Radon mitigation in Denver, CO
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Radon mitigation is one of the highest-volume residential service categories in Colorado. Most Front Range counties — Denver, Jefferson, Arapahoe, Adams, Boulder, Douglas — sit in EPA radon Zone 1 (highest risk), and the [Colorado Department of Public Health and Environment (CDPHE)](https://cdphe.colorado.gov/radon) estimates that roughly half of Colorado homes test above the EPA action level of 4 pCi/L. The granitic and metamorphic bedrock that produces the Front Range's topography also produces some of the highest residential radon concentrations in the country — and that's before you account for the high-altitude pressure differentials that pull more soil gas into homes.
This page covers what Denver-area homeowners should know before scheduling: the standard mitigation approach (sub-slab depressurization), CDPHE's contractor certification framework, and the Colorado-specific factors (basement housing stock, walkout-basement designs, altitude effects) that affect mitigation system design.
Colorado certifies radon mitigation contractors through the [Colorado Radon Program](https://cdphe.colorado.gov/radon-mitigation). Real-estate transactions in Colorado often include radon disclosure or testing as part of inspection.
How sub-slab depressurization works for Denver homes
The standard mitigation approach for nearly all Denver-area homes is sub-slab depressurization (SSD). Radon enters from soil gas through the basement slab, sump pits, drain tiles, and crawl-space areas. SSD installs a vent pipe through the slab and uses a continuous-running fan to create negative pressure under the slab — pulling soil gas through the pipe and exhausting it above the roofline rather than into the home.
The components: a 3-4" PVC pipe through the basement slab, sealed at the penetration; pipe routing through the home (often through a closet, utility area, or attic) to above the roofline; an in-line radon fan rated for the soil-gas volume; and a U-tube manometer or digital gauge that lets the homeowner verify the system is operating.
What varies between homes: number of suction points required (most Denver homes need one; large or compartmentalized basements may need two or three), fan size (different soil permeabilities need different airflow), and pipe routing (older Denver homes with finished basements have constraints on where the pipe can go).
Denver housing patterns that affect mitigation design
Several Front Range housing patterns require attention during mitigation design:
Walkout basements. Denver-area suburbs (Lakewood, Highlands Ranch, Castle Rock, parts of Aurora and Centennial) include many walkout-basement homes built into hillsides. Walkouts have partially exposed basement walls and complicated soil-gas paths. Mitigation design adjusts for the wall-exposure side and may require multiple suction points or specialized sealing.
Finished basements. Many Front Range homes have finished basements — drywall, drop ceilings, carpet, sometimes a bar or theater room. Mitigation requires breaking through finished floors at the suction point and routing the vent pipe behind walls or through closets without damaging finishes. Experienced Colorado mitigators design routes that minimize finish damage.
Altitude effects on fan sizing. Denver sits at 5,280 feet; some Front Range mitigations are at 6,000-7,500 feet. Lower air density at altitude means radon fans deliver less airflow per RPM than at sea level. CDPHE-certified mitigators select fans rated for high-altitude operation; less-experienced contractors sometimes specify sea-level-rated fans that under-perform.
Sump-pit interactions. Many Denver-area basements have drain-tile loops connecting to sump pits. The drain tiles act as a soil-gas pathway. Proper mitigation seals the pit lid, vents the pit through the radon system, and may require adjustments to the drain-tile design.
Slab-on-grade homes. Some Denver-area construction (especially newer suburban) is slab-on-grade rather than basement. SSD on slab-on-grade homes is straightforward but requires the slab penetration to be in the right location for the soil pattern under the home.
High altitude and the pressure differential effect
Denver's elevation produces a real pressure-differential effect that affects radon levels. Homes at higher elevations have lower exterior air pressure, which (combined with stack effect from heating systems) draws more soil gas into the home than equivalent low-elevation homes would. The same uranium-bearing bedrock at sea level might produce 2 pCi/L indoors; at 5,280 feet, the same geological source might produce 4-8 pCi/L.
