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.
If the number has not come down
Work these in order. The first four cost nothing and resolve a meaningful share of cases; the last three belong to a certified contractor.
- Confirm the test conditions
Closed-building conditions for 12 hours before the test and throughout it, whole-house fans and window units off, device on the lowest lived-in level at least 20 inches off the floor and away from exterior walls, vents, and the sump. More re-measurements fail here than anywhere else.
- Confirm the system is running
Listen for the continuous low hum at the fan. Check the breaker if there is silence. Do not open the fan housing, work on the roof discharge, or touch the wiring.
- Read the manometer
Offset fluid columns mean the system is developing suction. Level columns mean it is not — the fan, the pipe, or the discharge has a problem. Note the reading before you call anyone.
- Account for weather and duration
Cold weather, falling barometric pressure, frozen or saturated ground, and high wind all push readings up. Weather explains variation around a number; it does not explain a number well above the action level.
- Contact the installer with facts
Give them the result, its date, the placement, the conditions, and the manometer reading — then ask what the written scope says happens when a verification test fails.
- Ask for a diagnostic, not a guess
The professional next step is measuring sub-slab pressure field extension at test holes across the slab. That is what determines whether the answer is sealing, a different fan, or an additional suction point.
- Retest after any corrective work
Same location, closed-building conditions, after the system has run continuously for at least 24 hours. Without that, you have a change rather than a result.
Nothing on this list asks you to perform electrical, roofing, or fan work. Those are hazards and, in states with radon certification requirements, credentialed activities. Checking a gauge and listening for a hum is where homeowner troubleshooting ends.
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.
- Should I retest? — Time since the last test, plus what has changed in the house — the two things that decide it.
- Radon Testing Method Comparison Dataset — Which method answers which question — and which decisions each one cannot carry.
- If federal guidance changed — Which parts of a homeowner decision are anchored to the federal number, and which would not move at all.
- Licensing change mid-project — Open quote, scheduled install, or pending verification — how a regime change lands on each.
- Continuous monitors — The hourly log is the product. What it proves, and which decisions it still cannot carry.
- All homeowner guides — every decision page in one place.
- Editorial policy — the standards this page was reviewed against.
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.
Confidence and limits of this page
Every decision resource on this platform states its own boundaries. This is not a disclaimer — it is the part of this guide you should read before relying on it.
- What we knowClaims on this page we stand behind, each traceable to an approved source or to observable structure.
- National certification and state licensing are separate regimes, and which one legally governs depends on the state.
- Closed-building conditions, device placement, and duration are specified in published testing guidance rather than left to preference.
- Weather and season measurably affect indoor readings in the same house, which is why guidance treats a single short measurement as a screening step.
- What we do not knowQuestions you may reasonably have that this page does not answer.
- How far any individual short measurement sits from that house annual average, which cannot be recovered from the short measurement itself.
- Whether a given laboratory or device met its stated precision on your particular sample, which only duplicate measurements would show.
- What research would settle itWhat it would actually take to answer the above — naming it is the difference between honesty and hedging.
- Paired short and long measurements in the same houses under documented conditions, which would quantify how often a screening result changes the decision.
- Device-to-device duplicates across a range of humidity and temperature, the evidence that would establish real-world rather than laboratory precision.
- What we do not claimClaims you might expect to find here and will not, with the reason.
- We do not claim any device brand or laboratory is more accurate than another, because we hold no comparison measurements and a ranking without them would be invented.
- We do not claim a correction factor for season or weather, because applying one to a single house would imply a precision that does not exist.
- We do not state or estimate individual health risk, and no tool on this platform computes one.
- What we cannot independently verifyFacts this page relies on that we could not confirm ourselves.
- Whether a laboratory or professional currently holds the credential it advertises, which only the issuing authority can confirm at the moment you ask.
- The present content of the cited guidance documents, for the network reason stated in our methodology.
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.
Sources
- U.S. EPA — Radon
- U.S. EPA — A Citizen’s Guide to Radon
- U.S. EPA — Consumer’s Guide to Radon Reduction