Place your radon detector in the lowest level of your home that people regularly use, at least 20 inches above the floor and ideally 3 to 6 feet high. That’s the short answer, but the details matter. Poor placement is the most common reason for inaccurate radon readings, and a bad reading can mean either missing a dangerous problem or paying for mitigation you don’t need.
Start With the Lowest Lived-In Level
Radon is a radioactive gas that seeps up from the ground, so concentrations are highest closest to the earth. The CDC recommends placing your detector in the basement or the lowest level of your home. If you don’t have a basement, that means the ground floor.
The key word is “lived-in.” A finished basement where your kids play video games counts. An unfinished crawl space where no one spends time does not. Furnace rooms and utility closets are also poor choices, both because no one breathes that air regularly and because mechanical equipment can distort readings. You’re trying to measure what the people in your household actually inhale, so pick a room where someone spends at least a few hours per week.
The Right Height: Your Breathing Zone
The detector needs to sit at least 20 inches above the floor, with the ideal range being 3 to 6 feet. This range is called the “breathing zone” because it matches the height at which most people inhale air while sitting or standing. A detector placed directly on the floor can pick up higher concentrations that don’t reflect what you’re actually breathing. A nightstand, desk, dresser, or bookshelf in the middle of the room works well.
The CDC specifically recommends placing the device in the middle of the room rather than against a wall. Keep it at least four inches away from other objects so air can circulate freely around the sensor.
What to Stay Away From
Radon detectors are sensitive to airflow, humidity, and heat. Any of these can skew your results, sometimes dramatically. Avoid placing your detector near:
- HVAC vents, fans, and registers. Moving air dilutes radon near the sensor and gives you a falsely low reading.
- Windows and exterior doors. Even small drafts introduce outdoor air, which averages about 0.4 pCi/L compared to the national indoor average of 1.3 pCi/L.
- Kitchens and bathrooms. High humidity interferes with many detector types, and exhaust fans create airflow that disrupts measurements.
- Fireplaces and direct sunlight. Excess heat can affect the detector’s chemistry or electronics.
- Exterior walls. These tend to have more air infiltration and temperature variation than interior spaces.
The best spot is an interior room on the lowest lived-in floor, away from drafts and moisture, on a stable surface at breathing height.
Closed-House Conditions During Testing
Placement alone isn’t enough. For short-term tests (those lasting less than four days), you need to maintain what’s called “closed-building conditions” starting 12 hours before the test begins and continuing throughout the entire test period. That means keeping all windows shut on every level of the house, not just the floor being tested. Exterior doors should stay closed except for normal entry and exit.
Normal HVAC operation is fine. You don’t need to turn off your furnace or air conditioning. But whole-house fans, window fans, and anything that pulls outdoor air into the home should be turned off. These conditions keep the air inside your home stable enough for the detector to get an accurate reading.
Testing in Multi-Story and Multi-Family Buildings
If you live in a single-family home with multiple floors, one detector on the lowest lived-in level is typically sufficient for an initial screening. If that test comes back at or above the EPA’s action level of 4 pCi/L, you may want to test upper floors as well to understand how radon is moving through the house.
Multi-family buildings follow a different protocol. HUD guidelines call for testing 100% of all units that have a floor or wall in direct contact with the ground, including non-dwelling spaces like lobbies. For upper floors, at least 10% of units on each level above ground should be tested. If no units touch the ground at all (as in a building with underground parking), test the lowest occupied floor.
Short-Term vs. Long-Term Placement
Short-term tests run for 2 to 7 days and give you a snapshot. They’re useful for quick screening or real estate transactions, where a 48-hour minimum test is standard. For short-term testing during a home sale, the recommended approach is either placing two passive detectors side by side for at least 48 hours and averaging the results, or using a single continuous monitor for the same period. If the average hits 4 pCi/L or higher, mitigation is recommended.
Long-term detectors stay in place for 90 days to a full year and capture seasonal variation. Radon levels fluctuate with weather, ground moisture, and how often you open your home. A long-term test gives a much more reliable picture of your actual annual exposure. Place the detector in the same spot you’d use for a short-term test: lowest lived-in level, breathing zone height, away from drafts and moisture.
When Placement Rules Differ for Real Estate
During a home sale, radon testing follows stricter protocols because the financial stakes are higher and the potential for tampering exists. Many states require or strongly recommend that a certified radon measurement professional conduct the test rather than the buyer or seller doing it themselves. The detector placement rules are the same, but a professional will also document the device’s exact location, verify closed-house conditions, and use tamper-evident seals.
If you’re testing a home you’re considering buying and want to do it yourself, use a nationally recognized measurement device and follow the lab’s instructions carefully. Place it in the lowest level that could reasonably become living space, even if it’s an unfinished basement. A buyer might finish that basement someday, and lenders and inspectors want to know the worst-case scenario.
What the Numbers Mean
Once your test is complete, the result is measured in picocuries per liter (pCi/L). The EPA’s action level is 4 pCi/L, meaning you should install a mitigation system if your reading hits that number or higher. But the EPA also recommends considering mitigation for levels between 2 and 4 pCi/L, because there is no known safe level of radon exposure. The average American home reads about 1.3 pCi/L. Outdoor air averages around 0.4 pCi/L.
If your first short-term test comes back above 4 pCi/L, follow up with either a second short-term test or a long-term test before investing in mitigation. If both readings confirm the problem, a radon mitigation system (typically a vent pipe and fan that pulls gas from beneath the foundation) can reduce levels by up to 99%.