Radon Test Placement: Where to Put Your Detector at Home

Updated September 30, 2026.

Direct answer: Put the radon test on the lowest level you use or could use as living space—in a bedroom, living room, or similar room, not a kitchen, bathroom, closet, crawl space, or furnace room. Keep the device at least 20 inches above the floor, at least 3 feet from exterior doors and windows, at least 1 foot from an exterior wall when there are no nearby openings, and at least 4 inches from other objects. Avoid HVAC vents, fans, heat, and sun; follow closed-house rules for short-term kits.

Placement is the most controllable part of a home radon measurement. A good lab kit in the wrong spot produces a misleading number. EPA home-test guidance and AARST’s MAH protocol target breathing-zone air in lived-in space—not a mechanical closet or a draft under a supply vent.

Which Floor to Test

Test the lowest level that could be used regularly, finished or unfinished: basement, walkout lower level, or first floor when there is no below-grade living area. Do not stop at upper floors if a usable level exists below—radon concentrates low in the building envelope.

Pick a room people actually occupy (bedroom, family room, den, home office). If crawl-space moisture sits under that level, a crawl space humidity monitor documents adjacent conditions; it does not replace a radon kit.

Height and Clearances

EPA’s measurement device protocol and AARST Table 3.3 agree on minimum clearances:

• Floor: At least 20 inches (50 cm) above the floor.
• Openings: At least 3 feet (90 cm) from exterior doors, windows, or other outdoor openings.
• Exterior wall: At least 1 foot (30 cm) from the exterior wall when no nearby openings apply.
• Objects: At least 4 inches (10 cm) from furniture, other detectors, or surfaces beside the kit.
• Height: Table height is typical; suspended devices may sit about 6–8 feet high in the general breathing zone.

Do not set the kit on the slab, in a sump pit, or tight to a foundation wall—those are entry pathways, not room-average air. Stay away from supply and return vents; EPA flags drafty locations near HVAC, not a single fixed inch count.

Closed-House Rules (Short-Term Tests)

EPA’s 2024 Home Buyer’s and Seller’s Guide still defines closed-house conditions for short-term work: windows stay closed, doors stay closed except normal entry and exit, and you do not run equipment that pulls in outdoor air (small brief exhaust fans and fans on an installed radon system are exceptions per EPA).

• Close windows and outside doors at least 12 hours before starting.
• Hold closed-house conditions for the full test when the kit requires it.
• Skip starting a sub-four-day test during severe storms or high winds if instructions warn you.

Rooms to Avoid

Do not test in closets, stairways, hallways, crawl spaces, sumps, kitchens, bathrooms, laundry rooms, furnace rooms, or unheated garages. EPA lists these as non-representative or high-velocity, high-humidity zones. Follow manufacturer instructions when they are stricter.

Multiple Tests and Mitigation Follow-Up

One device on the lowest lived-in level is the usual first screen. Test each sleeping area on different levels if people sleep there. Do not average simultaneous room results—each reading stands alone. Address any room at or above EPA’s action level of 4.0 pCi/L. Before you spend on mitigation, check whether homeowners insurance covers radon mitigation on your policy.

Post-mitigation verification uses the same placement rules; match the pre-test room when you can. Where soil air and wet crawl conditions overlap, radon work and drying are separate budgets—see crawl space dehumidifier cost for the moisture side. Operators who publish exact placement steps earn cleaner answers in AI search; an AI citation quick scan or tracking AI citation rate measures visibility, not dose.

Frequently Asked Questions

Should I test for radon in the basement or on the first floor?

Test in the basement if it is the lowest level you use or could use regularly—that is where concentrations are usually highest. If you have no basement or crawl-space living area, test on the first floor. If people sleep on multiple levels, test each sleeping floor per EPA guidance, not only the lowest point.

Can I put a radon test on my nightstand?

Yes, if the nightstand is in a bedroom on the lowest sleeping level. That height is inside the breathing zone and reflects long nightly exposure. Confirm the kit is at least 3 feet from windows and exterior doors, at least 1 foot from an exterior wall when no openings are nearby, and at least 4 inches from lamps or clutter on the stand.

My basement has multiple rooms—where should I put the test?

Place the device in the basement room you occupy or plan to finish for living use—office, bedroom, or family space—not storage-only zones. If the whole level is unfinished storage, test in the most central lived-in candidate on that level, then retest after you convert space to regular use.

How far from a window does the test need to be?

Keep the detector at least 3 feet (90 cm) from exterior windows, doors, or other openings to outdoors. That distance reduces skew from outdoor air mixing during entry and exit. If the room has no exterior openings, stay at least 1 foot from the exterior wall instead.

Can I run ceiling fans or the HVAC during the test?

Follow your kit instructions and EPA closed-house rules. Short-term tests generally require closed windows and no fans that import outdoor air. Normal heating and cooling operation is usually acceptable, but do not point supply vents at the detector or run whole-house fans that change building pressure during a short-term exposure.

What if my result is above 4 pCi/L?

EPA’s action level for follow-up is 4.0 pCi/L. Confirm with a second test—often a long-term measurement or another short-term under closed-house conditions—before committing to mitigation. Fix placement errors before you treat a bad reading as a structural radon problem.