Southeast Idaho has a radon problem
Radon is a colourless, odourless radioactive gas produced by uranium decaying in soil and rock. It enters homes through the foundation and concentrates at the lowest level — which is precisely where you are about to build bedrooms.
The numbers here are not reassuring. Idaho Department of Health and Welfare guidance estimates that roughly two in five Idaho homes exceed 4.0 pCi/L — the level at which the EPA recommends taking action.
The EPA also notes there is no radon level considered entirely without risk, and identifies radon as a leading cause of lung cancer among people who have never smoked. Rather than rely on any figure you read online, including ours, test your own house. Levels vary house to house.
Why this matters specifically when finishing a basement
Three things change when you finish a basement, and all three point the same direction.
You seal it. Finishing tightens the envelope — insulation, drywall, sealed penetrations. A tighter basement exchanges less air with outside, and radon concentration rises.
You occupy it. An unfinished basement is somewhere you spend twenty minutes doing laundry. A finished one is where someone sleeps eight hours a night. Exposure is a function of concentration multiplied by time, and you've just multiplied the time by twenty.
You cover the access. The standard mitigation approach — sub-slab depressurisation — involves coring the slab, running pipe, and installing a fan venting above the roofline. Doing that in an open basement is straightforward. Doing it after the walls are up, the ceiling is closed and the flooring is down means opening finished work and routing pipe through rooms where you'd rather not see it.
Test first. Mitigate during construction if needed, while the slab and walls are open and pipe routing is invisible. Test again after completion to confirm. Doing it in that order costs a fraction of retrofitting, and it means the pipe runs where you want it rather than where it will fit.
Testing
Testing is cheap and easy, which makes skipping it hard to justify on a project of this size.
- Short-term test kits sit in place for 2–7 days and give a quick reading. Good for an initial screen before you plan.
- Long-term tests run 90 days or more and give a far better picture of annual average exposure, because radon levels fluctuate substantially with season, weather and how the house is being ventilated.
- Winter matters. Levels typically run highest in winter, when the house is closed up and the stack effect pulls soil gas in. A summer test can understate your real exposure — relevant in Pocatello, where basement finishing is a winter project anyway.
Place the test in the lowest lived-in level, in the area you plan to occupy, following the kit instructions on closed-house conditions. The Idaho Department of Health and Welfare has run radon awareness programmes and has historically offered low-cost or free test kits — worth checking availability.
Mitigation, briefly
The standard method is active sub-slab depressurisation. A suction point is cored through the basement slab, PVC pipe is run from that point up and out above the roofline, and an inline fan runs continuously to draw soil gas from beneath the slab and vent it outside rather than letting it enter the house.
Supporting work usually includes sealing slab cracks, the perimeter joint, and any sump or utility penetrations, so the fan depressurises the sub-slab area rather than pulling conditioned air out of the house.
Done during a basement finish, the pipe can be routed inside a framed chase or mechanical room and effectively disappears. Done afterward, it frequently ends up running down a corner of a finished room.
Common questions
My neighbour tested low. Do I still need to test?
Yes. Radon varies dramatically house to house, even between adjacent properties, because it depends on the specific soil and rock beneath your foundation, your foundation's construction, and how your house moves air. A neighbour's result tells you almost nothing about yours.
Is a newer house safer?
Not necessarily, and sometimes the reverse. Newer homes are built tighter, which reduces air exchange and can allow higher concentrations. Some newer construction includes passive radon-resistant features — a rough-in that can be activated with a fan — which is worth checking for before assuming you need a full installation.
Does mitigation affect my heating bill?
The fan runs continuously and uses a modest amount of electricity. A properly installed system draws from beneath the slab rather than from conditioned indoor air, so it shouldn't meaningfully increase heating load. A poorly sealed one can, which is another argument for doing the sealing work properly.
Does it matter at resale?
Increasingly, yes. Radon testing during real estate transactions has become common, and an elevated result discovered during a sale is a negotiation problem on someone else's timeline. A documented mitigation system with test results is an asset.