The post-mitigation retest failed. What now?

Confirm the result before acting on it

Before anyone opens a slab again, make sure the number is real. The verification test should be in the same location as the original, on the lowest lived-in level, under closed-building conditions, after the system has run continuously for at least twenty-four hours.

A test placed in a different room, run with a window open, or started an hour after commissioning is not a failure of the system. Repeating a properly conditioned short-term test is the cheapest step available and it resolves a meaningful share of these cases.

The standard diagnostic sequence

A contractor who knows what they are doing does not guess at the fix. They measure. The tool is a set of small test holes drilled through the slab at increasing distance from the suction point, with a micromanometer reading the pressure difference at each one. That maps how far the suction field actually extends.

Where the field falls short, the causes are usually identifiable: tight or inconsistent sub-slab material that resists air movement, an interior footing dividing the slab into sections that do not communicate, an addition on its own separate pour, or a large opening bleeding suction away before it can spread.

• Sub-slab pressure field extension measured at multiple points
• Inspection of the sump pit lid, floor drains, and floor-wall joint
• Verification that any crawl-space membrane is sealed at walls and seams
• Check for slab sections that do not communicate with the suction point
• Confirmation that the fan is matched to the measured static pressure

The remedies, roughly in order of cost

Sealing comes first because it is cheapest and frequently sufficient on its own when a specific opening is the problem — a sump lid that was never sealed, a floor drain without a trap, a gap at the floor-wall joint, an exposed soil section behind a finished wall. Sealing alone rarely solves an elevated reading, but sealing that stops a suction leak often does.

Next is fan selection. A fan matched to the wrong static pressure moves the wrong amount of air. Swapping to a unit sized to the measured conditions is a modest cost on an existing pipe run.

Last is an additional suction point, which means another core through the slab, more pipe, and tying the new run into the existing fan or adding a second one. This is the real expense, and it is the correct answer when the diagnostic shows part of the footprint is simply out of reach.

Who pays

There is no universal rule, and anyone who tells you otherwise is describing their own contract rather than yours. What governs is the language you signed.

A scope that guarantees a specific outcome — most commonly a post-mitigation result below 4 pCi/L, sometimes with a stated number of remedial visits included — puts the corrective work on the contractor. A scope that promises an installation, a fan, and a pipe puts it on you.

Read the warranty section carefully too, because outcome guarantees and equipment warranties are different things and are often confused. A five-year fan warranty says nothing about whether the design has to work.

Keeping the conversation productive

Approach the installer with data rather than frustration: the verification result and its date, the placement and conditions, the manometer reading, and the relevant clause of the scope. Ask specifically for a pressure field extension diagnostic and a written explanation of what it showed.

If the response is that the number is acceptable, that the weather explains it, or that you should simply retest until it comes down, get an independent certified contractor to run the diagnostic. Their report is what you need to resolve a billing dispute, and it is worth its cost even if the first contractor eventually does the work.

Setting the expectation before the next job

Whatever happens here, the lesson transfers. On the next radon quote you accept — for this house or a future one — the scope should say what happens if verification fails, how many corrective visits are included, and whether the post-mitigation test is inside the price.

That single paragraph is the difference between a defined procedure and an argument. It costs nothing to ask for it before signing and everything to negotiate for it afterward.