The Seller Aired It Out: A Short-Term Real Estate Test Mid-Transaction
A 48-hour continuous radon monitor (CRM) is running in the finished basement of a split-level home during a real estate transaction. Placement follows your SOP: lowest livable level, 20 inches above the floor, 3 feet from exterior walls, 4 feet from the sump pit, out of the HVAC supply stream. Closed-house conditions began 12 hours before deployment, and the seller signed the closed-house acknowledgment at deployment. At hour 31 the hourly values fall from 4.3 pCi/L to 0.7 pCi/L within a single hour, hold between 0.5 and 1.1 for five hours, then climb back to 4.0 pCi/L. The CRM's motion and tamper indicators show no device movement. The seller's agent later mentions the seller 'aired the place out' after painting a bedroom. The 48-hour average is 3.2 pCi/L. The buyer's agent needs the report by Friday to meet an inspection contingency.
1. Is this test valid to report? Walk through the evidence you would use and state the decision rule you are applying.
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Invalid. The decision rule is that a short-term test must be conducted under closed-house conditions for at least 12 hours before and throughout the test.
Two lines of evidence show those conditions failed:
- The seller's admission that the house was aired out.
- The hourly trace itself. Natural diurnal variation and instrument drift do not move a basement from 4.3 to 0.7 pCi/L in one hour and back again five hours later. That pattern is ventilation: outdoor air at roughly 0.5–1 pCi/L replacing indoor air.
The average of 3.2 pCi/L is not a measurement of this house under closed-house conditions. It is a weighted mix of a closed-house period and an open-house period, and it understates the closed-house concentration, which is the number the buyer actually needs.
Motion and tamper flags only detect device handling. The radon trace is the primary evidence, and it overrides a clean tamper log. Do not report 3.2 pCi/L as a valid result.
2. What do you do before Friday? Include what goes in the written report and what you tell each party.
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Same day, notify the buyer's agent, the seller's agent, and your client in writing that the test is invalid and why, with the hourly data attached.
The report should not simply omit the test. It should state that a device was deployed, list deployment and retrieval details, show the data, and give the invalidation reason. A silent gap invites the assumption that no test was ever performed.
Offer the remedy: a new 48-hour CRM test at the same location, with closed-house conditions documented for 12 hours before and throughout, starting as soon as painting and ventilation are finished. Discuss who bears the cost - typically the party who caused the disturbance, or as the purchase contract specifies.
If Friday cannot be met, propose options in writing: extend the contingency, hold funds in escrow pending a valid test, or test after closing with an agreed-upon remedy if the result is elevated. Never average the two periods or 'correct' the data to fit the deadline.
3. The buyer asks whether the 3.2 pCi/L average can be used since 'most of the test was fine.' Draft the core of your written response.
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The average cannot be used. The 3.2 pCi/L is an arithmetic mean across a period when the building was closed and a period when it was ventilated; it estimates neither condition. Using it would bias the decision toward 'no action' and could lead a buyer to skip mitigation on a home that may be well above 4 pCi/L when closed.
State plainly that a valid test is inexpensive compared with mitigation or with buying a home that has an undisclosed radon problem. Offer the restart as the fastest path to a defensible number. If the seller refuses access, document the refusal and recommend a post-closing test before occupancy, with the offer negotiated accordingly.
Also note that a single short-term test is a snapshot. A follow-up long-term test is appropriate regardless of the outcome, and the report should say so rather than implying the question is settled by any one test.