Closed-house conditions mean keeping every window closed, limiting exterior-door openings to normal entry and exit, and not running fans or equipment that bring outdoor air inside. For a 2–4-day short-term radon test, maintain these conditions for at least 12 hours before testing and throughout the test; continue normal heating or cooling only when the system recirculates indoor air.
What closed-house conditions require
The purpose is to measure radon without unusual ventilation artificially changing the result. The home may remain occupied, and people can enter and leave normally.
According to the EPA’s Home Buyer’s and Seller’s Guide to Radon, occupants should:
- Keep all windows closed.
- Keep exterior doors closed except for ordinary entry and exit.
- Avoid operating whole-house fans, window fans, or other equipment that brings in outdoor air.
- Run heating and cooling systems normally.
- For tests lasting less than one week, use only air-conditioning systems that recirculate indoor air.
- Leave the test device in place and undisturbed.
- Keep an active radon-reduction system operating properly throughout the test.
The EPA allows a radon-system fan to continue running. It also says small exhaust fans may operate for short periods, which generally accommodates necessary, brief household use. If a bathroom fan, range hood, clothes dryer, fireplace, fresh-air intake, or ventilation system may run extensively, ask the tester how it should be handled before the test begins.
The testing process
For a reliable short-term test, use this sequence:
- Choose the proper device and location. Follow the device instructions and place it in the lowest level that could be used regularly—not in a bathroom, kitchen, laundry room, crawlspace, hallway, closet, or another area with high humidity or strong airflow. See where a radon test should be placed for room-selection details.
- Establish closed-house conditions. For a test lasting 2–4 days, close the home at least 12 hours before the start time. EPA recommends maintaining closed-house conditions for 4–7-day short-term tests as well.
- Record the start. Note the date, time, device identification, exact location, and who placed the device.
- Maintain normal occupancy. Do not intentionally ventilate the home, relocate or cover the device, or change normal HVAC operation merely to influence the reading.
- Complete the full exposure period. EPA says radon tests should last at least 48 hours, although a device may require longer.
- Record and submit promptly. Note the stop date and time and follow the laboratory’s return instructions without delay.
During a real estate transaction, the buyer, seller, agent, inspector, and occupants should agree in advance on who controls the device, who receives the report, and how possible condition violations will be documented. For a broader transaction workflow, see radon testing during a home inspection.
Edge cases that can affect the test
Severe weather: EPA advises against conducting a short-term test lasting less than four days during severe storms or high winds. If those conditions occur, document them and ask the tester whether the measurement remains usable or should be repeated.
A window was opened: A momentary, accidental opening is different from hours of ventilation, but its significance depends on duration, timing, weather, device data, and the applicable protocol. Record what happened instead of concealing it and let the qualified tester evaluate the test.
The house has mechanical ventilation: Heat-recovery ventilators, energy-recovery ventilators, make-up-air systems, and outdoor-air HVAC settings need case-specific instructions. Do not switch permanently installed equipment on or off solely for the test without confirming the applicable protocol.
There is a mitigation system: Keep the system running normally. If its fan is not working properly, the EPA checklist says to repair it and then test. Buyers should also request information about the system and previous measurements.
The home is newly built or recently renovated: Radon-resistant construction does not replace testing. EPA’s new-home radon guidance recommends testing after occupancy even when radon-resistant features are present. Renovations or HVAC changes can also justify a new test because they may affect radon levels.
What to ask the tester or inspector
Before authorizing a test, ask:
- Which measurement protocol and device instructions will be followed?
- Does the state require the tester, laboratory, or device to have a particular approval, license, certification, or registration?
- Where will the device be placed, and why is that location appropriate?
- Exactly when must closed-house conditions begin?
- How should the home’s HVAC, ventilation equipment, exhaust fans, and radon system operate?
- What features detect interference, movement, power loss, or unusual measurement patterns?
- What happens if weather or occupant activity compromises the conditions?
- Will the report identify the device, test location, exposure period, tester, and any observed deviations?
The EPA’s radon standards-of-practice page identifies the current consensus standards for home measurements and professional testing. Search the certified radon provider registry
Red flags that may justify a retest
Question the result when:
- No one gave occupants written test instructions.
- The required pretest closed period cannot be confirmed.
- Windows were left open or outdoor-air fans ran for extended periods.
- The device was moved, covered, unplugged, or placed near a draft, exterior wall, high heat, or high humidity.
- The test ended before the device’s minimum exposure time.
- A severe storm or high winds affected a test lasting less than four days.
- An existing mitigation fan was switched off or malfunctioning.
- The report omits the location, start and stop times, device information, or known deviations.
- A seller, buyer, or tester cannot explain unusual changes shown by a continuous monitor.
EPA warns that disturbing the device or closed-house conditions may invalidate a result and may be illegal in some states. That is federal guidance—not a universal federal real estate law. State and local rules may impose different or stricter testing, licensing, disclosure, placement, or transaction requirements, so verify the current rules with the relevant state radon program and local authority.
Records to keep
Retain the complete laboratory or monitor report, device identification, test location, floor level, start and stop times, weather notes, closed-house instructions, occupant acknowledgment, and documentation of any interruption. Also keep earlier radon results, mitigation-system plans, installation and service records, fan or alarm information, and post-mitigation tests.
These records help buyers evaluate whether an older result still represents the house. They also give sellers and agents a clear basis for disclosure discussions if a reading is elevated. If the result is 4 pCi/L or higher, EPA recommends taking action to reduce radon; learn how a radon mitigation system works.
Can I open an exterior door during a radon test?
Yes. Closed-house conditions allow normal entry and exit, but the door should not be propped open or used for deliberate ventilation.
Can ceiling fans run during a radon test?
A ceiling fan normally recirculates indoor air rather than bringing air inside. Still, follow the device instructions and tester’s protocol, especially if the fan creates strong airflow near the monitor.
How long must windows stay closed before testing?
For a 2–4-day short-term test, EPA guidance calls for at least 12 hours of closed-house conditions before the test and throughout the measurement period.
What should I do if closed-house conditions were broken?
Document what happened, including when and for how long, and notify the tester or parties to the transaction. If compliance cannot be confirmed, a new test is generally the clearest way to obtain a defensible result.