--- title: "Radon Still High After Mitigation: Why It Happens and How to Fix It" canonical: "https://radonmitigationyorkpa.net/radon-guide/radon-still-high-after-mitigation/" pubDate: "2026-10-01T00:00:00.000Z" updatedDate: "2026-10-03T00:18:00.000Z" author: Radon Mitigation York PA description: "Radon still high after a mitigation system went in? Learn the common causes, how installers diagnose them, and what a clearance guarantee should cover." --- # Radon Still High After Mitigation: Why It Happens and How to Fix It If your radon is still high after mitigation, the system usually is not pulling enough suction under the whole foundation, or soil gas is getting in somewhere it does not reach. Common fixes are more sealing, a second suction point or a stronger fan. Check the test setup first, then call the installer to diagnose and retest, which a [written clearance guarantee](/radon-mitigation-guarantee/) should cover. ## First, make sure the test was valid Before anyone changes the system, confirm the result is real. A post-mitigation test can read high for reasons that have nothing to do with the system. - **Timing.** EPA guidance is to start the test no sooner than 24 hours after the system starts, and within 30 days. - **Closed-house conditions.** Windows and outside doors should stay closed, except for normal coming and going, during the test. For short tests, EPA guidance also calls for closed-house conditions for 12 hours before the test starts. - **Location.** The test should sit in the lowest lived-in level, away from drafts, outside walls, sump pits and the fan pipe. - **Fan running.** Check the U-tube manometer. If the liquid is level on both sides, the fan may be off or unplugged. If any of these were off, a retest may solve the problem. If the setup was correct, the system needs work. ## The most common reasons radon stays high ### Not enough suction under the slab A single suction point works well when crushed stone or loose soil lets air move freely under the slab. In some homes, dense clay or a buried footing blocks that air. One side of the basement gets suction, and the other side does not. The fix is often a second suction point or a larger pit under the first one. ### Open entry points Every opening in the foundation is a shortcut for soil gas, and it also lets basement air leak into the system instead of soil gas. Pennsylvania's mitigation standards call for sealing the floor-wall joint, open cracks, perimeter channel drains and openings around pipes. If those were missed, the fan works harder and pulls less from the soil. ### An unsealed sump pit An open or loosely covered sump pit links the basement to the drain tile and the soil around the house. Pennsylvania's standards call for sump pits that let soil gas in to be covered and sealed. A gasketed, airtight lid often makes a big difference. ### Block walls or stone foundations Hollow block walls can move soil gas up through their cores. Fieldstone walls have gaps all through them. In these homes, sub-slab suction alone may not be enough. Sealing the tops of block walls or adding wall suction can help. ### Undersized or failing fan Fans come in different strengths. A fan picked for an easy slab may not be strong enough for tight soil. Fans also wear out over time. A weak or failing fan shows up as a low manometer reading or a fan that is louder than it used to be. ### A crawl space or dirt floor that was missed Many York homes have a basement on one side and a crawl space or a dirt section on the other. If only the slab area was treated, gas from the crawl space can keep levels high. That area needs a sealed membrane with suction underneath. ### Radon from well water If the home is on a private well, some radon can come from the water. A common rule of thumb used by state radon programs is that about 10,000 pCi/L in water adds roughly 1 pCi/L to indoor air. That is rarely the main cause, but very high water levels are worth testing. ## How a good installer diagnoses the problem A careful installer will not just swap in a bigger fan. Expect these steps: 1. **Check the fan and gauge.** Is it running, and is the manometer reading normal for that system? 2. **Test sub-slab suction.** Small test holes in the slab show whether suction reaches the far corners. 3. **Look for open entry points.** The installer checks the floor-wall joint, cracks, the sump, floor drains and any dirt areas. 4. **Make the fix.** Seal, add a suction point, add a membrane or change the fan. 5. **Retest.** A new test, under the same rules, confirms the result. ## What a clearance guarantee should cover This is where the written guarantee matters. A clear guarantee names a target number, usually below 4.0 pCi/L, and promises extra work at no charge until the target is met. It should also include a retest after each fix. A guarantee is different from a warranty. The fan maker's warranty covers the fan itself. The installer's workmanship warranty covers the install. A clearance guarantee covers the result. We break down how these differ in [radon system warranties explained](/radon-guide/radon-system-warranty/). If your installer offered no guarantee, you may need to pay for the added work. It is still worth calling the original installer first, since they know the system. ## When high readings show up later Sometimes a system clears at first, then a later test reads high. Common causes include: - A fan that has slowed or failed - A finished basement project that cut new holes in the slab - A sump lid that was removed for service and not resealed - New cracks as a house settles The EPA and PA DEP both recommend retesting at least every two years and after big changes to the house. Retesting catches problems early. ## Key takeaways - Check the test setup first: timing, closed-house conditions, placement and a running fan. - The most common causes are weak suction, missed sealing and an open sump pit. - Older stone and block foundations and mixed basement and crawl space homes are more likely to need added work. - Good installers diagnose with suction tests, not guesses. - A written clearance guarantee means the fix and the retest cost you nothing. ## Frequently Asked Questions ### How long should I wait to retest after a fix? The same rule applies as for the first test. Start at least 24 hours after the system is running again, under closed-house conditions. ### Why did my level drop only a little? Usually because the design does not fit the house yet. The EPA notes that some systems can cut radon by up to 99%, so a small drop usually points to a fixable design or sealing issue. ### Should I add a second system? Usually not. Most fixes involve more sealing, more suction points tied into the same pipe, or a stronger fan. ## Need a fix in York? If your system is not clearing, we can diagnose it, fix it and retest. Our York radon clearance guarantee covers every system we install: below 4.0 pCi/L on the clearance test, or we keep working at no charge. Call [(717) 356-3035](tel:+17173563035) any time for a same-day callback.