Where to Place a Radon Detector in Your Home

By Steve Crews · Last updated June 21, 2026

Why your detector’s location changes everything

Carbon monoxide spreads quickly and fairly evenly through a home, so placement for those alarms is forgiving. Radon is different. It seeps up from the soil, it is denser than most indoor air, and it concentrates at low elevations where ventilation is often weakest. A detector on the second floor can read two to three times lower than the basement. Test the wrong level and you could spend years thinking you are fine while the space you actually sleep in reads above the EPA action level of 4 picocuries per liter (pCi/L).

The U.S. Surgeon General has urged all Americans to test their homes for radon. The EPA’s testing guidance goes a step further and specifies where: the lowest livable level of the home. “Livable” does not require finished walls or carpet. The EPA and AARST-NRPP, the national credentialing body for radon professionals, treat any space occupied at least 4 hours a day, or any area that could realistically serve as a bedroom or living space, as livable. That definition is wider than most homeowners expect.

Homes with basements

If your home has a basement and you use it, start your testing there.

What counts as “using” it? A home gym you visit three mornings a week, a workshop where you spend weekend afternoons, a play area where kids spend time after school, or an unfinished basement where the laundry machines are. All of these meet the threshold. A finished basement with a bedroom or home office is the clearest case of all: the space is both high in radon and heavily occupied, which is the combination that builds real cumulative exposure.

Within the basement, place the device on an interior wall between 3 and 5 feet off the floor, roughly centered in the space. Avoid:

• Within 3 feet of windows, exterior doors, or gaps in the building envelope (outside air pulls readings down)
• Directly under or within a few feet of an HVAC supply vent (forced air dilutes the local reading)
• Next to a sump pit or floor drain (these can be localized entry points, not representative of the room overall)
• Kitchens and bathrooms, even in basements, because humidity and exhaust fans shift readings throughout the day

A basement bedroom at nightstand height, on an interior wall, is almost always a solid default.

Homes on slabs and over crawl spaces

No basement means the main floor is your target. Radon enters through penetrations in the slab, through gaps at the slab-wall joint, and through crawl space vents and vapor barrier openings. It first concentrates on the ground floor before gradually mixing upward.

The same placement rules apply: 3 to 5 feet up, away from windows and HVAC vents, out of high-humidity rooms. A first-floor bedroom, den, or family room you occupy for several hours a day is a better location than an entryway or hallway where nobody lingers.

One note for crawl space homes: if your crawl space is passively vented with screened openings rather than sealed and encapsulated, the air beneath your first floor has direct soil-gas contact. Main-floor readings can run higher than homeowners expect. Test there first, then decide whether additional investigation is warranted.

Choosing the right room on the right floor

Two principles cover most situations.

Test where you sleep. A bedroom is the ideal location for a radon detector, especially a basement bedroom. Someone sleeping there 7 to 8 hours a night, 365 nights a year, is accumulating the most sustained exposure in the household. That is the space most worth knowing about. If you have children sleeping in a basement bedroom, that room moves to the top of the priority list.

Choose a room with normal airflow. A room that stays sealed by closed doors most of the day does not represent typical air quality in the rest of the space. A slightly open room, or one that shares airflow with adjacent areas, gives a more representative reading.

Rooms to avoid: kitchen, bathroom, laundry room, utility room, and spaces nobody spends time in. The goal is to measure what you actually breathe, not the average of the whole structure.

Testing multiple floors

One detector on the lowest occupied floor gives you the highest-risk reading. But a single number does not tell you what is happening upstairs.

For a more complete picture, run a second device on the main floor at the same time. You may find that upper levels read considerably lower, which is common, or that radon is elevated throughout the home, which is possible in areas with high uranium content in the soil. Buyers and home inspectors in radon-prone regions increasingly expect readings from multiple levels, so if you are preparing to sell, two simultaneous tests are worth doing.

For everyday monitoring, a continuous electronic radon detector placed on the lowest occupied floor covers your most important data point. These devices track levels over days and weeks, which matters because radon fluctuates: levels often run higher in winter when windows stay closed and the house is tighter. Our roundup of the best home radon detectors covers the monitors most worth considering, from basic plug-in units to more feature-rich options.

Short-term and long-term tests: placement stays the same

Whether you are running a 48-hour short-term charcoal canister or a 90-day alpha track test, the placement protocol is identical: lowest occupied floor, 3 to 5 feet up, away from drafts and humidity. What changes is operating conditions. Short-term tests require closed-house conditions for at least 12 hours before and during the test period, meaning no open windows or doors and no exhaust fans running on continuous settings. Long-term tests run under normal living conditions and tend to give a more accurate picture of your annual average exposure.

Our guide to short-term vs. long-term radon tests covers when each approach makes the most sense. If you want a step-by-step walkthrough of the EPA placement protocol for a mail-in kit, see our DIY radon testing guide.

Before buying a kit, check whether your state radon program offers free or subsidized options. Many do, and a quick search for your state name plus “radon test kit program” will find the local office. If you prefer to pick one up yourself, short-term charcoal kits run under $20 and continuous monitors typically fall between $100 and $250. Check price on Amazon.

FAQ

Do I need a radon detector on every floor? You need at least one on the lowest occupied floor, where exposure is typically highest. A second detector on the main floor gives a fuller picture, especially if you want to understand how radon moves through the home or are preparing to sell. Upper floors rarely need their own devices unless the lower levels genuinely go unoccupied.

My basement is unfinished. Do I still test there? Yes, if you spend time in it. Even occasional use for laundry, storage access, or weekend projects qualifies. Radon does not care whether the space has drywall. An unfinished basement in radon-prone soil can easily read above the EPA action level of 4 pCi/L. If you truly never enter the basement, move the test to the main floor instead.

Can I put a radon detector inside a closet? No. Closets have restricted airflow, and a reading taken behind a closed door does not represent what you breathe in the open room. Keep the detector in the occupied, open portion of the room, on an interior wall at breathing height.

How long do I need to leave the detector in one spot before I can trust the result? Short-term charcoal kits require 48 to 96 hours, or whatever the label specifies. Long-term alpha track tests run 90 days to a full year for the most accurate annual average. Continuous electronic monitors can remain in place indefinitely and are useful for watching how levels shift with seasons and weather. Do not move a test kit mid-test; repositioning it during the measurement period invalidates the result.