Post-Mitigation Radon Test Failed? Next Steps to Take

By Chris, Founder of RadonResources.com · Reviewed against current EPA and state radon program guidance

If your post-mitigation radon test came back at or above 4 pCi/L, or above the level your contractor guaranteed, the system usually needs adjustment rather than replacement. Start by checking that the test itself was valid. Then contact your mitigation contractor, confirm the fan is running and ask them to diagnose the system. Common fixes include adding a suction point, sealing open entry routes, changing the fan or treating a crawl space or block wall the first design missed. This article walks through how to check the result, why systems underperform and how to work with your contractor to bring the level down.

What counts as a failed post-mitigation test

A post-mitigation test confirms whether your radon system is doing its job. The EPA recommends fixing homes at or above 4 pCi/L, so a result at or above that level after mitigation means the system isn’t meeting the basic goal. Many contractors also guarantee a lower target in their contract, such as 2 pCi/L or less. If your result falls between 2 and 4 and your contract promised 2 or less, the system hasn’t met the agreement even though it’s below the EPA action level.

Check your contract or invoice for the guaranteed level and how long the guarantee lasts. That document shapes what you can ask the contractor to do at no extra cost.

Step 1: Make sure the test was valid

Before assuming the system failed, confirm the test was run correctly. A flawed test can produce a misleading number, and checking takes only a few minutes.

• Timing. The EPA says a post-mitigation test should start no sooner than 24 hours after the system begins operating and within 30 days of installation. A test started too early may capture radon that hadn’t yet cleared from the house.
• Closed-house conditions. Windows and exterior doors should stay closed except for normal entry and exit for at least 12 hours before and throughout a short-term test.
• Location. The test should be in the same location as the pre-mitigation test, so the two results can be compared. It should sit away from drafts, heat sources, high humidity and exterior walls.
• Fan operation. The system fan should have been running for the entire test. A tripped breaker or unplugged fan will make the test meaningless.

If any of these weren’t met, a properly run retest is a reasonable first move. If the test looks sound, move on to the system itself. Our radon testing guide covers testing conditions in more detail.

Step 2: Check the system’s warning device

Every active radon system should have a monitor, usually a U-tube manometer on the vent pipe, a gauge or an audible alarm. The EPA notes that this warning device must be placed where it can be easily seen or heard. On a U-tube manometer, the liquid levels in the two sides should be uneven, which shows the fan is creating suction.

If the liquid levels are even, the fan may be off, failed or disconnected, or the pipe may be blocked. Check the fan’s circuit breaker and switch, and listen for the fan at the exterior or attic location. A reading that’s uneven but different from what the installer marked at setup can also point to a change worth reporting. Note what you see and share it with your contractor.

Step 3: Contact your mitigation contractor

Call the company that installed the system and share the test report, the manometer reading and your contract terms. A reputable contractor expects some systems to need follow-up work. The EPA notes that many contractors guarantee to adjust or modify the system until it reaches a negotiated radon level, so a second visit is often covered.

Ask the contractor what diagnostics they’ll perform, what changes they expect to make and whether the work falls under the guarantee. Get any added cost in writing before work begins. If your contractor is unresponsive or out of business, a different certified radon mitigation professional can evaluate the system. Our guide to hiring a certified radon mitigator and our radon contractor checklist explain what to look for.

Why radon systems sometimes fall short

Most homes are mitigated with active soil depressurization, which uses a fan and vent pipe to create suction beneath the foundation and pull radon outside before it enters. According to the EPA, sub-slab suction systems commonly reduce radon by 50 to 99 percent. When a system lands at the low end of that range, the cause is usually one of a handful of design issues.

The suction doesn’t reach the whole foundation

A system works only where its suction field reaches. Tight soil, a lack of gravel under the slab, interior footings or grade beams can block suction from spreading. Florida’s radon certification training notes that continuous footings or grade beams beneath the slab can disrupt the pressure field. The fix is often an added suction point on the far side of the obstruction.

Part of the house was left out

Homes with more than one foundation type, such as a basement plus a slab addition or a crawl space, may need each area treated separately. A system that covers the basement slab but not an adjoining crawl space can leave a major entry route open. Crawl spaces are usually handled with a sealed plastic membrane and a suction point beneath it. Our article on crawl space encapsulation and radon explains how that works.

