TL;DR
- Two out of five Idaho homes have elevated radon levels, according to the Idaho Department of Health and Welfare. It has been found in every county in the state.
- The EPA action level is 4.0 pCi/L, and the EPA suggests considering mitigation even between 2 and 4. Outdoor air averages about 0.4.
- You cannot predict it from your neighbor's house. Homes built side by side, on the same soil, with similar designs can test very differently.
- Twin Falls County is badly under-tested. State records have shown only a few hundred homes tested countywide, which means the local picture is thin rather than reassuring.
- The building code provision for radon is optional. IRC Appendix F only applies where a jurisdiction specifically adopts it, so in most of Idaho nobody will require this of your builder.
- A passive system during construction runs roughly half to two thirds the cost of retrofitting later, and it's mostly gravel, membrane, and a pipe.
- Retrofitting means coring through finished concrete and routing pipe through finished living space, plus a fan that costs money to run every year.
- Test regardless. Even homes built with radon-resistant features should be tested, and Idaho offers free test kits.
This is one of those decisions that costs almost nothing during construction and a genuinely annoying amount afterward. It rarely comes up unless someone raises it, and in most of Idaho nobody is required to raise it.
So here's the picture, including the part where I tell you the honest limits of what a builder can promise.
How much of a problem is radon in Idaho?
Enough that the state runs a program about it.
Radon is a naturally occurring radioactive gas produced by uranium and radium in soil. It has no color, smell, or taste. It moves up through soil and rock and enters buildings through gaps and cracks in foundations and crawl spaces, where it accumulates.
The Idaho Department of Health and Welfare states that two out of five Idaho homes have elevated radon levels, and that radon has been found in every county in the state. Roughly 43 percent of Idaho counties are classified as EPA Zone 1, the highest predicted radon potential.
The health numbers come from the EPA, which estimates radon is responsible for about 21,000 lung cancer deaths a year in the United States, and identifies it as the leading cause of lung cancer among people who don't smoke. Idaho's health department offers a blunt comparison: a reading of 4 pCi/L is equivalent to smoking eight cigarettes a day.
For scale, outdoor air averages around 0.4 pCi/L. The EPA recommends fixing a home at 4.0 pCi/L or higher, and suggests considering mitigation between 2 and 4.
Locally, South Central Public Health District, which covers Twin Falls, has published Idaho survey results showing roughly a third of tested homes above 4.0 pCi/L in earlier state screenings, with study averages varying between about 3.3 and 5.7 pCi/L depending on the survey. Different studies found different results, which is itself worth knowing.
Why you can't guess from your neighbor's house
This is the part that makes radon different from most building science problems.
The health district puts it plainly: homes built side by side, on the same soil or rock, or with similar designs, can have very different radon levels. Construction type, foundation details, how the house is ventilated, and small variations in geology all matter.
So "our neighborhood is fine" isn't information. Neither is "the builder down the road has never had a problem." The only thing that tells you about your house is a test in your house.
And Twin Falls County is thinly tested. State records reported in local coverage showed only a few hundred homes tested countywide, with the state actively asking for more. That's not evidence the county is safe. It's evidence nobody knows much yet.
Nobody is going to require this of your builder
Here's the code reality, and it surprises people.
The International Residential Code does contain a radon standard. It was Appendix F in the 2018 edition and was renamed Appendix BE in the 2024 edition. But it's an appendix, which means a jurisdiction adopting the IRC has to explicitly adopt that appendix for it to apply. The code text itself says the techniques are required in areas where designated by the jurisdiction.
In other words, unless your city or county specifically adopted the radon appendix, radon-resistant construction is optional. Your builder isn't cutting corners by leaving it out. It simply isn't on the checklist, and the inspector won't ask.
Worth knowing: the 2021 edition of the IRC added a requirement for post-construction radon testing, with mitigation if levels come back high, for jurisdictions that adopt it.
