Is Radon Worse in Winter? Seasonal Variation Explained

Why Radon Is Higher in Winter

Two physical effects combine to raise radon levels in winter:

The Stack Effect

Warm air rises. In winter, the temperature difference between your heated indoor air and cold outdoor air is large — sometimes 50–70°F. This creates a strong convective draft called the stack effect: warm air exits through upper-level openings (attic, upper windows, exhaust vents), and replacement air is drawn in from below — including radon from soil gas entering through the foundation.

The stronger the stack effect, the more radon gets pulled in. In extreme cold climates (Minnesota, Wisconsin, North Dakota), winter stack effect is significantly stronger than summer, and radon levels can be 50–100% higher in January than in July.

Tighter Building Operation

In summer, windows open, doors are used more, and there's more natural ventilation. In winter, homes run sealed — windows closed, every penetration weatherstripped. Less natural ventilation means radon that enters accumulates to higher concentrations instead of being diluted.

Modern energy-efficient homes amplify this effect. Tight construction reduces air infiltration — which improves energy efficiency but also means radon has fewer pathways out.

How Much Does It Vary?

Seasonal variation differs by home and climate, but typical ranges:

A home that tests at 3.5 pCi/L in August might test at 5.5–6.0 pCi/L in January in Minnesota. That's a below-action-level summer reading and a clearly elevated winter reading. Testing only in summer would give a false sense of security.

When to Test

Best time: October through March (heating season). This captures the highest typical exposure period and gives the most conservative, representative result. The EPA protocol requires windows to be closed for at least 12 hours before and during the test.

Summer testing: Valid and useful. If you test in August and come back above 4.0 pCi/L, you should mitigate — winter levels will be higher. If August results are below 2.0 pCi/L in a mild climate, you're likely fine year-round. The gray zone is 2.0–4.0 pCi/L in summer — that might be 3.0–6.0 pCi/L in winter, straddling the action level.

Real estate transactions: Testing during a home purchase often happens at any time of year. A summer test showing 3.0 pCi/L in Minnesota should be interpreted conservatively — winter levels are likely higher. Some buyers request a credit or mitigation even at sub-4.0 pCi/L summer results in high-radon states for this reason.

Does Mitigation Performance Vary Seasonally?

No — or only minimally. A properly installed sub-slab depressurization system maintains consistent negative pressure year-round regardless of outdoor temperature or building operation mode. The fan runs continuously and compensates for seasonal stack effect changes automatically.

Post-mitigation levels typically show less seasonal variation than pre-mitigation levels because the system is the dominant pressure driver, not the weather.

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