Radon Testing Checklist: Placement, Closed-House Conditions, and Interpretation
Radon results are only as good as the conditions they were measured under, and the most common reason a test gets challenged is a protocol failure rather than a device failure. This checklist covers closed-house conditions, placement rules, duration, quality assurance, and how to present a result so the buyer understands what it does and does not establish.
Who orders this
Real estate transactions in EPA Zone 1 and Zone 2 counties, new construction with passive systems, post-mitigation verification, and any home where a prior test was near the action level. Radon is the second leading cause of lung cancer in the United States, and elevated levels occur in every state.
Closed-House Conditions
The condition period is the part clients most often break. Establish it in writing and document it at pickup.
- Closed-house conditions established at least 12 hours before the test begins
- Conditions maintained for the entire test duration
- All exterior windows closed
- Exterior doors closed except for normal entry and exit
- Written notice left with the occupant stating the requirements
- Whole-house fans, window fans, and attic fans off
- HVAC operated normally — permanently installed systems may run
- Fireplaces and combustion appliances not operated except for normal use
- Signed condition-compliance acknowledgment collected at retrieval
- Tamper indicators noted at retrieval
Device Placement
Placement rules exist because radon concentration varies sharply with height, airflow, and proximity to surfaces.
- Lowest level of the home suitable for occupancy, whether currently finished or not
- 20 inches to 6 feet above the floor
- At least 3 feet from exterior walls
- At least 3 feet from windows, doors, and other openings
- At least 1 foot from the ceiling
- At least 4 inches from any other object
- At least 4 feet from heat sources, HVAC vents, and direct sunlight
- Away from areas of high humidity — not in kitchens, bathrooms, or laundry rooms
- Not in a crawlspace, closet, or unoccupiable utility space
- Location photographed and documented with room identification
- Placement height and clearances recorded
Test Duration and Device Type
Short-term tests answer a transaction question. Long-term tests answer a health question. Say which one you performed.
- Short-term test minimum 48 hours
- Device type — continuous radon monitor, charcoal canister, alpha track, or electret
- Continuous monitors preferred in real estate transactions for their hourly data and tamper detection
- Hourly readings reviewed for spikes suggesting a condition violation
- Start and stop timestamps recorded
- Barometric pressure and temperature logged where the device supports it
- Long-term test (90+ days) recommended for occupancy decisions rather than transactions
- Device calibration date current and documented
Quality Assurance
A QA program is what separates a measurement from a guess, and it is the first thing challenged in a dispute.
- Duplicate measurements at the required frequency (commonly 10% of tests)
- Spike or known-exposure checks per program requirements
- Blank measurements for passive devices
- Annual calibration records for each continuous monitor
- Chain of custody for devices sent to a laboratory
- Measurement professional certification current (NRPP or NRSB)
- State radon certification where separately required
Interpreting and Reporting
Give the number, the action level, and the practical next step. Avoid implying that a result below the action level means zero risk.
- Result reported in pCi/L, averaged over the test period
- EPA action level of 4.0 pCi/L stated for reference
- Statement that EPA suggests considering mitigation between 2 and 4 pCi/L
- Explicit note that there is no known risk-free level
- Hourly data chart included for continuous monitor tests
- Any condition violations disclosed and their likely effect on the result
- Recommendation for a certified radon mitigation contractor if above action level
- Post-mitigation retest recommended within 30 days of system installation
Most common findings — and why they matter
These come up often enough that it is worth knowing the consequence, not just the observation.
Closed-house conditions broken during the test
An open window can halve a reading. Without a continuous monitor's hourly data, a violated test looks identical to a clean one.
Device placed too close to an exterior wall or window
Produces artificially low readings from dilution near the opening, which is precisely the result a motivated seller wants.
Testing on an upper floor
Radon concentrates at the lowest level. A second-floor test is not comparable to the standard and does not answer the question.
No retest after mitigation
System installation is not proof of performance. A retest within 30 days is the only confirmation the system actually works.
Reporting a sub-4.0 result as 'safe'
There is no established threshold below which radon carries no risk. Report the number and the reference level without editorializing.
Frequently asked questions
What radon level requires action?
The EPA action level is 4.0 pCi/L. EPA also suggests considering mitigation between 2 and 4 pCi/L. There is no level known to be risk-free, so the reporting should give the number rather than a verdict.
How long does a radon test take?
A short-term transaction test runs a minimum of 48 hours after a 12-hour closed-house period. Long-term tests run 90 days or more and better represent actual annual exposure.
Can radon testing be done at the same time as a home inspection?
The device is usually placed at the inspection and retrieved 48+ hours later. That second trip is why radon is priced as a separate service rather than bundled at no cost.
Does a radon mitigation system guarantee low levels?
No. A properly designed sub-slab depressurization system usually reduces levels substantially, but performance depends on soil, slab condition, and installation quality. Only a post-mitigation retest confirms the result.
Do I need certification to perform radon measurement?
In most states, yes — NRPP or NRSB certification, and several states require separate state licensure on top of it. Verify with your state radon program before offering the service.
Turn this checklist into a finished report
Walk the property, photograph what you find, and narrate your observations. FieldScribe analyzes the photos, transcribes the voice notes, and drafts the report in professional language — you review, edit, and export the PDF. First report is $9.99 with no subscription.
Related checklists
This checklist is provided free for professional and personal use. It is a general reference, not a standard of practice — inspection requirements vary by state, jurisdiction, and client agreement, and licensing rules for specialty inspections differ. Verify current requirements with your state board and the relevant standard of practice before relying on it in the field.