Open entry routes compete with the system

Sump pits without a sealed cover, open floor drains, large cracks and gaps around pipes can let the fan pull air from the house instead of from the soil. That weakens suction under the slab. Sealing alone usually doesn’t fix elevated radon, but sealing major openings helps an active system work as designed. Hollow concrete block walls can also carry soil gas into a basement and may need their own treatment.

The fan doesn’t match the soil

Fans are sized for the conditions beneath the house. Tight soil needs a fan that creates strong suction at low airflow, while loose gravel needs higher airflow. A fan that’s mismatched with the soil may run correctly and still not reduce radon enough. Swapping to a better-matched fan is a common adjustment.

Radon is coming from another source

In homes with private wells, radon in water can release into the air when water is used. A soil depressurization system won’t address that. If a system checks out but indoor radon stays high, testing the well water is worth considering.

What the follow-up work usually involves

A contractor addressing a system that’s underperforming will typically start with diagnostics. This can include drilling small test holes in the slab and measuring suction with a micromanometer to see where the pressure field reaches and where it falls short. Based on those readings, common changes include:

• Adding one or more suction points
• Upgrading or replacing the fan
• Sealing the sump, floor drains and major cracks
• Extending the system to a crawl space or another foundation area
• Adding treatment for hollow block walls

Changes to suction can also affect fuel-burning appliances. The EPA notes that contractors may need to consider backdrafting, in which furnace or water heater exhaust flows back into the house. Our article on whether radon comes from a furnace covers that issue. After any modification, run a new post-mitigation test under the same conditions to confirm the change worked.

Keep testing after the system passes

Once a follow-up test shows the level is under control, the EPA recommends retesting at least every two years to make sure radon stays low. Radon can rise during the heating season, and fans wear out over time. A long-term test running more than 90 days gives a better picture of year-round performance than a single short-term test. Our radon test kit comparison covers long-term kits and continuous monitors, and our article on why radon rises when you heat your house explains seasonal changes.

A system that has had to be adjusted can still reduce radon well. Lowering an elevated level reduces your future exposure and the long-term lung cancer risk tied to it, even though it can’t undo exposure already received.

Frequently asked questions

How long after radon mitigation should I test?

The EPA recommends starting a post-mitigation test no sooner than 24 hours after the system begins running and within 30 days of installation. Use closed-house conditions and place the test in the same location as your pre-mitigation test so the results are comparable. A long-term test afterward gives a better sense of how the system performs across seasons.

Should I pay for a contractor to fix a system that didn’t work?

That depends on your contract. Many mitigation contracts guarantee a specific radon level and include adjustments needed to reach it. Review the guarantee terms, share your test results with the contractor and ask for any extra charges in writing before work begins. If the contract doesn’t include a guarantee, discuss the expected cost and results upfront.

Can radon still be high if the manometer shows suction?

Yes. A manometer only shows that the fan is pulling air through the pipe. It doesn’t show whether suction reaches the entire foundation. A system can run correctly and still miss areas because of footings, tight soil, a crawl space or open entry points. Diagnostic testing under the slab can find where suction falls short.

Should I test the system myself or use an independent tester?

Either can work if the test follows proper protocols. Some homeowners prefer a test from a certified measurement professional who isn’t the installer, especially when the result affects a guarantee or a home sale, because it removes questions about who placed the device. A homeowner test is reasonable for routine checks after the system passes.

Getting your radon level where it belongs

A failed post-mitigation test is a signal that the system needs adjustment, not that your home can’t be fixed. Confirm the test was valid, check the warning device and bring the results to your contractor so they can diagnose and modify the system. Once a new test shows the level is down, keep testing every couple of years. If you need a second opinion or a new contractor, you can connect with a certified radon mitigation professional in your area.

Sources and further reading

• U.S. Environmental Protection Agency, Consumer’s Guide to Radon Reduction (post-mitigation test timing, retesting, warning devices, guarantees, typical reductions, backdrafting).
• U.S. Environmental Protection Agency, A Citizen’s Guide to Radon (action levels, testing conditions).
• Florida Department of Health, Radon Certification Training, Unit 4: Mitigation Systems (pressure field diagnostics, causes of underperformance, post-mitigation testing).
• U.S. Environmental Protection Agency, Radon Reduction Techniques for Existing Detached Houses (active soil depressurization design and diagnostics).

This article is for general educational purposes and is not medical advice. Long-term radon exposure causes lung cancer, but individual risk depends on the level, the length of exposure and smoking history. Reducing elevated radon lowers future exposure; it does not reverse exposure already received. For help with your system, consult a certified radon mitigation professional.