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Fall is the right time to test for radon in Denver — specifically September through early November, before homes seal up for winter. Closed windows and heating systems pull more radon indoors, which makes winter readings higher and harder to act on quickly.
Why Does Denver Have a Radon Problem in the First Place?
Denver sits in EPA Radon Zone 1, the highest-risk category, with predicted indoor levels above 4.0 pCi/L. Colorado’s bedrock — granite and gneissic formations rich in uranium — continuously breaks down and releases radon gas into the soil beneath homes across the metro area.
We typically see roughly half the homes tested in the Denver metro come back above the EPA action level of 4.0 picocuries per liter. The state average hovers around 6.3 pCi/L — well above that threshold. Homes near the foothills in Jefferson County and along the Front Range tend to run even higher than homes closer to the city center, though elevated readings show up across the entire metro, including Denver proper, Lakewood, Aurora, and Arvada.
Colorado’s climate compounds the problem. Denver’s 300-plus days of sunshine come with cold winters and dramatic temperature swings. Homes seal tightly from October through March, and that’s exactly when radon concentrations climb.
Why Is Fall the Best Time to Test?
Fall testing gives you the most actionable results, with enough time to schedule mitigation before winter arrives.
A radon test takes 48 hours to 7 days for a short-term charcoal canister, and lab results typically come back within a week of mailing. If you test in late September or early October and results come back elevated, most licensed contractors in the Denver metro can schedule an installation appointment within two to three weeks. Sub-slab depressurization systems — the standard fix — usually take four to six hours to install in a typical home.
That timeline matters. If you wait until December, you’re testing during peak radon season with fewer available installation slots. Radon professionals across the metro see a noticeable spike in calls from November through February, which can push scheduling out by several weeks. Getting your test done in September or October puts you ahead of that rush.
For Denver homeowners curious about how radon testing works and what to expect, understanding the process before you buy a kit makes the results easier to interpret.
Where Should You Place the Test in Your Home?
Place the test on the lowest level of your home that you regularly use — this is where radon concentrations are highest and where the health risk is most direct.
That means a finished basement gets tested before a main floor. An unfinished basement still counts if family members spend time there, even occasionally. Radon enters through cracks in the foundation slab, gaps around utility penetrations, and through exposed soil — so lower floors accumulate more gas before it disperses upward.
Keep the test away from exterior walls, windows, and HVAC vents. It should sit at breathing height, around 20 inches off the floor or higher, and remain undisturbed for the full test period. Most short-term kits require closed-house conditions — windows and doors kept shut except for normal entry and exit — for at least 12 hours before testing begins and throughout the test period.
In our experience, the most common testing mistakes are placing canisters too close to drafts or skipping the closed-house requirement. Both errors push results lower than actual conditions, which means a false sense of security heading into winter.
How Much Does Radon Testing Cost in Denver?
Short-term DIY charcoal canister kits run around $15 to $40 and are available at most hardware stores. Professional short-term testing costs $150 to $250 and includes placement, retrieval, and a lab-certified report.
| Test Type | Method | Cost | Timeframe |
|---|---|---|---|
| Short-term DIY | Charcoal canister | $15–$40 | 2–7 days |
| Short-term professional | Charcoal canister or CRM | $150–$250 | 2–7 days |
| Long-term DIY | Alpha track detector | $25–$60 | 90 days–1 year |
| Continuous monitor (device) | Electronic CRM | $150–$300 | Ongoing |
| Mitigation system install | Sub-slab depressurization | $800–$2,500 | 4–6 hours |
If your short-term test comes back at or above 4.0 pCi/L, a follow-up long-term test or continuous radon monitor can confirm the result before committing to mitigation. If results come back between 2.0 and 4.0 pCi/L, Colorado health guidance recommends considering mitigation — particularly for homes with children or occupants who spend significant time on lower floors. You can review what radon levels actually require action in Denver homes to understand what your number means.
What Happens If You Skip Pre-Winter Testing?
Skipping fall testing means either testing during peak radon season with no time to act, or waiting until spring after months of elevated exposure.
Winter conditions in Denver homes consistently produce higher radon readings. The stack effect — natural air pressure differences that draw soil gases upward through foundation gaps — intensifies when heating systems run constantly and homes stay sealed. What reads at 3.8 pCi/L in September might read 5.5 pCi/L in January.
Testing in winter isn’t wrong, but the results often reflect worst-case conditions, and scheduling a mitigation installation between November and February is harder. Contractors are busier, and some installation steps — like exterior pipe routing on a below-zero morning — are more complicated than they are in October.
When Should You Call a Professional Instead of Using a DIY Kit?
Call a licensed radon professional if your DIY test comes back at or above 4.0 pCi/L, if you’re buying or selling a home, or if a previous mitigation system is already installed and due for inspection.
Colorado requires radon professionals to maintain licensing through DORA (the Department of Regulatory Agencies) and carry professional liability insurance. A licensed contractor will assess your foundation type, slab condition, and sub-slab communication before designing a mitigation system — details a DIY installation won’t account for.
If your home already has a mitigation system, fall is also the right time for an annual fan inspection and U-tube pressure check. These checks typically take under an hour and cost little to nothing if the system is running correctly. If the fan has failed, you may have been living without protection without realizing it.
What Denver Homeowners Should Do Next
Order or purchase a short-term radon test kit and place it on your lowest occupied level before mid-October. Follow the closed-house protocol, mail the canister to the lab, and expect results within a week.
If results come back below 2.0 pCi/L, plan to retest in two years. Between 2.0 and 4.0 pCi/L, schedule a long-term test or consult a licensed contractor about your options. At 4.0 pCi/L or above, contact a licensed radon mitigation contractor promptly — most installations in the Denver metro can be completed within a few weeks when scheduled in September or October.
RadonSweep connects Denver homeowners with licensed Colorado radon professionals who know the metro’s soil conditions and building stock. If you’re ready to test or need help understanding your results, the process starts with a simple conversation — no pressure, just information.
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FAQ
Q: Can I test for radon myself, or do I need a professional?
DIY charcoal canister kits are EPA-approved and cost $15–$40. They’re reliable for screening. Use a professional if you’re buying or selling a home or if results come back elevated.
Q: How long does a radon test take in Denver?
Short-term tests take 2–7 days. Lab results typically return within a week of mailing. The full process from kit purchase to results usually takes 10–14 days.
Q: Do new Denver homes need radon testing?
Yes. New construction can test higher than older homes depending on foundation design and soil conditions. Building age doesn’t predict radon levels — geology and construction type do.
Q: Will opening windows lower my radon levels before testing?
No — and you shouldn’t. Open windows invalidate short-term test results and only temporarily reduce radon. Levels return once windows close. Closed-house conditions are required for accurate testing.
Q: How quickly can a mitigation system be installed after testing?
Most licensed contractors in the Denver metro can schedule installation within two to three weeks. The job typically takes four to six hours for a standard sub-slab depressurization system.
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Sources
- U.S. Environmental Protection Agency: Radon Testing and Mitigation Guidelines, EPA Action Level Standards
- Colorado Department of Public Health and Environment: Radon Program — Radon FAQs and State Guidance
- Colorado Geological Survey: Radon in Colorado — Geology and Occurrence
- Colorado Department of Regulatory Agencies (DORA): Radon Program Licensing Requirements
- Colorado Environmental Public Health Tracking: Indoor Radon Statistics and Zone Classifications