How to Test for Radon in Your Home: A Plain-Language Guide
Testing for radon in your home is a procedural sequence with three distinct phases: pre-test preparation, the exposure itself, and post-test handling. Most procedural guides cover the middle phase well but skim the pre-test prep that determines whether the result is reliable. This guide gives equal attention to all three phases, with specific emphasis on the 12-hour closed-house preparation window, the placement strategy that produces representative results, and the mail-in protocol that preserves accuracy. This guide summarizes EPA Indoor Air Quality and Colorado Department of Public Health and Environment guidance current as of 2026.
How to test for radon in your home: the three phases
Phase one is pre-test preparation. The home must be in closed-house condition for at least 12 hours before the test starts and must remain in closed-house condition throughout the exposure. This is the procedural requirement that homeowners most often violate. The 12-hour pre-test window matters because indoor radon concentration takes time to equilibrate with the home in its closed state. Opening windows the morning of the test invalidates the result.
Phase two is the exposure. The detector sits at a specific location for a specific duration (48 hours minimum for short-term protocol). Placement rules matter; deviations bias the result.
Phase three is post-test handling. The detector is sealed, the documentation is completed, and the sample is shipped to the analyzing lab on the same day the test ends. Delays in shipping affect charcoal canister results. The full sequence walked through below produces a reliable, defensible measurement. The radon testing pillar guide provides regional context for Colorado homeowners.
Phase one: pre-test preparation
Start preparing 12 to 24 hours before the planned test start. Close every window in the home. Close every exterior door except for normal brief entry and exit. Close storm doors if you have them. Check the garage door (the home-to-garage door should be closed if the garage is unconditioned).
Configure HVAC for normal operation. Heating and cooling should run as they do in everyday use. Do not change thermostat settings dramatically; do not run the system harder than usual. If the home has a heat-recovery ventilator (HRV) or energy-recovery ventilator (ERV), operate it on its normal setting (these mechanical ventilation systems are exempt from the closed-house rule because they are part of normal operation).
Turn off whole-house fans, attic fans, and window-mounted exhaust fans. Briefly using a bathroom exhaust fan during a shower is acceptable; running it constantly is not. Do not use combustion appliances unusually heavily (do not run the wood stove at high heat to dry out the basement, for example). Live in the home normally; just close it.
Why 12 hours matters
Indoor radon equilibrates with the closed building envelope over a period of hours. When windows are open, outdoor air dilutes indoor radon and pushes the measurement toward zero. When windows close, soil gas entry continues at a rate determined by foundation and pressure conditions, and indoor concentration rises toward a steady-state value within hours. The 12-hour pre-test window ensures the home reaches that steady state before measurement begins. EPA Protocol explicitly requires the 12-hour pre-test prep period.
Phase two: detector placement and start
Place the detector at the lowest occupied level of the home. For most single-family homes that means the basement, finished or unfinished. For homes without a basement, the main floor. The placement spot should be in a regularly occupied or potentially-occupied room, not a kitchen or bathroom (humidity and cooking fumes interfere with some detector chemistries).
Position the detector 20 inches to 6 feet above the floor on a shelf, furniture surface, or small platform. Stay at least 3 feet away from exterior walls, doors, and windows. Stay at least 3 feet away from heating and cooling vents. Avoid direct sunlight, which can heat the detector. Avoid drafty corners. Avoid closets and cabinets where airflow is restricted.
Open the detector per the manufacturer instructions. Most charcoal canisters have a foil seal that gets peeled back. Record the start date and time on the information card. Document the location with a photo (timestamped photos help if any question arises later about the test conditions). The how to do a radon test step-by-step covers each placement decision.
During the exposure: what to do and what to avoid
The home should operate as it does in normal closed-house use. Run heat or cooling. Cook in the kitchen. Take showers (use the bathroom exhaust fan briefly while showering; turn it off when done). Run the dishwasher and clothes washer. Open the front door to leave for work and return at the end of the day. These are all normal activities and do not invalidate the test.
