Place the radon test in the lowest level of the house that is or could be used regularly, such as a basement bedroom, family room, playroom, office, or workshop. Set the device at least 20 inches above the floor, where it will remain undisturbed and away from drafts, exterior walls, heat, and humidity. Do not place it in a bathroom, kitchen, laundry room, hallway, closet, crawlspace, or furnace room.
How to choose the right level and room
Start with the purpose of the test. For a home sale, purchase, or inspection, test the lowest level a buyer could reasonably use as living space—even if it is unfinished today. The EPA Home Buyer’s and Seller’s Guide to Radon gives examples including bedrooms, play areas, family rooms, dens, exercise rooms, and workshops.
Use this process:
- Identify the lowest usable level. A basement that could become an office, bedroom, gym, or recreation room generally matters more than an upstairs room, even if nobody currently sleeps downstairs.
- Choose a representative room. Prefer an open, regularly usable room rather than a confined utility or storage area.
- Find a stable location at least 20 inches above the floor. A table, shelf, or purpose-built stand may work if the device instructions allow it.
- Keep the device away from problem conditions. Avoid exterior walls, direct sunlight, fireplaces, radiators, HVAC registers, fans, open windows, and areas with high humidity.
- Follow the device instructions. The manufacturer or laboratory may specify additional separation distances, exposure times, or mounting requirements.
- Document the exact location. Record the level, room, approximate position, height above the floor, and start and stop times.
For a detailed walkthrough beyond placement, see how to test for radon at home.
Placement checklist
Before starting, confirm that the device is:
- On the lowest level that is or could be used regularly
- In a bedroom, family room, den, office, playroom, exercise room, or similar usable space
- At least 20 inches above the floor
- In a location where children, pets, occupants, or contractors will not move it
- Away from exterior walls, drafts, high heat, and high humidity
- Away from doors, windows, supply registers, return vents, and fans
- Outside kitchens, bathrooms, laundry rooms, closets, hallways, stairways, crawlspaces, and furnace rooms
- Positioned according to the device manufacturer’s instructions
During a short-term test, closed-house conditions are important. EPA guidance generally calls for keeping windows closed and exterior doors closed except for normal entry and exit. For a test lasting two to four days, those conditions should begin at least 12 hours before testing and continue throughout the test. Heating and cooling systems may operate normally, but equipment that brings outside air into the house can affect the measurement.
Basements, crawlspaces, additions, and unusual layouts
Finished or unfinished basements: Finish level is not the deciding factor. If an unfinished basement could be used regularly after a renovation, a real estate test should generally be conducted there. EPA also advises testing before and after finishing or renovating a lower-level area.
Crawlspaces: Do not put a standard indoor-air radon device inside a crawlspace. The measurement is intended to represent air people may breathe in usable indoor space. Test the lowest occupiable room above or beside the crawlspace, following the device instructions or a qualified tester’s protocol.
Split-level homes: Select the lowest area that could be occupied regularly. If separate foundation areas or wings have limited airflow between them, one device may not adequately characterize the whole house. A qualified tester can determine whether multiple locations are appropriate.
Homes with multiple foundations: A basement, slab-on-grade addition, and crawlspace section may behave differently. The EPA-listed radon standards of practice include a residential measurement standard that addresses planning tests involving multiple home locations. Ask the tester to explain whether the structure requires more than one device.
New construction: Radon-resistant construction does not replace testing. EPA’s new-home radon guidance recommends testing after occupancy even when radon-resistant features were installed.
Homes with mitigation systems: Test with the system operating normally. If its fan or indicator suggests a malfunction, have the system evaluated before relying on the result.
What buyers, sellers, and agents should ask
Placement should be agreed upon before the device is deployed, especially when a purchase contract or inspection contingency is involved. Ask:
- Which level and room will be tested, and why?
- Could the buyer use a lower level as living space?
- Does the home’s foundation layout call for multiple devices?
- What device and test duration will be used?
- What closed-house instructions will occupants receive?
- How will movement, power loss, or other interference be detected?
- Will the report identify the device, room, height, dates, times, and test conditions?
- Does the tester meet current state or local requirements?
Search the certified radon provider registry
For transaction-specific planning, review the guide to radon testing during a home inspection.
Placement and testing red flags
Question the result—or consider retesting—if you find any of these conditions:
- The device was placed on an upper floor while a basement could be used regularly.
- It was left on the floor or placed in a bathroom, kitchen, closet, hallway, crawlspace, or utility room.
- It sat next to an open window, exterior door, HVAC register, fan, fireplace, or dehumidifier exhaust.
- Someone moved, covered, unplugged, or tampered with it.
- Required closed-house conditions were not maintained.
- Severe weather or unusual ventilation conditions conflicted with the device or testing protocol.
- The report does not identify where the measurement occurred.
- A seller’s earlier result covers a different level than the buyer intends to occupy.
- The home was renovated or its HVAC system changed after the previous test.
Federal EPA publications provide guidance, not a single nationwide licensing or transaction law. States and local jurisdictions may impose different testing protocols, disclosure duties, professional qualifications, or contract requirements. Verify the current rules with the applicable state radon program and local authorities.
Records to keep
Keep the laboratory or professional report, device type and serial number if provided, floor and room tested, placement height, test dates and times, and closed-house instructions. Preserve notes about weather, HVAC operation, power interruptions, open windows, device movement, and anyone who entered the property.
Also retain earlier results, renovation dates, foundation or HVAC changes, and all mitigation-system installation, maintenance, warranty, and post-mitigation test records. These details help a buyer or tester decide whether an older result still represents the home.
Frequently asked questions
Should a radon test go in the basement or on the first floor?
Use the basement if it is or could be used regularly. Otherwise, test the lowest regularly usable level, usually the first floor.
Can I place a radon test on the floor?
EPA guidance says to place the device at least 20 inches above the floor. Follow any additional height requirements supplied with the device.
Can a radon test be placed in a utility room?
Generally, choose a representative living area instead. Furnace rooms, laundry rooms, and other spaces with heat, humidity, or high airflow are poor locations.
What happens if the result is 4.0 pCi/L or higher?
EPA recommends taking action to reduce radon at 4 pCi/L or higher. Start by reviewing whether the test was placed and conducted correctly, then consult guidance on what a 4.0 pCi/L radon level means.