Why Is My Radon Still High After Mitigation? A Troubleshooting Guide

A radon mitigation system that is doing its job should pull a home well under the EPA action level of 4 pCi/L, and most reputable contractors guarantee a result under 2 pCi/L. So when you pay for an install, wait for the system to settle, run a retest, and the number comes back high anyway, it is reasonable to feel both confused and a little cheated.

The good news: this is almost always fixable, and in most cases it is the contractor’s job to fix it at no extra charge. A high reading after mitigation is not a sign that radon is unbeatable in your home. It is a sign that something specific is wrong, and radon problems are mechanical, which means they are diagnosable. This guide walks through how to confirm the problem is real, the six reasons systems underperform, and what a competent contractor owes you on a callback.

First, Confirm the Reading Is Actually Real

Before you call anyone, make sure the high number reflects the home and not the test.

A post-mitigation test has to follow the same rules as any other radon test, and a surprising number of “system failures” are really test failures. Run the retest in the lowest occupied level of the home, in a room where someone actually spends time, not in a closet, a crawl space, or directly next to the system’s suction pipe. Keep closed-house conditions: doors and windows shut except for normal entry and exit, for the 12 hours before a short-term test and for its full duration. Do not adjust the HVAC mid-test, and do not place the detector on a windowsill or near an exterior door. Our radon testing guide covers placement in detail, and the DIY versus professional testing guide explains which kits meet AARST measurement standards.

There is one more confirmation worth doing, and it takes ten seconds: read the manometer.

Is the Fan Actually Running? Read the Manometer, Not the Outlet

Every code-compliant system has a U-tube manometer, a small clear gauge mounted on the vent pipe, usually filled with colored liquid. It is the single most useful diagnostic tool you own, and most homeowners never look at it.

When the system is working, the liquid in the two sides of the U sits at different heights. That offset is the proof that the fan is creating negative pressure in the pipe and pulling soil gas from beneath your slab. When the two columns sit at the same level, there is no suction, which nearly always means the fan has died or the pipe is blocked.

Two things to understand about the manometer. First, it tells you the system is moving air, not what your radon level is. A perfectly offset manometer on a system that still reads high means the fan is fine but the suction is not reaching enough of the soil. Second, do not rely on whether the fan is plugged in or humming. Radon fans can spin while a cracked housing or a blocked pipe kills the actual suction. The manometer is the truth; the outlet is not.

If the columns are even, you have likely found your problem before you have even finished reading this guide. Skip to the warranty section: a fan that failed inside its warranty window is a straightforward replacement.

The Six Most Common Reasons a System Underperforms

If the retest is valid and the manometer shows suction but levels are still high, the cause is almost always one of these six.

1. An Undersized or Failing Fan

Radon fans are sized to the resistance of the soil under a specific home. A fan that was marginal for your foundation on install day, or one that is degrading after years of service, may move air without generating enough vacuum to pull radon from the whole footprint. Fans typically last 5 to 10 years and weaken before they die outright. Our radon mitigation fan guide explains how fans are matched to homes and when an upsize is warranted.

2. Missed Entry Points or Not Enough Suction Points

This is the most common real cause. A single suction point can only depressurize so much area beneath a slab, and the reach depends on how connected the sub-slab gravel layer is. Homes with footings that divide the slab, additions poured at a different time, or poorly draining fill often need two or more suction points to cover the whole foundation. If the original install used one point on a home that needed two, parts of the slab were never under suction, and radon kept entering there.

3. Leaky or Disconnected Ductwork

The vent pipe relies on every joint being airtight. A glue joint that was rushed, a coupling that worked loose, or a crack from a knock in the basement bleeds suction out of the system before it reaches the soil. The symptom is often a manometer reading that is lower than it was at install. A contractor can find leaks quickly with a smoke pencil or by ear.

4. Foundation Changes Since the Install

Your house is not static. A new sump basin without a sealed lid, a finished-basement renovation that opened a wall cavity, a fresh crack from settling, or a poured addition can all create a new radon pathway or a leak in the depressurized zone. If your system worked for years and then a retest spiked, ask yourself what changed in the foundation. Our maintenance guide covers the retest-every-two-years discipline that catches these shifts.

5. Short-Circuiting

A well-designed system pulls air from beneath the slab. A poorly sealed one pulls conditioned air out of the house through a gap in the slab, around the suction pipe, or through an open sump. This “short-circuit” wastes the fan’s capacity on house air instead of soil gas, so radon keeps coming in elsewhere. The fix is sealing: the suction point penetration, slab cracks, the sump lid, and any open block-wall cavities all have to be closed so the vacuum acts on the soil.

6. Sub-Membrane Seal Failure in Crawl Spaces

If your home has a crawl space, the system depends on a heavy polyethylene membrane laid across the crawl floor and sealed tightly to the walls and piers. A torn membrane, a seam that lifted, or a section that was never fully sealed lets the suction pull crawl air instead of soil gas. Crawl-space systems are more sensitive to install quality than slab systems for exactly this reason. If your contractor flagged moisture or grading issues that needed fixing first, those can also undermine the seal over time.

What a Good Contractor Does on a Callback

When you call the installer back, a competent one does not just swap the fan and hope. The professional move is to re-run a Pressure Field Extension (PFE) test: drill small test holes at points away from the suction location, apply a micromanometer, and measure whether the negative pressure field actually reaches those areas of the slab. The PFE result tells the contractor exactly where the system is and is not working, which turns “your radon is still high” into “the northeast corner of the slab is not under suction, so we need a second point there.”

Expect the callback to include a fresh diagnostic, a clear explanation of the cause, the specific fix (an added suction point, resealing, a fan upsize, membrane repair), and a follow-up test to verify the result. If the contractor cannot tell you why the system underperformed, that is a reason to get a second certified opinion. Our guide on choosing a radon contractor covers how to vet for NRPP or NRSB certification and a written guarantee.

When It Is Not the System: Other Radon Sources

Soil-suction systems address radon coming up through the ground, which is the source in the overwhelming majority of homes. A few homes have a second contributor that a sub-slab system cannot touch.

The most common is well water. Radon dissolved in groundwater off-gasses into the air during showers, dishwashing, and laundry, and a soil-side system does nothing for it. If you are on a private well and your levels stay stubbornly elevated after a verified-good system, test the water. Aeration or granular activated carbon treatment handles waterborne radon. Our guide on radon in well water explains when water is worth testing and how it is treated.

Building materials are a far rarer contributor. Some stone and concrete products emit trace radon, but the amounts are almost never enough to keep a home above the action level on their own. Rule out the system and, if relevant, the water before chasing this.

Your Warranty and What It Should Cover

This is the part homeowners most often leave on the table.

A reputable radon contractor guarantees the system will hold the home under 4 pCi/L, and many guarantee under 2. If your post-mitigation or follow-up test comes back high and you are inside that guarantee, the corrective work, adding a suction point, sealing a missed entry, repairing a membrane, or replacing a defective fan, is the contractor’s responsibility, not a new invoice. The fan itself usually carries a separate manufacturer warranty of around 5 years.

Get all of this in writing before the install. The single best protection against a high reading after mitigation is hiring a certified contractor who commits to a guaranteed result and includes post-mitigation testing in the original quote. If your number is still high, see our companion guide on what radon level is dangerous for context on how much urgency a given reading actually warrants, then put the fix back on the contractor who guaranteed it.

A high radon reading after mitigation is frustrating, but it is a solved category of problem. Confirm the test, read the manometer, identify which of the six causes fits, and hold your contractor to the guarantee. Most homes that read high after a first install are fully corrected with one focused callback.