What to avoid: opening windows for fresh air, propping doors open for any extended period, running whole-house fans or attic fans, running exhaust fans continuously, running portable air purifiers that affect indoor air composition, lighting candles or incense in the test area (combustion byproducts can affect some detector chemistries), and disturbing the detector physically.
If a weather event during the test creates unusual conditions (extreme high winds, severe storms with rapid pressure changes), note the dates in case the result needs interpretation. Most short-term tests are unaffected by typical weather, but unusual events occasionally produce biased results.
Phase three: ending the test and shipping
At the end of the specified test duration (typically 48 hours for charcoal canister), reseal the detector per the manufacturer instructions. For a charcoal canister, place the foil cap back over the opening and tape it securely; some kits include a snap-on cap. Record the end date and time on the information card. Complete any remaining sections of the card: home address, contact email for results, test location within the home.
Place the sealed detector and completed card in the supplied envelope. Ship the same day the test ends. Use first-class mail at minimum; expedited shipping (Priority Mail) for kits where the lab is more than a few states away. Same-day shipping minimizes the time interval between test end and lab receipt, which preserves charcoal canister accuracy.
What if you miss the 12-hour pre-test window
If the home has been in open-windows mode and you remember to close it five hours before the planned test start, the practical answer is to delay the test start by seven hours so the full 12-hour window is satisfied. The kit chemistry does not start counting until you open the seal; you can buy the kit ahead of time, prepare the home, and start the test once the 12-hour prep is complete.
If you have already started the test but realize the home was in open-windows mode for part of the previous 12 hours, the result will be biased low. Decide whether to continue and accept the low-bias result, or abort and start fresh after proper preparation. Charcoal canisters cannot be paused and resumed; once exposed, the canister has begun absorbing.
Common failure modes and how to avoid them
Failure mode one: opening windows during the test. Solution: tell everyone in the home that windows must stay closed for the test duration. Put a note on the windows.
Failure mode two: placing the detector in the kitchen or bathroom. Solution: choose a living room, bedroom, office, or family room location.
Failure mode three: placing on the floor or against an exterior wall. Solution: place on a shelf 20 inches to 6 feet above the floor, at least 3 feet from exterior walls.
Failure mode four: forgetting to seal the canister at the end. Solution: set a phone alarm for the end of the test duration.
Failure mode five: delaying shipping. Solution: have a stamped envelope ready before the test ends; drop in the mail the same day.
Failure mode six: testing during unusual conditions (party in the basement with the door propped open, family member smoking heavily in the test area, contractor with an open door). Solution: schedule the test for a quiet stretch of normal home operation.
What to expect from the lab and the result
Lab analysis takes five to seven business days. Results return via email or postal mail, depending on the kit. The result includes the radon concentration in pCi/L, the test duration, any quality control notes, and a disposition statement (EPA action level exceeded, consider mitigation, or acceptable).
EPA action level: 4.0 pCi/L. At or above this level, mitigation is recommended. Between 2.0 and 4.0 pCi/L, mitigation is worth considering. Below 2.0 pCi/L, no immediate action needed; re-test every two to five years. For action steps see the test radon in home action guide.
Verification and follow-up
For an initial result at or above 4.0 pCi/L, EPA recommends a confirmatory measurement. A second short-term test at the same location is acceptable. A 90-day alpha-track is preferred because it captures annual-average exposure better than another two-day window. The combination of two short-term tests averaged, or a long-term test, gives the basis for the mitigation decision.
For homes that test below 2.0 pCi/L initially: re-test every two to five years. Re-test after significant home changes: new HVAC, new windows, foundation repair, basement finishing, addition construction. These changes affect pressure dynamics and soil-gas entry.
References
- EPA Citizen’s Guide to Radon — U.S. Environmental Protection Agency
- EPA radon measurement protocols — U.S. Environmental Protection Agency
- Colorado radon program — Colorado Department of Public Health and Environment
- CDC radon health information — Centers for Disease Control and Prevention
Front Range homeowners who want a NRPP-certified professional radon test or a mitigation referral can get in touch here.