Post-Mitigation Clearance With an Installer's Monitor
A homeowner's pre-mitigation test in the basement averaged 12.4 pCi/L. A contractor installs a sub-slab depressurization system with three suction points and leaves a continuous monitor in the basement from the day before installation through 18 hours after the fan was started. The contractor reports a 0.9 pCi/L average and tells the homeowner the system is cleared. The homeowner then hires an independent measurement professional for a post-mitigation test. At the visit, the fan is running and the manometer reads -1.2 inches of water column. The homeowner closed the windows 12 hours ago, but the basement door was open to the upstairs for two hours yesterday during cleaning. The independent professional plans a 48-hour test and must decide whether the contractor's monitor data can be used, when the test should start, and what to do if the result is 4.2 pCi/L.
1. Can the contractor's 0.9 pCi/L average be used as a post-mitigation clearance result? Why or why not?
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No. The contractor's monitor ran through installation, when doors were likely open, the system was being adjusted, and closed-house conditions were not established for the required 12 hours before the measurement period. The average also includes the period before the fan started, so it does not represent post-mitigation conditions. In addition, a clearance test should not begin until at least 24 hours after the system is completed and any adjustments are finished; this monitor was read only 18 hours after startup. The contractor is also not independent of the mitigation work, which is a conflict even if the device is accurate. The 0.9 pCi/L value can be treated as a diagnostic indication that the system is moving soil gas, but it is not a valid clearance result and should not be reported as one.
2. When should the independent test start, and what conditions must be met?
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The test should start after the system has been completed and stable for at least 24 hours, following state requirements if they are stricter. Since only 18 hours have passed, wait at least another 6 hours before beginning. Then establish closed-house conditions for 12 hours before the start and maintain them for the full 48-hour test. Normal entry and exit are allowed, but windows and exterior doors should remain closed except for brief use, and the basement door should be kept closed as it normally would be. The fan must remain running, and you should record the manometer reading at the start and end, note any system adjustments, and verify the test location is in the lowest livable level. Document the cleaning episode and confirm that conditions were restored before the 12-hour precondition began.
3. If the 48-hour result is 4.2 pCi/L, what does that mean and what should you recommend?
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A 4.2 pCi/L result is at or above the 4 pCi/L action level, so the system has not been verified as reducing radon below the action level. Do not describe it as a clearance or as close enough. Recommend that the contractor evaluate the system: check fan performance, suction field, manometer reading, sealing at the slab and sump, and whether additional suction points or sealing are needed. After any work, wait the required period and retest under closed-house conditions. Because 4.2 pCi/L is near the action level and measurement uncertainty exists, a confirmatory test may be appropriate if no system defect is found, but the first step is system evaluation, not dismissal. Report the result accurately to the homeowner and note the conditions and device information.