The best time to test for radon in Colorado is during the fall or winter months, when homes are closed up and radon levels are naturally higher. This produces the most accurate and conservative reading of your home’s typical radon exposure. That said, radon testing is beneficial year-round, and there are specific situations, such as buying or selling a home or moving into a new property, when testing should happen regardless of the season.
Knowing when to test for radon matters because timing affects how accurate your result is. A test done in July with windows open can look very different from the same home tested in January. Here’s what drives those differences and how to get a reading you can actually rely on.
Why Does the Time of Year Affect Radon Levels?
Radon levels in a home aren’t fixed. They shift throughout the year based on how the home is ventilated and how air pressure behaves inside versus outside.
In warmer months, open windows and doors allow fresh air to dilute radon before it builds up. In colder months, homes seal up tight, and that trapped air gives radon nowhere to go. At the same time, temperature differences between indoor and outdoor air intensify the stack effect, which pulls soil gases upward through foundation cracks and into the living space more aggressively.
The result: radon levels in the same home can be two to three times higher in winter than in summer. A summer test with open windows may underreport your actual year-round risk in a way that gives false reassurance.
Why Fall and Winter Are the Best Times to Test for Radon
Testing during fall and winter gives you the most conservative, protective baseline. Here’s why that matters.
When a home is sealed for heating season, it simulates the conditions under which occupants are most exposed. Windows stay closed. Exhaust fans run less. The heating system creates negative pressure that draws more soil gas in from below. In Colorado specifically, the heating season is long, often stretching from October through April, which means “worst case” conditions apply for a significant portion of the year.
Colorado’s altitude and dry air also play a role. Homes at elevation tend to have greater temperature differentials between indoors and outdoors, which intensifies the stack effect even more than in lower-altitude states. If you want to know the highest radon level your home is likely to reach, test in winter.
For a deeper look at why seasonal conditions matter in Colorado specifically, our post on fall and winter radon testing walks through the data and what a cold-weather test typically reveals.
Is Spring or Summer Testing Still Worth Doing?
Yes, with a few caveats.
When Spring and Summer Testing Makes Sense
If you’re buying a home in June, you test in June. If you just moved in during August and the home has never been tested, you test now. A test done under less-than-ideal seasonal conditions is still far more useful than no test at all. It gives you a baseline, and if the result comes back elevated even in summer, that’s a strong signal that winter levels will be higher still.
How to Account for Seasonal Variability in Warm-Weather Results
The key is understanding what a warm-weather result means. A summer reading of 3.0 pCi/L in a home with open windows could represent a winter reading of 6.0 pCi/L or more under sealed conditions. If you test in spring or summer and get a result of 2.0 pCi/L or higher, consider following up with a winter test, or go ahead and mitigate. In Colorado, the conservative approach is almost always the right one.
When You Should Test Regardless of Season
Some situations call for testing no matter what month it is.
Before or During a Home Purchase
Inspection deadlines don’t wait for ideal testing windows. If you’re buying a home, test during the inspection period.
After Completing Home Renovations
Major renovations that affect the foundation, basement, or HVAC system can change how radon enters and circulates through a home. Test again after the work is done.
After Installing or Modifying a Mitigation System
A post-mitigation test confirms the system is working. This isn’t optional; it’s the only way to verify the installation was effective.
If Your Home Has Never Been Tested
In Colorado, this is urgent regardless of season. Roughly half of homes in the state test above the EPA’s 4.0 pCi/L action level. Not knowing your level isn’t a neutral position.
Every Two Years as Routine Monitoring
Radon levels shift over time as homes settle and conditions change. A biennial retest is the standard recommendation, even in homes with mitigation systems in place.
How Closed-House Conditions Affect Test Accuracy
For a radon test to produce a valid result, the home needs to maintain closed-house conditions. Windows and exterior doors should be kept closed for at least 12 hours before testing begins and throughout the full test period except for brief, normal entry and exit.
This is straightforward in winter, when windows are already closed. In summer, it requires more deliberate effort. In Colorado, where summer weather can swing from 90°F afternoons to 50°F evenings, the temptation to open windows overnight is real. Even a few hours of open windows can significantly lower the radon reading and produce a result that underestimates actual risk.
If you’re testing in warmer months, plan the test period carefully and communicate closed-house requirements clearly to everyone in the home before the test begins.
Short-Term vs. Long-Term Testing and Seasonal Timing
Short-term tests run for 48 to 96 hours and are the standard for real estate transactions. They’re fast and practical, but they capture only a narrow window of conditions. A long-term test, which runs for 90 days or more, captures seasonal variation naturally and gives a far more accurate picture of annual average exposure.
For homeowners who aren’t under a transaction deadline, a long-term test started in fall, running through the heart of heating season, gives the most complete and representative result. For time-sensitive situations, a properly conducted short-term test in winter is the next best option.
Understanding how long radon tests take and what each test type measures helps you choose the right approach for your situation. A licensed professional can also advise on which test type makes sense given the time of year and the reason you’re testing.
Test for Radon This Season With Better Colorado Radon
Fall and winter are the best times to test, but the right time to test is whenever you’re ready to know. In Colorado, where elevated radon is common and the heating season is long, there’s no bad time to get an accurate result. There is only waiting longer than you should.
Better Colorado Radon provides professional radon testing for Colorado homeowners year-round, with results and documentation you can act on with confidence.
Book Today to schedule your radon test with Better Colorado Radon.
