How Long for Radon to Drop After Mitigation

What is Radon Mitigation and how does it work?

A standard active soil depressurization system pulls radon from under your slab and vents it above the roofline. Levels typically drop within minutes of the fan turning on, but the official post-mitigation test should wait at least 24 hours.

We see it happen on every install we do here in Huntsville. The crew seals the suction pit, runs the pipe up the side of the house, mounts the fan, and the moment that fan kicks on the pressure under your slab reverses. Radon that was being drawn into your living space now gets pulled outside. On most homes the indoor reading falls from above 4.0 pCi/L to well below 2.0 pCi/L in the first hour.

The Alabama Department of Public Health notes that Madison County is one of 15 counties with the highest radon potential in the state. That means homes here sit on ground that releases more radon than average. A mitigation system is not a maybe, it is the fix the EPA and the U.S. Surgeon General recommend for any home testing at or above 4.0 pCi/L. We install systems that bring that number down to around 1.3 pCi/L, which is the national indoor average.

The system itself is simple but specific. A three- to four-inch PVC pipe goes through the slab into a pit of gravel below. A fan mounted in the attic or outside the house creates suction. The pipe exits above the roofline so the gas disperses safely. That whole setup runs about $800 to $1,500 installed nationally, though complex homes can reach $2,500 or more. In Huntsville, with our clay soils and varying foundation types, we often see costs land toward the upper end of that range.

How Radon Mitigation works, start to finish

A mitigation system creates a low-pressure zone under your foundation and vents the gas outside. The fan runs continuously, and the pressure change is immediate.

Our crew starts by testing the slab for the best suction point. We drill a four-inch hole, dig out a small pit, and seal the pipe into the slab with a heavy-duty urethane sealant. The pipe runs up the exterior of the house, usually through the garage or up a corner where it is least visible. The fan goes either in the attic or on the outside wall, depending on your home's layout and local building codes.

Once the fan is wired and running, the manometer, the little fluid-filled gauge on the pipe, shows negative pressure. That tells us the system is pulling from under the slab. We check all the cracks and joints in the basement or crawlspace to make sure the suction is holding. On homes with crawlspaces, we lay a heavy vapor barrier and seal the perimeter before running the pipe.

The entire install takes a single day for most homes. We have done them in half a day on simple slabs. The fan runs 24/7 and uses about as much electricity as a 60-watt light bulb. There is no maintenance beyond checking the manometer once a month and making sure the exterior vent is not blocked by snow or debris.

1. Test the slab.We find the best spot for suction using a soil-gas probe and a manometer.
2. Drill and seal.A four-inch hole through the slab, a gravel pit below, and a sealed pipe connection.
3. Run the pipe.PVC pipe routed up the exterior or through the garage to above the roofline.
4. Mount the fan.Fan installed in the attic or outside, wired to a dedicated circuit.
5. Verify the drop.We run a short test to confirm the pressure change and check for leaks.

Why Radon Mitigation matters for your home

Radon is the second leading cause of lung cancer after smoking, and it is a colorless, odorless gas you cannot detect without a test. Mitigation is the only way to bring high levels down to safe ranges.

Indoor radon averages about 1.3 pCi/L nationally. Outdoor air sits around 0.4 pCi/L. When your home tests at 4.0 pCi/L or higher, the EPA says to fix it. In Huntsville, where Madison County is flagged as high-potential, we see plenty of homes testing at 8.0, 12.0, even 20.0 pCi/L. Those numbers are not rare here.

A mitigation system does not just lower the number, it changes the airflow dynamics of your home. By pulling from under the slab, the system reduces the stack effect that draws soil gases inside. That also helps with moisture and other soil gases, though the main job is radon.

The cost of a system, which runs $800 to $2,500 depending on complexity, is a one-time expense. Compare that to the long-term health risk of living with elevated radon. We have homeowners who put off mitigation for years because they think it is a hassle. Then they see the post-mitigation test come back at 1.1 pCi/L and wonder why they waited.

Common problems and what causes them

Most issues come down to weather, home tightness, or system design. A post-mitigation test that shows high levels usually has a fix that is straightforward once you find the cause.

Weather is the biggest wildcard. Heavy rain saturates the ground and can temporarily push radon levels up. Barometric pressure changes affect how much gas is pulled from the soil. That is why we always tell homeowners to wait at least 24 hours after a system install before testing, and ideally pick a day with stable weather. A test during a thunderstorm might show a higher number than the system delivers.

Home tightness matters. A leaky house lets more outside air in, which dilutes radon but also makes the system work harder. A very tight home, like a new energy-efficient build, can trap radon more effectively if the system is not sized right. We adjust fan strength and pipe diameter based on the home's air exchange rate.

System design problems are rare with a proper install, but they happen. The suction pit might be too shallow, the fan might be undersized for the square footage, or a crack in the slab might be letting radon bypass the suction. We have seen homes where a previous crew ran the pipe through an unconditioned attic without insulating it, causing condensation that froze and blocked the pipe in winter. That is why we insulate and pitch every pipe to drain.

If levels do not drop as expected, we start with a smoke test to check for air leaks around the pipe seal and foundation cracks. Then we check the manometer to confirm the fan is pulling the right pressure. If the fan is running but the pressure is low, the suction pit might need to be enlarged. On rare occasions, a second suction point is needed for a large or split-level foundation.

Common questions about Radon Mitigation

Here are the questions we hear most from Huntsville homeowners, with the answers we give on every job.

We have been doing this long enough to know what people worry about. Some of it is straightforward, some of it surprises even us. Here is what comes up most often.

The fan runs 24/7 and uses about as much electricity as a 60-watt light bulb.

Frequently Asked Questions