27% higher than older, draftier traditional houses. Without mechanical ventilation to compensate, a tighter building envelope directly translates to higher radon accumulation, especially during seasons when you keep everything sealed up.
How Much Higher Winter Levels Actually Get
The size of the seasonal swing varies by location and building type, but the pattern is remarkably consistent. A review of indoor radon studies compiled data from multiple countries and found winter maximums across the board. In one study of shops in Jordan, average winter radon was 45 Bq/m³ compared to about 17 Bq/m³ in summer, roughly a 2.7x increase. Measurements in homes in northern India showed winter averages of 65 Bq/m³ versus 40 Bq/m³ in summer. Another Indian study found winter averages of 197 Bq/m³ compared to 141 Bq/m³ in summer, with some individual readings reaching 249 Bq/m³ in the cold months.
The ratio varies, but a home that tests at 2 pCi/L in summer could plausibly test at 4 to 6 pCi/L in winter. That’s the difference between a level that seems fine and one that crosses the EPA’s action threshold.
What This Means for Testing
The EPA recommends fixing your home if radon levels reach 4 pCi/L (150 Bq/m³) or more, and suggests considering action even between 2 and 4 pCi/L, since there is no known safe level of radon exposure. Because winter produces peak concentrations, testing during the colder months gives you a picture of your worst-case exposure.
For an accurate winter test, your home needs to maintain “closed house conditions.” Keep all exterior doors and windows closed for at least 12 hours before you begin testing, and keep them closed for the entire testing period. Normal entry and exit through doors is fine. You don’t need to seal your home beyond what’s typical for cold weather, just avoid opening windows or propping doors for extended periods during the test.
If you tested in summer and got a borderline result (say, 2 to 3 pCi/L), retesting in winter is worth doing. You may find your levels are well above the action threshold during the months when you spend the most time indoors with the least ventilation.
Winter Risks for Existing Mitigation Systems
If you already have a radon mitigation system, winter introduces its own problems. Most residential systems work by actively pulling air from beneath the foundation slab and venting it outside through a pipe. Moisture vapor from the soil gets pulled up along with radon, and in cold weather, that moisture can freeze inside the vent pipe. Ice buildup is especially common in northern climates on vent pipe runs longer than 10 feet or when metal downspout material was used instead of PVC pipe, since metal offers less insulation.
Typically, condensation forms just above the fan and drips back down. But when temperatures stay consistently below freezing, that condensation can turn into an ice blockage that partially or fully clogs the vent. A blocked system can’t depressurize the soil effectively, which means radon levels inside your home can spike back up precisely when they’re already at their seasonal worst. If you have a mitigation system, check your manometer (the small U-tube gauge on the pipe) periodically during cold snaps. A change in the fluid levels can indicate a blockage. Most ice jams are temporary and clear on their own when temperatures rise, but persistent blockages may need attention from a radon professional.
Simple Steps to Reduce Winter Radon
If you don’t have a mitigation system and your levels are elevated, opening a window on the lowest occupied level for even a few minutes a day can help reduce concentrations by improving air exchange. This is a temporary measure, not a fix, but it illustrates how much sealed conditions contribute to the problem. Sealing visible cracks in your foundation can reduce the entry points, though this alone rarely solves a radon issue since the gas finds its way through openings too small to see.
For homes that consistently test above 4 pCi/L, an active soil depressurization system is the standard solution. These systems reverse the pressure differential that drives radon indoors, pulling the gas from beneath the slab and venting it safely above the roofline. They’re effective year-round, reducing radon levels by up to 99% in most homes, and they run continuously to counteract exactly the kind of seasonal pressure changes that make winter so problematic.