When to Test for Radon: Seasons, Timing, and Triggers
Knowing when to test for radon matters almost as much as testing itself. A 48-hour kit deployed during an open-window summer afternoon can read 1.8 pCi/L. The same home with the same family during a closed-window January morning can read 7.2 pCi/L. Both readings are real. Neither is wrong. The difference is occupancy pattern, building pressurization, and stack-effect ventilation. This guide summarizes EPA and CDPHE guidance current as of 2026 and is not a substitute for professional radon measurement. Front Range homeowners — Colorado is EPA Zone 1, the highest radon risk tier — have the most reason to think carefully about timing.
The short answer: any time is better than never
EPA’s Citizen’s Guide to Radon recommends testing every home, full stop, and recommends winter testing when possible because closed-house conditions tend to produce conservative (higher) readings. A test that comes in below 2 pCi/L during winter closed-house conditions is reasonably trustworthy as a year-round picture. A test that comes in below 2 pCi/L during a summer with windows open most of the day tells you less, because summer occupancy patterns are not the dangerous ones. Lung exposure accumulates during sleeping hours, and most homes sleep with windows closed.
That said, a test deployed in any season is better than a test never deployed. CDPHE estimates roughly half of Colorado homes test above EPA’s 4 pCi/L action level. A summer test that comes back at 9 pCi/L is just as actionable as a winter test at the same number — the home needs mitigation either way. The danger of summer-only testing is the false negative: a test that reads below action level in summer but would have read well above it in winter. Repeat testing during the heating season catches that pattern.
Why winter testing reads higher
Stack effect drives the seasonal difference. Warm indoor air rises in winter, escapes through the upper floors of the home, and creates negative pressure at the lower levels. Negative pressure pulls soil gas — including radon — through foundation cracks, sump pits, plumbing penetrations, and the slab itself. Summer reverses the pattern partially: outdoor temperature is closer to indoor temperature, stack effect weakens, and open windows often vent the lowest levels of the home. The difference between a closed January house and an open July house can be a factor of 2 to 4 on radon concentration.
EPA’s protocol for real-estate transaction testing — a faster, more decisive test than the general screening test — specifies closed-house conditions for at least 12 hours before deployment and throughout the test. Windows and exterior doors stay closed except for normal entry and exit. HVAC operates normally. No whole-house fans, no attic fans, no continuous bath exhaust. These conditions approximate winter behavior even when the test runs in July, which is why summer real-estate tests can still be valid when properly performed.
Best testing seasons for Front Range homes
Late October through early April produces the most conservative radon readings in Colorado. Furnaces run, windows stay closed, and stack effect is at maximum. A passive 48-hour charcoal canister deployed in this window gives the most defensible result for a homeowner deciding whether to mitigate. April through October readings are still useful, especially when paired with closed-house conditions, but should be interpreted cautiously. A summer reading at 3.5 pCi/L should prompt a winter retest before a “no mitigation needed” decision.
Continuous radon monitors (CRMs) sidestep the seasonal question entirely by logging hourly concentrations over weeks or months. A homeowner with a CRM sees the seasonal pattern directly: levels climb in October, peak in January or February, and fall through April. The companion radon monitor for home guide walks through long-term monitoring options and what the data actually shows over a full year.
Life events that should trigger a test
Five situations warrant testing regardless of when the home was last measured.
Buying or selling a home
Real-estate transaction testing should follow EPA protocol — closed-house, 48-hour minimum, ideally by a certified measurement professional with chain-of-custody documentation. Buyers in Colorado almost always include radon in inspection contingencies; sellers benefit from pre-listing tests to avoid surprises.
After mitigation system installation
A post-mitigation test confirms the system is working. EPA recommends a follow-up test within 30 days of installation and annual retests thereafter. Sub-slab depressurization systems can lose performance from fan failure, blocked vent stacks, or seal degradation.
Major home renovation
Basement finishing, foundation work, slab penetrations for new plumbing, sump-pit modifications, and HVAC reconfiguration all change the radon pathway. Retest within six months of completion to confirm levels haven’t shifted.
New occupants moving in
A test from five years ago in a different occupancy pattern may not reflect current conditions. A new family with children sleeping in basement bedrooms warrants fresh measurement.
Two to five years since last test
EPA recommends retesting every two years even with no other triggers. Radon levels can drift as foundations settle, seals age, and soil moisture cycles change. A home that read 2 pCi/L in 2024 can read 5 pCi/L in 2026 without any visible change.
Real-estate transaction timing
Inspection contingencies typically run 7 to 14 days in Front Range purchase contracts. A 48-hour charcoal canister or 48-hour electret ion chamber test fits comfortably in that window, with two days for deployment, two days for the actual test, one to three days for laboratory analysis, and one day for report delivery. CRM testing is faster — results are available immediately when the monitor is retrieved. The decision framework at is radon testing really necessary walks through when buyers should insist on the test versus when sellers can reasonably push back. In Colorado the answer is almost always: insist.
Occupancy effects to know about
Testing in a vacant home produces lower readings than testing in an occupied home. The reason is HVAC behavior — an empty home with thermostats set to vacation mode runs less, which reduces airflow and theoretically allows more radon accumulation but also reduces the air mixing that distributes accumulated radon to breathing zones. EPA’s real-estate protocol assumes normal occupancy. Tests in vacant homes during transactions are technically valid under closed-house protocol but should be interpreted with awareness that the post-occupancy reality may differ.
Multi-level homes warrant testing on the lowest occupied level. A basement bedroom with regular overnight occupancy needs a basement test. A finished basement used only for storage warrants a first-floor test where the family actually sleeps. The radon test basement placement guide covers test location selection in detail.
Test type by timing window
Three test categories map to different timing situations.
Short-term screening (2 to 7 days)
48-to-96-hour charcoal canisters or electret ion chambers. Best for first-look screening and real-estate transactions. Results within a week. Vulnerable to short-term weather and ventilation anomalies.
Long-term measurement (90 days to 1 year)
Alpha-track detectors or continuous radon monitors. Best for confirming whether a screening result was a fluke and for understanding seasonal patterns. Most representative of true annual exposure.
Continuous monitoring (ongoing)
Permanent home CRMs log hourly readings indefinitely. Best for homes near or just above the 2 pCi/L EPA consideration level, where seasonal variation might cross the action threshold and the homeowner wants ongoing visibility.
When NOT to test
A handful of conditions reduce test validity enough to defer.
During severe weather. Major storms with rapidly changing barometric pressure can produce anomalous readings. EPA recommends waiting for stable weather.
Immediately after major HVAC service. A furnace just replaced or a duct system just modified hasn’t reached steady state. Wait two weeks.
During active foundation work. Cracks, sealing, or sump modifications change the radon entry pathway in real time. Wait until work completes.
In a home with windows that won’t stay closed for 48 hours. A short-term test under closed-house conditions cannot work if the conditions cannot be maintained.
References
- EPA Citizen’s Guide to Radon — U.S. Environmental Protection Agency
- CDPHE Radon Program — Colorado Department of Public Health and Environment
- CDC Radon Health Information — Centers for Disease Control and Prevention
- American Lung Association Radon Overview — American Lung Association
Front Range residents weighing whether to test now or wait for heating season can connect with a vetted radon measurement professional for advice tailored to your home’s situation.