Where to Put a Radon Test Kit for an Accurate Reading
A do-it-yourself radon kit is accurate hardware, but where you set it down decides whether the number it returns means anything. Put the same canister on a windowsill instead of a side table and the result can read noticeably low, because outdoor air leaking past the sash dilutes the sample. The rules below come from the EPA and AARST measurement protocols, and they answer the one question this page exists for: exactly where in the house a short-term test belongs.
Start on the lowest level you actually live on
Radon seeps in through the foundation and is denser than air, so concentrations are highest near the ground. Test the lowest level of the home that is regularly occupied — not automatically the lowest level that exists. The EPA’s Citizen’s Guide to Radon frames it as the lowest “lived-in” level, which for most homes breaks down like this:
- Finished basement used as a bedroom, family room, or office — test the basement. This is the common Front Range case; many Colorado homes finish the basement as primary living space.
- Unfinished basement used only for storage or laundry — test the first floor instead. Measuring an unoccupied basement overstates what the people upstairs actually breathe.
- Slab-on-grade or crawlspace home — test a regularly occupied room on the first floor. Never place the kit inside a crawlspace; it is sealed off from living space and reads nothing representative.
- Walk-out or garden-level basement — if that level is finished and lived in, it is your test floor.
Within that floor, pick a room where someone genuinely spends time: a living room, den, or bedroom. Skip the kitchen, bathroom, and laundry room. Their moisture, heat, and exhaust fans distort the reading, and their air is not where you spend the hours the test is meant to represent.
How high and how far from the walls
Once the room is chosen, the spot inside it matters just as much:
- Height: 20 inches to 6 feet off the floor. A side table, shelf, or bookcase around table height (roughly 20–30 inches) sits in the breathing zone of a seated person, which is what you want to sample. Setting it on the floor invites humidity, condensation, and dust onto the charcoal media; setting it near the ceiling is too far above where anyone breathes.
- Away from exterior openings: at least 3 feet from windows and exterior doors and roughly a foot off exterior walls. These are the leak points where inside and outside air trade places, and a kit parked there samples that exchange rather than the room.
- Off the walls, toward the room: keep it a few inches clear of other objects and out of tight corners, where air goes stagnant and reads unlike the rest of the space.
The target that satisfies all of this at once: a stable table near the middle of a lived-in room, table height, a few feet from any window or exterior wall, where it will not get bumped.
Spots and equipment to keep clear of
Moving air is the enemy of a short-term test — it mixes room air unevenly and skews the sample. Keep the kit well away from:
- HVAC supply registers and return grilles
- Ceiling fans and whole-house fans
- Fireplaces, wood stoves, and space heaters
- Bathroom exhaust fans and kitchen range hoods
- Dryer vents and frequently cycling sump pumps
- Direct sunlight and any other source of steady heat or draft
You do not need to shut the furnace off — leave the HVAC in its normal mode. Just don’t place the detector in the path of a register or fan.
Closed-house conditions during the test
Placement only holds up if the house is closed while you measure. For a short-term test (2 to 7 days), keep closed-house conditions for 12 hours before the test starts and for the entire test window:
- Windows shut, exterior doors shut except for normal comings and goings
- No whole-house fans or window fans pulling outdoor air
- HVAC running as it normally would
If someone throws a window open or runs a whole-house fan during the 12-hour lead-in, restart the clock. The EPA’s 4.0 pCi/L action level — the point at which fixing the home is recommended — is calibrated against closed-house results, so an open-window test isn’t a fair comparison. Because of that requirement, short-term kits are best run in cooler months when windows stay shut anyway. For what the result means once you have it, see what counts as a safe radon level.
Leave it alone, then mail it fast
Do not pick the kit up, peek at it, or move it to another room mid-test. Relocating it disturbs the integrated sample and voids the result. If it gets knocked over — a curious kid, a cat on the table, a cleaning crew tidying up — the test is done; start fresh with a new kit and a new 12-hour lead-in rather than trying to salvage it. Warn everyone in the house, and any cleaners, that the little box is a 48-hour measurement and stays put. A quiet corner of an occupied room, out of the daily traffic path, is the sweet spot.
When the exposure window ends, the placement job isn’t quite over. A charcoal canister keeps adsorbing and begins to release radon after the test, so seal it in the return mailer and send it the same or next day — most labs need it back within a few days for a valid analysis. A kit that sits on the counter for a week undoes careful placement. Alpha-track detectors are more forgiving on timing but still shouldn’t linger. Which kit type you’re holding changes the details here; our guide to radon gas test kits and the walkthrough on how to test for radon at home cover the differences.
A note for Front Range homes
Colorado sits heavily in EPA Radon Zone 1, the highest-potential tier, and the state health department reports that roughly half of Colorado homes test above the action level — well above the national average. The placement rules don’t change, but the odds of a high reading do, so getting the spot right is worth the extra minute. For local context and testing options across the region, start with our Colorado radon testing guide.
Frequently asked questions
Can I set the radon test on the floor?
No. Floor-level humidity, condensation, and dust interfere with the charcoal and drag the reading off. Use a stable table or shelf 20 to 30 inches up — table height — which matches the breathing zone of a seated person.
Which room is best if my basement is finished?
Test the finished basement, since it’s both the lowest level and a lived-in one. Pick a family room, bedroom, or office rather than a bathroom or laundry area, and set the kit near the center on a table a few feet from any exterior wall or window.
How far from a window does the kit need to be?
At least 3 feet from windows and exterior doors, and about a foot off exterior walls. Air leaks in around these openings and dilutes the sample, which is why windowsill placement is the single most common mistake.
Do I have to keep windows closed the whole time?
Yes for a short-term test — closed for 12 hours before and throughout the measurement. If a window is opened or a whole-house fan runs, restart the 12-hour lead-in. Long-term tests of 90 days or more average across normal living and are far less sensitive to this.
What if I realize I placed it in the wrong spot?
Don’t move it and continue — that invalidates the run. Start over with a fresh kit and a fresh 12-hour closed-house period in the correct location.
This page reflects EPA, AARST, and CDPHE radon-measurement guidance current as of 2026; for unusual homes, a certified radon professional can advise on placement.