What is a radon mitigation system?
Updated August 2026
A radon mitigation system, in the sub-slab depressurization form nearly every house gets, is a sealed plastic pipe and a continuously running fan that pull radon gas out of the soil under your foundation and dump it above the roofline before it can seep indoors. Radon is a colorless, odorless radioactive gas produced as uranium in soil and rock decays, and the EPA identifies it as the second-leading cause of lung cancer in the United States after smoking. The EPA sets its action level at 4 pCi/L; the average indoor reading nationwide sits near 1.3 pCi/L.
Where it is
Follow the white pipe. A finished system starts as a 3-inch or 4-inch PVC riser coming out of the basement slab (or out of the sealed poly membrane in a crawl space), runs up through a closet, the garage, or along an exterior wall, and terminates above the eave, away from windows. The fan is a squat plastic canister spliced into that pipe, always mounted in unconditioned space such as an attic, garage, or outside wall, so a cracked joint cannot leak soil gas into a bedroom. Look at eye level on the basement pipe for the manometer: a small clear U-shaped tube of colored liquid.
What it does
The fan keeps the gravel or soil beneath the slab at slightly lower pressure than the house, so radon takes the pipe instead of the crack in your basement floor. A correctly designed system typically drops a house from the 8 to 20 pCi/L range down under 2 pCi/L, and it does so on 20 to 90 watts of electricity, about $2 to $8 a month. Failure is silent, which is the whole problem: when the fan dies, the liquid in the manometer settles level in both legs, the faint hum from the pipe stops, and indoor radon climbs back toward its original level within a day or two. Grinding or rattling from the fan housing means the bearings are going.
Maintenance
Look at the manometer once a month, on the same trip as the furnace filter; if the two liquid columns are even, the system is not pulling. The fan check takes ten seconds, and the full service list adds the pipe joints, the slab caulk, and the crawl space membrane seams. Retest the house every 2 years, and again after foundation work, a new HVAC system, or finishing a basement. A short-term charcoal kit costs $15 to $30, a lab-analyzed long-term test $25 to $50, and a professional continuous monitor test $150 to $300. Fans themselves run 5 to 10 years.
What it costs to replace
A replacement radon mitigation system runs $800–2,500 to install a complete system, or $150–400 to swap a worn-out fan.
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Frequently asked questions
What radon level is considered dangerous?
The EPA recommends fixing a home that tests at 4 pCi/L or higher and says to consider fixing between 2 and 4 pCi/L, since there is no level with zero risk. Outdoor air averages roughly 0.4 pCi/L and indoor air about 1.3 pCi/L nationally. Radon varies enormously house to house on the same street, so your neighbor's result tells you nothing useful about yours. Test in the lowest lived-in level, with windows closed, for at least 48 hours.
Should the radon fan run all the time?
Yes. Leave it on 24 hours a day, year round, including while you are on vacation. The suction field under the slab collapses within hours of the fan stopping, and indoor readings usually return to pre-mitigation levels inside 12 to 48 hours. Running the fan continuously costs about $2 to $8 a month in electricity, which is far less than paying for a second test and a service call after someone flips the breaker off for the winter.
How do I know the system is actually working?
The manometer gives you the daily answer and a test kit gives you the real one. Uneven liquid legs mean the fan is generating suction; the installer usually marks the correct reading on a label. That gauge proves airflow, not results, so buy a short-term kit every couple of years and confirm the number is still under 4 pCi/L. Set a reminder rather than trusting memory, because nothing about a failing radon system smells, sounds, or feels different upstairs.
Can a crawl space get a radon system?
Yes, using sub-membrane depressurization. The contractor lays heavy reinforced polyethylene across the dirt floor, seals it to the foundation walls and around piers, and pulls suction from underneath it with the same pipe and fan arrangement. Expect the crawl space version to land at the higher end of the $800 to $2,500 range because of the sealing labor. It also cuts ground moisture, which helps the joists and the flooring above it.