A radon mitigation system, also called a radon venting or reduction system, is a pipe and fan that pull radon-laden soil gas from under your home and vent it outside, above the roof, where it quickly dilutes. It stops radon before it gets in, rather than trying to clear it from the air afterward. Some systems lower indoor radon by up to 99%, and most homes can be fixed for about the cost of other common home repairs (EPA, Consumer’s Guide to Radon Reduction).
EPA recommends fixing a home when a long-term test, or the average of two short-term tests, shows 4 pCi/L or more, and considering it between 2 and 4 pCi/L. If you haven’t confirmed your level yet, start with how to test for radon.
Why radon gets into a house
Radon comes from uranium breaking down in soil and rock. The air pressure inside a home is usually a little lower than the pressure in the soil around the foundation, so the house acts like a vacuum and draws soil gas in through cracks and other openings. Soil gas is the main source of high indoor radon in the US; well water and building materials add a much smaller share in most homes (EPA, Consumer’s Guide).
A mitigation system reverses that pressure difference. The fan creates a slight vacuum under the foundation, so soil gas flows into the pipe instead of into the house.
The parts of a typical system
- Suction point. A hole through the basement floor or slab, or under it from outside, into the gravel or soil beneath. Often one is enough; how many a home needs depends on how easily air moves under the slab and how strong the radon source is.
- Vent pipe. Carries the gas from the suction point up and out of the house, inside or along the outside wall.
- Radon fan. Draws the gas through the pipe. It goes in an unconditioned space such as the attic, the garage or the outside of the house, never in or below a living area.
- Exhaust. Releases the gas above the roof line.
- Warning device. Tells you if the system stops working. Usually this is a manometer, a liquid gauge on the pipe; alarms, lights and dial gauges are also used.
- Sealing. Cracks and openings in the foundation are sealed so the fan pulls from under the slab, not from the room.
- Label. The system should be clearly labeled so nobody alters it by accident.
All of these come from EPA’s description of a soil suction system (Consumer’s Guide).
Types of systems, by foundation
The right system depends mostly on how your home is built. Many homes combine two foundation types, such as a basement under part of the house and a slab or crawlspace under the rest, and may need a combination of methods (EPA, Consumer’s Guide).
| Foundation | Method | How it works |
|---|---|---|
| Basement or slab | Sub-slab suction (sub-slab depressurization) | Pipe through the slab into the gravel or soil below. EPA calls it the most common and usually the most reliable method |
| Basement or slab with drain tile | Drain-tile suction | Suction on the perforated pipes that carry water away from the foundation |
| Basement with a sump | Sump-hole suction | The sump is capped so it still drains water and doubles as the suction point |
| Basement with hollow block walls | Block-wall suction | Depressurizes the hollow cores of the walls, often together with sub-slab suction |
| Crawlspace | Sub-membrane suction | A heavy plastic sheet covers the earth floor, and the pipe and fan draw radon from under it. EPA calls it the most effective method for crawlspace homes |
| Crawlspace | Crawlspace depressurization | A fan draws air from the crawlspace itself. Usually less effective than sub-membrane suction, and can raise energy costs |
Active or passive. An active system uses a fan. A passive system relies on natural air currents and is usually built into new homes as a radon-resistant feature. Passive systems generally don’t work as well on high levels, and EPA recommends adding a fan if a new home still tests at 4 pCi/L or more (Citizen’s Guide).
Methods that rarely work on their own
- Sealing alone. EPA does not recommend it, because sealing by itself hasn’t been shown to lower radon significantly or consistently. Entry points are hard to find, and normal settling opens new ones.
- House or room pressurization. A fan pushes air into the lowest level to keep radon out. It depends heavily on the house and on keeping lower doors and windows shut, so EPA suggests it only after the common methods have failed.
- Heat recovery ventilator (HRV). Brings in outdoor air while recovering heat. It can help, especially in a basement, but adds heating and cooling costs.
- Opening windows. Temporary at best: radon levels usually return to where they were within about 12 hours of closing up.
Source for all four: EPA, Consumer’s Guide.
What a contractor does before installing
The contractor should inspect the house and review your radon test results and how the test was done. If that isn’t enough to design the system, they may run diagnostic tests. A smoke test shows where air is moving in through cracks, drains and sumps. A soil communication test uses a vacuum in one small hole and smoke in another to see how easily air moves under the slab. A contractor may also check that fuel-burning appliances won’t backdraft once the system runs (EPA, Consumer’s Guide).
Installation rules to check
EPA lists these requirements for a soil suction system and points to the ASTM E2121 installation standard. Your state may add its own rules (Consumer’s Guide):
- The exhaust vents above the roof surface and at least 10 feet above the ground.
- The exhaust is at least 10 feet from windows, doors and other openings, unless it is at least 2 feet above them.
- The fan is not in or below a living area. An outdoor fan must be rated for outdoor use under local codes.
- Electrical work meets local codes.
- A warning device is installed where you can easily see or hear it, and the contractor checks that it works.
- The system is clearly labeled.
After installation: test again
Test within 30 days of installation, but no sooner than 24 hours after the system starts running. A 2- to 7-day test is recommended, with windows and doors closed for 12 hours before and during it. The contractor may test their own work, but EPA recommends an independent follow-up test as well, done by you or by a separate tester (EPA, Consumer’s Guide).
Living with a system
- Never turn the fan off. It must run all the time for the system to work.
- Check the gauge regularly. If it shows the system isn’t working, call a contractor.
- Expect to replace the fan eventually. Fans may last five years or more, and manufacturer warranties tend not to exceed five years.
- Retest at least every two years to make sure levels stay low.
- Running costs are modest. A fan adds slightly to the electric bill, and most systems lose some heated or cooled air.
- Remodeling? Finishing a basement or adding a foundation can affect the system. Ask your contractor first, and retest in the lowest lived-in area afterward.
Source: EPA, Consumer’s Guide.
Choosing a contractor
EPA recommends a qualified radon mitigation contractor, because a system installed without the right equipment and knowledge can raise radon levels or create other hazards. Many states license, certify or register radon professionals. Your state radon program can give you a list, and states that receive EPA radon grants list only providers credentialed by the state, the National Radon Proficiency Program (NRPP) or the National Radon Safety Board (NRSB) (EPA).
When you compare estimates, EPA suggests asking for (Consumer’s Guide):
- proof of certification, liability insurance and the licenses your area requires;
- references, and before-and-after test results from past jobs;
- an explanation of how the system will work, and any fee for diagnostic tests;
- a warning device, a post-installation test, and any guarantee to reach a set radon level (for example 2 pCi/L), including how long it lasts and whether it transfers if you sell;
- a written contract with the total cost, permits, timeline and cleanup.
For typical prices, see radon mitigation cost. To see how common high results are near you, look up your county below.