How to Place a Radon Test Kit So the Number Is Real
A hardware-store radon kit costs about fifteen bucks and, placed correctly, gives you a number as trustworthy as anything a pro's continuous monitor produces. The catch is that most bad radon results don't come from bad kits — they come from bad placement and a house that wasn't kept closed. Get the setup right and the cheap kit tells the truth. Get it wrong and you've wasted a week testing your ventilation instead of your radon.
Test the lowest level you actually use
Radon seeps up from the soil, so it's always highest at the bottom of the house and thins out as it rises. Test the lowest level of your home that you live in or could reasonably finish — a finished basement, or the ground floor if the basement is unfinished but you'd ever put a bedroom or rec room down there. Don't test the second floor and call it safe; you'll get a comfortingly low number that has nothing to do with where you sleep or where a future buyer's inspector will test.
Where the kit goes in the room
Placement inside the room matters more than people expect. The rules that keep a result honest:
- At least 20 inches off the floor — roughly breathing height when seated. On a shelf or table, not on the ground.
- At least 3 feet from exterior walls, windows, and doors — anywhere outside air leaks in and dilutes the reading.
- At least 4 inches from any other object, so air moves freely around the detector.
- Away from drafts, heat, and humidity — not near a fireplace, vent, heat register, dehumidifier, or in direct sun. All of those skew the result.
- Not in a kitchen, bathroom, or laundry — humidity and exhaust fans throw the number off.
A quiet corner of a basement bedroom or family room on a shelf is about perfect.
The closed-house rule — the part most people blow
For a short-term test (the 2-to-7-day charcoal kits), you have to keep the house under closed-building conditions for 12 hours before the test starts and for the entire test. That means:
| Keep closed | Still fine to run |
|---|---|
| Windows and exterior doors (except normal in-and-out) | Furnace and central A/C |
| Window fans and whole-house fans | Normal bathroom fans (briefly) |
| Anything that pulls in outside air | Standard daily comings and goings |
Radon builds up in a sealed house exactly the way it does over a normal closed-up winter — that's the condition you're trying to measure. Leave a window open and you're measuring a breezy day, not your home. That's also why winter is the best time to test: the house is naturally shut up anyway.
Log it and mail it fast
Write down the exact start and stop time, seal the kit back up the moment the test window ends, and get it in the mail to the lab the same day if you can. Charcoal kits keep absorbing (and can release) radon after the test period, so a kit that sits on your counter for a week before mailing drifts off its real value. The lab needs it promptly to read it accurately.
Joe — the septic-ignoring, well-neglecting legend himself — ran a radon kit, got a reassuring 1.8, and bragged about it for a year. What he left out: he ran it in July with the basement window open and the box sitting on the concrete floor two feet from the door. Every rule, broken. When a buyer's inspector tested it right during closing, it came back 6.1. Joe learned that a test is only as good as the setup.
Bottom line
The kit isn't the weak link — the setup is. Lowest lived-in level, up off the floor, away from walls and drafts, house kept closed, mailed promptly. Do that and a $15 kit gives you a number you can act on. If it comes back at or above 4.0 pCi/L, confirm with a second test and then look at mitigation. The EPA's plain-language guide walks through kit types and placement step by step: A Citizen's Guide to Radon.