What to do with that: ask your building department whether the radon appendix has been adopted where you're building, and ask your builder whether they're including radon-resistant features regardless. The second question matters more, because the answer to the first one is usually no.
What a passive system actually is
Less exotic than it sounds. Four components, all installed before or during the slab pour.
A gas-permeable layer. Clean aggregate under the entire slab, which gives soil gas somewhere to move laterally instead of pushing up through the concrete.
A soil-gas-retarder membrane. Plastic sheeting over the aggregate and under the slab. In crawl spaces, over the crawl space floor.
Sealing and caulking. All openings and penetrations in the foundation floor sealed, which is good practice regardless of radon.
A vent pipe. A minimum 3 inch diameter gas-tight ABS or PVC pipe embedded vertically into the sub-slab aggregate before the slab is cast, typically with a T fitting to keep the opening in the permeable material, running up through the conditioned space and terminating above the roof.
Plus one small thing that matters a lot later: an electrical junction box roughed in near the pipe, in case a fan is needed.
Without a fan, the system is passive. It relies on natural convection, warm air rising in the vent pipe, to draw soil gas from under the slab and out above the roof. If testing later shows levels are still elevated, adding a fan converts it to an active system, and that upgrade is quick and inexpensive because the pipe and gravel are already there.
That's the whole point. You're not installing a radon system so much as installing the infrastructure that makes a radon system easy if you turn out to need one.
The cost comparison
This is the argument, and it's a strong one.
During construction, a passive system runs roughly 50 to 70 percent of the cost of retrofitting a mitigation system later, according to published guidance. And you're mostly paying for gravel you'd arguably want anyway, sheeting, a length of pipe, and labor while the slab is open.
Afterward, published national figures for a professional retrofit mitigation installation cluster around $1,200, with typical ranges from roughly $800 to $2,500 depending on complexity. Those are national averages, not quotes, and complex structures cost more.
But the price isn't really the point. The disruption is.
A retrofit means coring through finished concrete and routing pipe through completed living space. That pipe has to get from under your slab to above your roof, and it does that through your house. Closets, if you're lucky. A corner of a finished room, if you're not. There's also a fan, mounted outside the conditioned space, that runs continuously.
And a fan-powered system has an operating cost. Published guidance for northern climates puts it around $200 a year when you account for both the fan's electricity and the heating of outside air drawn in to replace what the system pulls out. That's every year, indefinitely.
Compare that to a length of PVC installed before the concrete was poured.
The basement connection
Worth flagging for anyone planning a basement finish, because it's the intersection people miss.
Radon concentrates at the lowest level of a home. Testing guidance is specifically to test the lowest lived-in area. So finishing a basement does two things at once: it creates habitable space exactly where radon is highest, and it covers the slab and walls with finishes that make future mitigation more invasive.
If you're planning to finish a basement, test first. It's cheap, it's quick, and knowing the number before the walls close up is worth a great deal. We cover the rest of the basement finishing process, including the permits and the deed statement that catches people, in a separate guide.
Test anyway. Even with a passive system.
I want to be clear about the limits here, because this is a health question and I'm a builder.
No builder can promise you a radon level. A passive system reduces entry and prepares the house for mitigation. It does not guarantee a result. The EPA recommends that all homes be tested, including those built with radon-resistant features. That recommendation exists precisely because outcomes vary.
How to test:
- Idaho's Department of Health and Welfare offers free test kits, generally one per address every two years, with discounted kits and library lending programs as alternatives.
- The Idaho Radon Hotline is 1-800-445-8647.
- Test the lowest level of your home where people spend time.
- A short-term test runs 3 to 7 days and is best done in winter, when houses are closed up. A long-term test, up to a year, gives a better picture of your annual average.
- If a short-term test comes back high, the usual guidance is a second test before deciding, or a long-term test if you can wait.
On a new build, the best time to test is after construction is complete and the house is closed up and operating normally.
What to ask your builder
Has the jurisdiction adopted the radon appendix? Usually no, in which case the next question matters more.