Practical implication: Front Range homes typically test higher than equivalent low-elevation homes with the same soil. The mitigation system has to do more work — pull more soil gas and maintain higher pressure differential — to get below the EPA action level. CDPHE-certified mitigators understand this and specify accordingly. Out-of-state contractors familiar with low-altitude mitigation sometimes under-spec systems that work less well in Front Range conditions.
Testing — before and after
Before mitigation: get a confirmed radon test result.
Short-term test (charcoal canister or alpha-track, 2-7 days, inexpensive): closed-house conditions, basement/lowest-occupied level, away from windows and exterior walls. Results below 2 pCi/L: low risk, retest every 5 years. Results 2-4 pCi/L: borderline; consider mitigation or longer-term test. Results above 4 pCi/L: EPA action level — mitigate.
Long-term test (alpha-track passive monitor, 90+ days): more accurate annual average. Useful when short-term test was borderline or when seasonal variation matters.
Real-estate transactions: most Front Range transactions include radon testing. Colorado's Real Estate Commission disclosure requirements typically capture radon. Test results above 4 pCi/L typically trigger mitigation as part of the closing.
After mitigation: post-system testing is essential. Schedule a closed-house test within 24-72 hours of fan activation, then again at 30 days. The properly-installed system should produce results well below 2 pCi/L on both tests. If post-mitigation results are higher than expected, the certified mitigator returns to diagnose — typically a sealing or suction-point issue.
Frequently asked questions
Should I test for radon in Denver?▾
Yes. CDPHE estimates roughly half of Colorado homes test above the EPA action level of 4 pCi/L, and most Front Range counties are EPA Zone 1 (highest risk). Testing is inexpensive (an over-the-counter short-term kit), takes 2-7 days, and produces a clear answer. Real-estate transactions in Colorado often require radon disclosure or testing.
Why are Denver radon levels so high?▾
Two factors combined: granitic and metamorphic bedrock under the Front Range contains uranium-bearing rock that releases radon as it decays, and high altitude produces pressure differentials that pull more soil gas into homes than equivalent low-elevation locations. The same uranium content that elevates radon at sea level produces materially higher indoor concentrations at Denver elevation. Geology plus altitude is the explanation.
What rock gives off radon?▾
Granite, shale, and other igneous and metamorphic rocks containing trace uranium are the primary geological sources. Colorado's Front Range geology — granitic batholith intrusions, metamorphic schists and gneisses, and shales — is among the highest-radon source rock in the country. Even homes built on apparent flatland in the metro area have elevated radon because the underlying bedrock is uranium-bearing.
In what month is radon highest?▾
Radon levels are typically highest in winter, when homes are sealed and stack-effect pressure differences (warm interior, cold exterior) draw more soil gas in. In Denver specifically, the cold-dry winter combined with heated indoor air produces the strongest stack effect of the year. Short-term tests should be conducted in closed-house conditions; long-term tests over 90+ days give more representative annual averages.
How often should I retest after mitigation?▾
Initial post-mitigation test 24-72 hours after fan activation, plus a confirmation test at 30 days. After confirmation, retest every 2-5 years to verify the system is still operating. Replace the fan when the U-tube manometer readings indicate failure. Modern radon fans typically last 10-15 years; high-altitude duty cycles are typically the limiting factor in Denver.
My new construction home has a passive radon stack — do I need to test?▾
Yes. Newer Front Range construction often includes a passive radon-resistant new construction (RRNC) stack pre-installed during build. Passive stacks reduce radon but don't guarantee levels below 4 pCi/L. Test every new home. If results are above 4 pCi/L, a CDPHE-certified mitigator can activate the passive stack with an in-line fan rather than installing a full new system.
Can I install my own mitigation system in Colorado?▾
CDPHE requires certified contractors for radon mitigation. DIY installation does not satisfy the certification framework and creates real-estate-disclosure problems at sale. For real performance and resale safety, hire a CDPHE-certified mitigator.