Post-mitigation testing
A radon mitigation system is the only home repair with a built-in pass mark, and the post-mitigation test is that mark. Without it you own equipment; with it you own a documented result.
It is a small cost, it is frequently the first line dropped from a bargain bid, and it is the single most important item in the whole quote.
When it should happen
After the system has run continuously for at least twenty-four hours. Concentrations typically fall within a day or two of operation, and testing before the system has stabilized produces a number that means little.
It should not be delayed indefinitely either. Testing within the first weeks, while the installation is fresh and the contractor is still engaged, is what makes a remedy conversation straightforward if one is needed.
Where it should be placed
In the same location as the original measurement — the lowest lived-in level, the same room, ideally the same spot, twenty inches or more off the floor, away from exterior walls, drafts, and the sump. Closed-building conditions apply exactly as they did the first time.
Moving the detector between the before and after tests is the most common way a homeowner ends up unable to compare the two numbers. If the original location is unavailable for some reason, document why.
Who should perform it
The installing contractor commonly does it, and that is normal practice. Some homeowners prefer an independent measurement on the reasonable grounds that the person checking the work has no stake in the outcome.
Either arrangement is defensible. What is not defensible is no test at all, or a test performed by someone with a financial interest whose protocol and location are undocumented. Ask for the result on paper with the credential on the same page.
What if it comes back high
It happens, and it is a design problem rather than a dead end. The usual remedies are an additional suction point where the diagnostic underestimated what the slab needed, further sealing where the system is losing pressure, or a fan better matched to the sub-slab conditions.
What determines how painful this is for you is whether the written scope named a remedy before work started. When it does, the situation is a procedure. When it does not, it is a negotiation with someone who has already been paid.
Keep testing after that
File the verification result with the written scope and the warranty, then retest every two years and after anything that changes how the house breathes — a new furnace, replaced windows, a finished basement, a foundation repair, or a fan replacement.
That accumulating file is what answers a future buyer without another negotiation, and it is what distinguishes a maintained system from an installed one.
Post-mitigation testing: common questions
Should the verification test be short-term or long-term?
Short-term is standard for verification, because you need a prompt answer about whether the system performed and the system is running under controlled conditions. A long-term detector afterward is a useful supplement for a house you are staying in, since it captures the annual picture rather than the first few days.
What number should the post-mitigation test show?
Below EPA's 4 pCi/L action level at minimum, and mitigation professionals generally aim substantially lower than that. Some contracts specify a target. What matters is that the threshold and the remedy are agreed in writing before installation rather than debated afterward.
The contractor says a manometer reading proves it works. Is that enough?
No. A manometer shows that the fan is creating pressure difference — that the system is pulling. It says nothing about the indoor concentration, which is the thing you actually care about. Both matter, but only the post-mitigation measurement answers the question you paid to have answered.
Keep reading
- What to expect after installation — Verification, noise, energy, maintenance, and retest intervals.
- How testing works — Devices, placement, closed-building conditions, and what invalidates a result.
- Warranty questions — What the warranty covers, for how long, and what triggers a claim.
- Cheapest isn't always best — The three omissions that turn a bargain into a repeat job.
- All homeowner guides — every decision page in one place.
Twenty-four hours of runtime, same location as the original, closed-building conditions, result on paper with the credential. That document is the deliverable — everything else in the job is how it got produced.
Sources
- U.S. EPA — Radon
- U.S. EPA — A Citizen’s Guide to Radon
- U.S. EPA — Consumer’s Guide to Radon Reduction
Insist the verification test is in the quote
Independent licensed contractors quote the design and the verification together. Free to request, and the proof is priced in rather than added later.