Finished a basement, added new windows, or updated HVAC in Centennial or nearby Arapahoe Ridge? Renovations can change how radon enters and collects in your home. If you are planning to sell or simply want peace of mind, knowing when and how to retest is essential in Colorado’s high‑risk radon zone. In this guide, you will learn when to retest, which test to use, how to read results, what Colorado disclosure law requires, typical costs, and where to find local resources. Let’s dive in.
Why retesting matters here
Colorado has some of the highest radon levels in the country. The U.S. Environmental Protection Agency (EPA) recommends action at 4.0 pCi/L and careful follow up when results are between about 2.0 and 4.0 pCi/L. You can review the EPA’s guidance on how often to test or retest.
Both Boulder County and Arapahoe County are designated EPA “Zone 1,” meaning highest potential for elevated radon. Local agencies report that about half of homes test above 4.0 pCi/L in these counties, including neighborhoods in and around Centennial and Boulder’s Arapahoe Ridge. See local overviews from Boulder County Public Health and Arapahoe County Public Health.
When to test after renovations
Test before major work
If you plan to finish a basement, convert a crawlspace, pour a new slab, or change foundation elements, test first. Pre‑renovation testing lets you design mitigation into the project, which can be simpler and more cost effective. The EPA’s remodeling guidance explains why testing early helps manage indoor air quality during projects. Review EPA remodeling and indoor air tips.
Always test after work is complete
The CDC advises testing before and after renovations, since changes in airflow, sealing, and use of lower levels can raise or reveal radon. This is especially important if you finished a basement or tightened the building envelope. See CDC guidance on when to test for radon.
New concrete and timing
There is no single federal waiting period after pouring new concrete. Plan to test once the home is in normal use. If you want the most stable picture after major work, consider a long‑term test to capture seasonal and daily variation.
Which test to use
Short‑term tests
Charcoal canisters and 48–96 hour devices offer quick screening right after work finishes. For transactions or defensible documentation, a professional Continuous Radon Monitor (CRM) is preferred because it records hourly data. Learn more about test options from CDPHE.
Long‑term tests
Alpha‑track detectors run 90 days or more to estimate the annual average. If a short‑term result is near 4.0 pCi/L or you want a more representative average after big changes, long‑term testing is a smart move. CDPHE and EPA both recommend long‑term tests when accuracy matters.
Professional CRM testing
For real estate, post‑mitigation checks, or any situation where you need a defensible result, hire a certified professional who uses a CRM. CDPHE recommends certified testers for transaction testing.
How to run and place tests
Place the test in the lowest lived‑in level, or the lowest level that could be used as living space after your renovation. Follow closed‑house conditions for short‑term tests by keeping windows and exterior doors closed 12 hours before and during the test. Set the detector 2–7 feet above the floor in a regularly used room, away from exterior doors, windows, kitchens, bathrooms, fans, or direct sunlight. For step‑by‑step instructions, follow CDPHE’s testing guidance.
What your results mean
- 4.0 pCi/L or higher: Confirm with a follow‑up test and plan mitigation. After installation, run a post‑mitigation test to verify reduction. See CDPHE’s Radon FAQ for next steps.
- 2.0 to 3.9 pCi/L: Consider a long‑term test to establish the average. Decide on mitigation based on household risk and how you use lower levels.
- Under 2.0 pCi/L: Retest periodically, and always retest after major renovations or changes in occupancy.
Most Colorado mitigations cost about 1,000 to 2,000 dollars depending on design. Sub‑slab depressurization is the most common and effective method. Sealing alone is not a reliable fix. CDPHE covers typical approaches in its testing and mitigation overview.
Colorado disclosure rules for sellers
Colorado law requires sellers to disclose known radon information, including prior test results, any mitigation, and to provide the state brochure. This requirement is reflected in CRS 38‑35.7‑112. If you renovated after your last test, buyers will often request a fresh test, especially if you finished a basement or changed foundation or HVAC elements. Keep clear, written records of any testing and mitigation to streamline negotiations.
Local resources for Centennial and Boulder
- Arapahoe County residents, including Centennial: Check county radon statistics and programs for free or low‑cost kits on the Arapahoe County Public Health radon page.
- Boulder County and Arapahoe Ridge: Find testing options, guidance on radon‑resistant new construction, and local resources on Boulder County’s radon pages.
Quick homeowner checklist
- Test before you start major lower‑level or foundation work.
- Retest right after renovations that affect basements, crawlspaces, slabs, HVAC, or air sealing.
- Choose a professional CRM for transactions or when you need defensible results.
- Use a long‑term test if short‑term results are near 4.0 pCi/L.
- If elevated, hire a Colorado‑certified mitigator and keep all reports.
- If selling, include test results and mitigation documents in your disclosures.
- Retest after mitigation to confirm performance, and retest in the future after any major changes.
If you are planning renovations or a sale in Centennial or Boulder County, proactive radon testing protects health and reduces transaction friction. For guidance on timing tests around your project and preparing buyer‑ready documentation, connect with Ken Posen - Compass for local insights and trusted referrals to certified pros.
FAQs
Do I need to retest for radon after finishing a basement in Centennial?
- Yes. Finishing a basement can change airflow and radon pathways. Test after completion, and use a professional CRM if you need a defensible result for a sale.
Which radon test is best after renovations in Arapahoe Ridge?
- Use a short‑term professional CRM for quick documentation, then follow with a long‑term test if the result is near 4.0 pCi/L or you want the best annual average.
How soon should I test after new concrete or slab work in Boulder County?
- Test once the home is back to normal living conditions. If you want the most stable picture after major slab work, add a long‑term test.
What if my post‑renovation radon level is 4.0 pCi/L or higher?
- Confirm the result and plan mitigation with a certified professional. After installation, run a follow‑up test to verify the reduction.
What radon records do I need to provide when selling in Colorado?
- Provide known test results, mitigation history, and the state radon brochure. Keep clear documentation for any post‑renovation testing to support buyer confidence.
