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Radon-resistant construction: what to ask for before the slab is poured
The cheapest day to deal with radon is pour day
Radon comes out of the ground, and a house touches the ground at exactly one place: the slab and foundation. That is why radon protection is a construction-stage decision, not a furniture decision. Idaho’s own health department puts the price tag on it plainly:
“Building a new home? Radon-resistant features cost $300–$500 and are easiest to install during construction.”
— Idaho Department of Health and Welfare, radon in Idaho (page last updated 9-16-2026). Retrieved 5 October 2026
The EPA’s builder-facing page — last updated August 6, 2026 — agrees on the economics: “The cost to the builder of including these features is typically less than the cost to mitigate the home after construction.” After the slab is poured and the walls are closed, the same job means drilling, pipe-fishing, and sealing from the wrong side of the drywall.
Five features, all ordinary materials
EPA’s list is short, and nothing on it is exotic:
| Feature | What it does |
|---|---|
| Gravel layer | A 4-inch layer of clean, coarse gravel below the slab — the “gas permeable layer” — so soil gases including radon can move freely under the house instead of up through it |
| Plastic sheeting or vapor retarder | Heavy duty plastic sheeting (6 mil. polyethylene) on top of the gravel, blocking soil gas entry and keeping concrete out of the gravel |
| Vent pipe | A 3-inch or 4-inch solid PVC Schedule 40 pipe run from the gravel layer up through the conditioned space and roof, venting soil gas above the house — labeled “Radon System” |
| Sealing and caulking | All openings, cracks and crevices in the foundation floor and walls sealed with polyurethane caulk |
| Junction box | An electrical outlet roughed into the attic so a vent fan can be added later without opening walls |
The vent pipe is the only piece a homeowner will ever notice, and EPA’s guidance is that “This pipe should be labeled "Radon System."” — so nobody mistakes it for a plumbing vent years from now. The pipe is plumbing-adjacent by nature (EPA notes it is similar to a drain-waste-vent), which is why it belongs in the same coordination conversation as the state plumbing permit and slab-stage inspections.
A proven, ordinary practice
This is not boutique construction. EPA tracks it:
“More than 1.5 million new homes have been constructed since 1990 with radon-resistant features, based on an annual survey of builders conducted by the National Association of Home Builders (NAHB) Research Center.”
— US EPA, Radon-Resistant Construction Basics and Techniques (last updated August 6, 2026). Retrieved 5 October 2026
The system works passively — no fan running, no power draw. EPA’s builder guide Building Radon Out (April 2001) describes the basic passive sub-slab depressurization system as one that “effectively reduces radon levels by an average of about 50%” — and that was the 2001 vintage of the technique, before it became routine.
Is it required here? No — and that is the point
Idaho has no statewide radon-construction mandate. The state health department says it in one sentence:
“These radon control methods are not required by law in Idaho unless a city or county has specifically adopted them into their local building codes.”
— Idaho Department of Health and Welfare, radon in Idaho (page last updated 9-16-2026). Retrieved 5 October 2026
The methods referenced are the IRC’s Appendix F radon appendix — a standard appendix that only has force where a jurisdiction adopts it. So on an unincorporated corridor lot, nothing in the permit path (see the county building permit process) will require these features. If you want them, you ask for them — and EPA expects exactly that:
“A new home buyer may ask the builder about these features, and if not provided, may ask the builder to include them in the new home.”
— US EPA, Radon-Resistant Construction Basics and Techniques (last updated August 6, 2026). Retrieved 5 October 2026
The one thing you still do after move-in
Radon-resistant is not radon-proof, and no one can know a finished home’s level without a test. EPA’s 2001 builder guide says it flatly:
“After occupancy, all homes should be tested for radon, even those built with radon-resistant features.”
— US EPA, Building Radon Out: A Step-by-Step Guide (April 2001). Retrieved 5 October 2026
The attic junction box exists for exactly that day: if a post-move-in test comes back high, a mitigator drops a fan onto the pipe that is already running through your roof — an upgrade, not a demolition.
Put it in the schedule
- Raise radon features at design time — they cost $300–$500 at the slab stage and far more later.
- Confirm the gravel, sheeting and stubbed vent pipe before the pour, alongside the footing and frost-depth work.
- Have the vent pipe run through warm interior space and labeled “Radon System,” with the attic junction box wired.
- Test the finished home after move-in; activate the fan only if the result calls for it.
- Want it standard on your build? (208) 656-1516.
The features ride along with work the build already does — gravel under the slab, sheeting under the concrete, sealing at the foundation — which is why EPA calls the marginal cost small relative to the overall cost of a custom home and why they pair naturally with energy-code insulation choices: the same tight envelope that saves heat also slows soil gas.
Common questions
What is radon-resistant construction?
A set of five features installed while a home is built: a 4-inch clean gravel layer under the slab (the gas permeable layer), heavy duty 6 mil plastic sheeting over it, a 3- or 4-inch solid PVC Schedule 40 vent pipe run from the gravel through the roof and labeled 'Radon System', polyurethane sealing of foundation openings, and an attic junction box for a possible future fan. EPA says no special skills or materials are required.
How much do radon-resistant features cost in new construction?
The Idaho Department of Health and Welfare - page last updated 9-16-2026 - says radon-resistant features cost $300 to $500 and are easiest to install during construction. EPA adds that the builder's cost of including them is typically less than the cost to mitigate the home after construction.
Are radon-resistant features required in Idaho?
No. The Idaho health department states these radon control methods are not required by law in Idaho unless a city or county has specifically adopted them into their local building codes - a reference to the IRC's Appendix F, which only has force where adopted. On unincorporated land, nothing in the permit process requires them; they are a buyer's or builder's choice.
Can I ask my builder to include radon features?
Yes - EPA says a new home buyer may ask the builder about these features, and if not provided, may ask the builder to include them in the new home. The request needs to happen before the slab is poured, because three of the five features live under the concrete.
Does the radon system need a fan?
Not at first. The basic version is a passive sub-slab depressurization system - the vent pipe alone, using natural stack effect - which EPA's 2001 Building Radon Out guide says effectively reduces radon levels by an average of about 50 percent. The attic junction box is roughed in so an in-line fan can be added later if post-occupancy testing calls for more reduction.
Do I still need to test for radon if the house is built radon-resistant?
Yes. EPA's guidance is that after occupancy, all homes should be tested for radon, even those built with radon-resistant features. The features lower the odds; only a test of the finished, lived-in house tells you its actual level.