Yes. If you are about to finish a basement anywhere along the Front Range, test for radon first. Testing costs little, takes days, and the result changes how the space should be built. Finding a problem after the drywall is up is the expensive version of the same discovery.
The EPA's map of radon zones places every county along the corridor from El Paso to Larimer in Zone 1, its highest-potential category. That map predicts county averages, not individual homes, and the EPA is clear that every home should be tested regardless of zone. Two houses on the same street can test very differently. The only way to know about yours is to measure it.
Why the order matters
Radon is a radioactive gas that comes up out of the soil and enters through cracks in the slab, the joint where the slab meets the foundation wall, sump pits and gaps around pipes. A finished basement does not stop it. Carpet, drywall and a nice sectional only change what the problem costs to fix.
The most common fix, an active sub-slab depressurization system, runs a pipe from under the slab, through the house or up the outside, to a fan that vents above the roofline. In an open, unfinished basement, routing that pipe is simple. In a finished one, it means a chase through a closet, a soffit where nobody wanted one, or an exterior run that changes how the house looks. A sealed sump lid, another common part of the system, is easy to plan around before framing and awkward to retrofit into a finished floor.
A testing checklist before you finish
- Test the lowest level you plan to live in. That is the basement itself, even while it is unfinished, because that is where you will spend time once it is done.
- Use a short-term test first. Short-term tests run from 2 to 90 days and give a quick reading. Long-term tests run more than 90 days and give a better picture of the year-round average.
- Keep closed-house conditions. For a short-term test, windows and outside doors stay closed except for normal coming and going, starting 12 hours before the test and for its duration. That is easiest in fall and winter, when the house is closed up anyway.
- Place the kit correctly. Follow the kit's instructions: away from drafts, exterior walls, heat sources and high humidity, at roughly breathing height.
- Follow up on a high result. If the first short-term test comes back high, run a second test, either another short-term test or a long-term one, before deciding what to do.
What the number means for your plan
Results come back in picocuries per liter (pCi/L). The EPA recommends fixing a home at 4 pCi/L or higher and suggests considering a fix between 2 and 4 pCi/L. Below that, finishing can go ahead on the normal plan.
| Result | What it means | What changes in the finish |
|---|---|---|
| Below 2 pCi/L | Low reading | Proceed normally. Retest after finishing, since sealing and new living patterns can shift results. |
| 2 to 4 pCi/L | The EPA suggests considering a fix | Decide before framing. If mitigation is likely, reserve a pipe route and keep the sump accessible. |
| 4 pCi/L or higher | The EPA recommends fixing | Install mitigation before finishing, or at least before walls close, and test again afterward. |
Planning the finish around a mitigation system
If mitigation is in your future, the layout decisions are straightforward when you make them early.
- Pick the pipe route first. A route through a storage room or mechanical closet keeps it out of living space. An exterior route avoids interior chases but shows on the outside of the house.
- Keep the sump serviceable. A sealed lid still needs access for the pump. Plan a closet or a removable panel, not a floor that covers it.
- Seal what you can see. Visible cracks and the slab-to-wall joint are easier to seal while the floor is bare.
- Think about noise. Fans run constantly. Keep them away from bedrooms and the theater wall.
We'd settle all of this during design, before a single wall goes up. It is one of the decisions our basement finishing planning covers alongside egress, ceiling height and moisture.
What about a basement that is already finished?
Testing still applies, and so does mitigation. The work is harder but common. If you are already planning to rework an older finished basement, that is the moment to deal with radon, because walls and ceilings will be open anyway. Our page on remodeling an existing basement covers what else to look for when the old finish comes off.
Common questions
Does a new house mean low radon?
No. Newer construction can still test high. Some homes have radon-resistant features built in, such as a passive vent pipe, and those can often be activated with a fan if a test comes back high.
Can I test while the basement is under construction?
Test before construction starts, then again after the space is finished and lived in. Dust, open walls and changing ventilation during the work make mid-construction readings hard to trust.
Who should install a mitigation system?
Use a mitigation contractor who works to the current national standards and can explain where the pipe and fan will go. Ask how the system will be labeled and how you will know it is running.
Published by Basement Finishing Parker CO. A basement finishing and remodeling company based in Parker, Colorado, working along the Front Range from Colorado Springs to Fort Collins. About the company.
