Radon Still High After Mitigation? Here Is Why, and What to Do

1. First, confirm you actually have a problem

Before you assume the system failed, make sure the number you are looking at is reliable. A single short-term reading can be thrown off by how and where the test was placed, by weather, or by the house not being closed up during the measurement. If your high result came from a quick consumer test in an unusual spot, repeat the test under proper closed-house conditions before drawing conclusions. We cover this in more detail in section 7.

Once you have a result you trust that is still at or above 4.0 pCi/L with a system installed, you have a real problem to solve, and the rest of this guide walks through the reasons in roughly the order worth checking them. A working radon mitigation system is designed to bring most homes below the action level, so a persistent high reading is not something to live with.

2. The fan is off, unplugged, or failing

This is the single most common reason a previously working system reads high again, and it is the easiest to overlook. A radon fan runs continuously, and over years of service it can wear out and stop. It can also be switched off at its dedicated breaker, tripped, or unplugged during electrical work, a roof job, or attic access, and then never turned back on. Because the system is quiet, nobody notices until a retest comes back high.

If the fan has simply been turned off, restoring power and confirming the manometer shows suction may be the whole fix. If the fan is genuinely worn out, it needs replacement, which is a routine job. Our radon fan replacement guide covers the signs of a failing fan and what replacement involves.

3. The fan is undersized for the home

Radon fans come in a range of strengths. Some are built to move a lot of air against low resistance, others to pull hard against tight, compacted soil. If the fan installed on your home is too weak for the soil and slab conditions underneath it, it can run perfectly and still fail to develop enough suction across the foundation to pull radon out faster than it enters. The system looks like it is working, but the numbers stay high.

This is a matched-equipment problem, not a broken-part problem. The correction is usually a stronger fan suited to the conditions, sometimes paired with an additional suction point. A professional measures the suction the system is actually achieving and selects equipment that can hold the slab under negative pressure. To understand how the suction mechanism is supposed to work, see how radon mitigation works.

4. Unsealed cracks, sumps, and openings

An active radon system works by holding the area under your slab at slightly lower pressure than the home above it, so soil gas is captured and vented instead of seeping inside. That negative pressure only holds if the big openings in the slab are sealed. A sump pit without a sealed cover, an open floor-wall joint, gaps around plumbing penetrations, or large cracks all let the fan pull air through those gaps instead of through the soil, which bleeds off suction where you need it.

Sealing is not a standalone radon fix, but it is an essential partner to the fan. When a system underperforms, unsealed openings are one of the most frequent reasons, and sealing the sump, joints, and major cracks often lets a properly sized fan finally pull the home below 4.0 pCi/L. This is usually a low-cost correction relative to the result it produces.

5. Not enough suction points for the foundation

A single suction point pulls on the soil around it, and how far that influence reaches depends on what is under the slab. Loose gravel spreads suction widely, while tight or poorly connected fill limits how much of the foundation one point can cover. If a large slab, or a section blocked by an interior footing, sits outside the reach of the one point that was installed, radon keeps entering through the area the system cannot influence.

Homes with more than one foundation type compound this. A house that mixes a basement with a slab-on-grade addition, or a basement with an attached crawl space, often needs a suction point and an approach for each area. An inaccessible foundation section, such as a sealed-off room or a crawl space that was never addressed, can leave a radon source the original system simply never reached. The fix is adding suction points, or adding crawl space membrane and suction, so the whole footprint is covered.

6. The system was designed for a different layout

Sometimes the equipment and the install are fine for what they were designed to do, but they were designed for the wrong situation. A system laid out for a simple basement may not account for a finished area, a later addition, or a crawl space that contributes radon. Renovations after the system went in can change the airflow and pressure balance of the home, and a system that worked at install can drift out of spec as the house changes around it.

This is why a diagnostic visit looks at the whole home, not just the fan. The goal is to match the system to how the house is actually built and used today, which may mean relocating or adding a suction point, upsizing the fan, or extending the system to an area it never covered. For a sense of what a well-functioning system should achieve, see our overview of radon mitigation effectiveness.

7. It may be a measurement issue, not a system issue

Not every high reading means the system failed. A test device placed too close to a window, in a draft, near an exterior door, or in a part of the home that is not representative can return a misleading number. Short-term tests are also sensitive to conditions during the measurement window, so an open window, a running whole-house fan, or major weather swings can skew the result.

The point is not to explain away a real problem. It is to make sure you are diagnosing the right thing. If a careful retest still comes back high with the fan confirmed running, the issue is in the system and one of the earlier sections applies.

8. What you can check yourself

A few checks are safe and useful for any homeowner before calling for service:

• Read the manometer. Find the U-shaped gauge on the vent pipe. Offset liquid columns mean the fan is pulling suction. Level columns mean it is not.
• Confirm the fan has power. Make sure the dedicated switch or breaker for the radon fan is on and was not turned off during other work.
• Check for fan operation. Listen for the steady hum and feel for light vibration at the fan housing, usually in the attic or outside on the exterior pipe.
• Look at the obvious openings. Note an uncovered sump pit, an open floor-wall gap, or large slab cracks, which point to a sealing gap.
• Retest properly. Run a fresh test in the lowest lived-in level, away from windows and drafts, under closed-house conditions.

These checks will not measure suction precisely or size a fan, but they often reveal a fan that is simply off, and they give a professional a clear starting point if a visit is needed.

9. When to call a professional

Call for a diagnostic visit when the fan is confirmed running, the manometer shows suction, and a proper retest still reads at or above 4.0 pCi/L. At that point the problem is in how the system is sized, sealed, or laid out, and those are not do-it-yourself fixes. A professional measures the suction the system is achieving, checks coverage across the whole foundation, identifies unsealed openings, and determines whether the answer is a stronger fan, additional suction points, better sealing, or extending the system to an area it never covered.

A failing or noisy fan, a system that worked before but now reads high, or a brand new system that never got below the action level are all worth a service call. After any adjustment, the work is not finished until a retest confirms the home reads below 4.0 pCi/L. EraseRadon diagnoses underperforming systems and corrects them across metro Atlanta, and you can read more on our radon mitigation services page or contact us for help.

10. Frequently asked questions