Where Should a Radon Test Kit Be Placed?

By Chris, Founder of RadonResources.com · Reviewed against current EPA and state radon program guidance

Place a radon test kit in the lowest lived-in level of your home, in a room you use regularly such as a bedroom, living room or finished basement den. Set it at least 20 inches above the floor, about 1 foot from exterior walls, 3 feet from exterior doors and windows and 4 inches from other objects. Keep it out of kitchens, bathrooms, laundry rooms and closets, and away from drafts, heat, humidity and sump pumps. This article explains each rule, how placement changes for a home sale, and the mistakes that most often skew a result.

Start with the lowest lived-in level

Radon comes from the soil, so it usually concentrates on the lowest floors of a home. The EPA and state radon programs recommend testing on the lowest level where people regularly spend time. In a home with a finished basement, that’s usually the basement. In a home built on a slab, it’s the main floor.

“Lived-in” is the key word. A basement used as a family room, office or bedroom counts. A basement used only for storage and the furnace may not, for a homeowner’s own test. Testing where you actually spend time gives a better picture of your real exposure. If you’re thinking about finishing an unused basement, testing it first can tell you whether to plan for mitigation before the walls go up.

Don’t place a test in a crawl space. People don’t live there, and the reading wouldn’t reflect the air in the rooms above.

Choose the right room

Within the lowest lived-in level, pick a room used regularly, such as a bedroom, living room, playroom or den. Massachusetts’ radon program, like others, advises against kitchens, bathrooms, laundry rooms and closets. These rooms have exhaust fans, moisture, heat or limited air movement that can affect the reading, and they don’t represent where people spend most of their time.

A bedroom is often a good choice, because people spend a large share of their hours there. Just make sure the device won’t be bumped, moved or covered during the test.

The placement distances that matter

State testing checklists, including those from Arkansas and Massachusetts, give the same basic distances. Place the device:

• At least 20 inches above the floor, on a table, shelf or dresser. This puts it in the breathing zone and avoids floor-level drafts.
• At least 1 foot from exterior walls. Outside walls can have air leaks that dilute the reading.
• At least 3 feet from exterior doors, windows and other openings in outside walls.
• At least 4 inches from other objects, so air can circulate around the device.

Also keep the device away from heat sources, direct sunlight, electronics, high humidity, sump pumps and heating or cooling vents that blow directly on it. Some state guidance also recommends keeping devices off stone surfaces such as granite, which can give off small amounts of radon on their own.

Keep the house closed during the test

Placement alone isn’t enough. Short-term tests need closed-house conditions so the result isn’t diluted by outdoor air. Keep windows and exterior doors closed for at least 12 hours before the test starts and throughout the test, except for normal entry and exit. Close fireplace dampers, and don’t run window fans or whole-house fans. Run your heating or air conditioning as you normally would.

Massachusetts also advises against short-term testing during severe storms or periods of high wind, because rapid weather changes can affect radon levels. Our article on why radon rises when you heat your house explains how air pressure inside a home affects readings, and our guide to radon testing in winter vs. summer covers how the season affects your result.

How placement changes for a home sale

Testing during a real estate transaction follows slightly stricter rules. Instead of the lowest level people currently use, the test usually goes in the lowest level that could be used as living space. Arkansas’ checklist notes that this typically means the basement, unless it has a dirt floor or a ceiling too low to finish without major structural changes. The idea is to capture what a buyer could be exposed to if they use the space.

Transaction tests are usually run by a certified measurement professional, often with two devices side by side or a continuous monitor that records hourly readings. Larger homes may need more than one test location. Arkansas’ guidance recommends additional tests when the tested level is larger than 2,000 square feet or the home has more than one foundation type or HVAC system. Our guide for real estate agents on radon covers how testing fits into a sale.

Common placement mistakes

A few habits cause most questionable results:

• Setting the kit on the floor. Floor-level placement can be affected by drafts and is easy to disturb.
• Testing in the furnace room or near the sump. Air movement and moisture in these spots can push readings up or down.
• Moving the kit mid-test. Once a test starts, leave it in place until the end.
• Opening windows during the test. Fresh air dilutes radon and can make a high level look low.
• Testing an unused level for your own peace of mind. A storage basement may read higher than where you actually live, which can overstate everyday exposure.
• Forgetting the start and stop times. Passive kits sent to a lab usually need accurate dates and times to calculate the result.

Follow the instructions that come with your kit, since some devices have their own requirements for test length or handling. Our radon test kit comparison covers short-term kits, long-term kits and continuous monitors, and our radon testing guide walks through the full process.

Should you test more than one floor?

One test on the lowest lived-in level is the standard starting point. Testing an upper floor as well can be useful if you want to know whether your bedrooms read differently from the basement, or if you’re deciding whether to mitigate based on how you use the house. The EPA’s action guidance is based on the lowest lived-in level, though, so that’s the result to compare against 4 pCi/L.

The EPA recommends fixing a home at or above 4 pCi/L and considering action between 2 and 4 pCi/L. No radon level is considered completely safe. If your result is near the line, our article on what 4.0 pCi/L actually means explains how to interpret it and when a follow-up test makes sense.

Placement after mitigation

If your home has a radon mitigation system, place the post-mitigation test in the same location as the original test. That way the before-and-after results can be compared directly. The fan should be running for at least 24 hours before the test begins and throughout the test. If a post-mitigation result is still high, our article on what to do when a post-mitigation test fails walks through the next steps.

Frequently asked questions

Can I put a radon test in my basement if no one uses it?

For your own home, the standard is the lowest level people regularly use. An unused basement may read higher than your living space and overstate daily exposure. Testing it can still be helpful if you might finish the space later. For a home sale, the test usually goes in the lowest level that could be used as living space, which often includes an unfinished basement.

Can I put a radon test kit in a bedroom?

Yes, if the bedroom is on the lowest lived-in level. Bedrooms are often a good choice because people spend many hours there. Place the kit on a dresser or table at least 20 inches off the floor, away from exterior walls, windows, heaters and fans, where it won’t be disturbed.

Can I hang a radon test kit?

Some kits can be suspended, which can help when there’s no suitable table or shelf. If you hang a device, follow the manufacturer’s instructions and keep the same placement distances from walls, windows and objects. Make sure it’s secure and won’t swing in a draft or get bumped.

Does a radon test kit need to stay in one spot?

Yes. Leave the device in the same spot for the full test period. Moving it partway through can change what it measures and make the result unreliable. If a kit is accidentally knocked over or moved, consider starting over with a new test.

Getting a result you can trust

Where you place a radon test affects how well the result reflects the air you breathe. A regularly used room on the lowest lived-in level, a raised surface away from walls and openings, and a closed house give you a number you can rely on. To see what others nearby have measured, you can browse radon readings homeowners have reported. If your test comes back elevated, you can connect with a certified radon professional for a follow-up test or mitigation estimate.

Sources and further reading

• U.S. Environmental Protection Agency, A Citizen’s Guide to Radon (lowest lived-in level, action levels, testing basics).
• Massachusetts Department of Public Health, Testing Your Home for Radon (rooms to avoid, placement distances, closed-house conditions, weather).
• Arkansas Department of Health, Radon Testing Checklist (placement distances, real estate placement, additional test locations, closed-house conditions).
• U.S. Environmental Protection Agency, Consumer’s Guide to Radon Reduction (post-mitigation testing).

This article is for general educational purposes and is not medical advice. Long-term radon exposure causes lung cancer, but individual risk depends on the level, the length of exposure and smoking history. Follow your test kit’s instructions, and consult a certified radon professional for testing during a real estate transaction or for mitigation.