Will you include radon-resistant features even though they're not required? Gas-permeable layer, membrane, sealed penetrations, vent pipe, junction box.
Where will the vent pipe run? This is a design decision, and it's much easier to route it through an interior wall or a closet chase when the house is still on paper.
Will you rough in the junction box for a future fan? It's a tiny item that saves a real headache.
What will this add to the estimate? Ask for it as a line item so you can see what you're actually deciding about.
If you're buying an existing home rather than building, the equivalent question is simply whether it's been tested, and the answer is usually no.
Building or remodeling in the Magic Valley?
We build custom homes, additions, ADUs, shops, and remodels across Twin Falls, Jerome, Kimberly, Buhl, Filer, Burley, Rupert, and unincorporated Twin Falls County.
We'll tell you what radon-resistant construction would add to your project and where the vent would run, and we'll tell you plainly that it doesn't replace testing. If you're finishing a basement, we'll suggest testing before the walls go up.
We're a registered Idaho contractor, RCE-65510, insured and warrantied.
Tell us about your project → or call (208) 731-1729
I'm a builder, not a health professional or a certified radon measurement provider. For testing and mitigation specifics, work with Idaho's radon program and a certified professional.
Frequently asked questions
Is radon a problem in Idaho? The Idaho Department of Health and Welfare reports that two out of five Idaho homes have elevated radon levels, and that it has been found in every county. Around 43 percent of Idaho counties are EPA Zone 1, the highest predicted potential category.
What level is considered too high? The EPA recommends fixing a home at 4.0 pCi/L or higher, and suggests considering mitigation between 2 and 4. For context, outdoor air averages about 0.4 pCi/L.
Does my builder have to install radon protection? Generally no. The IRC's radon provisions sit in an optional appendix, formerly Appendix F and now Appendix BE, which only applies where a jurisdiction specifically adopts it. Ask your building department whether it's been adopted locally, and ask your builder whether they'll include the features regardless.
What does a passive radon system consist of? A gas-permeable aggregate layer under the slab, a plastic soil-gas-retarder membrane over it, sealed foundation penetrations, and a minimum 3 inch gas-tight vent pipe embedded in the aggregate running up through the house and terminating above the roof. A junction box roughed in nearby allows a fan to be added later.
How much cheaper is it to do during construction? Published guidance puts a passive system at roughly 50 to 70 percent of the cost of retrofitting later. The larger saving is in disruption, since a retrofit means coring finished concrete and routing pipe through finished living space.
What does retrofit mitigation cost? National figures cluster around $1,200, with typical ranges from roughly $800 to $2,500 depending on the house. Those are national averages rather than local quotes. A fan-powered system also has an operating cost, often cited around $200 a year in northern climates once you include heating the replacement air.
Does a passive system guarantee low radon? No. It reduces entry and makes mitigation easy if needed. The EPA recommends that all homes be tested, including those built with radon-resistant features.
My neighbor tested fine. Am I fine? Not necessarily. Homes built side by side, on the same soil, with similar designs can test very differently. The only meaningful test is one in your own home.
How do I test? Idaho's health department offers free test kits, generally one per address every two years, and the Idaho Radon Hotline is 1-800-445-8647. Test the lowest level where people spend time. Short-term tests run 3 to 7 days and are best in winter. Long-term tests, up to a year, give a better annual average.
I'm finishing my basement. Should I test first? Yes. Radon concentrates at the lowest level, which is exactly the space you're about to make habitable, and finishes make future mitigation more invasive. Testing before the walls close up costs almost nothing and tells you whether to rough anything in while it's easy.
Sources: Idaho Department of Health and Welfare radon program; South Central Public Health District published radon information; U.S. Environmental Protection Agency radon guidance and risk assessments; 2018 International Residential Code Appendix F, Radon Control Methods; published industry and state guidance on radon-resistant new construction costs, accessed September 2026. Cost figures are national published averages, not Bolton Built quotes. Radon levels vary by home, so test rather than estimate.
