Radon mitigation: how it works and what it costs

If your test came back at 4 pCi/L or higher - or between 2 and 4 and you use the lower level - mitigation is the fix. The good news is that it is a mature, well-understood job that a certified installer does in a day. This guide explains the system you will most likely be sold, the variants for other foundations, realistic cost drivers, and how to make sure it keeps working.

First: confirm the number

Don't mitigate off one short-term test unless it was very high (above 8 pCi/L or so). Radon swings with weather and season. Run a second short-term test, a 90-day long-term test, or a continuous monitor for a few weeks; the EPA's guidance is to act on the average of two short-term tests or on a long-term result. See how to test.

How active sub-slab depressurization works

Radon enters because the soil under a house is at slightly higher pressure than the basement. A mitigation system reverses that. The installer cores a 4-5 inch hole through the basement slab, excavates a small pit beneath it, and inserts a PVC pipe. The pipe runs up through the house (or out the rim joist and up the exterior wall) to a continuously running in-line fan, then vents above the roofline, away from windows. The fan pulls soil gas from under the entire slab and exhausts it outdoors before it can enter. A U-tube manometer on the pipe shows the suction is present at a glance.

Well-designed systems commonly cut indoor radon by 80-99%. The most important variable is how well suction "communicates" under the slab: a gravel bed under the concrete lets one suction point cover the whole footprint, while dense clay or a slab poured directly on soil may need two or more points or a stronger fan. Good installers test this with a vacuum and smoke pencils before they choose the fan.

Variants for other foundations

• Crawlspace (sub-membrane depressurization). A heavy plastic sheet is sealed across the dirt floor and up the walls; the suction pipe draws from beneath the sheet. Usually the most labor-intensive and therefore the most expensive variant.
• Slab-on-grade. Same as sub-slab, but the suction point is often placed in a closet or garage and the pipe run is shorter.
• Block-wall depressurization. For hollow-block foundation walls that act as radon pathways; suction is applied to the wall cavities, sometimes in addition to sub-slab.
• Drain-tile / sump depressurization. If the house has a perimeter drain loop or a sump pit, the installer can seal the sump with an airtight lid and draw suction from it, covering the whole perimeter cheaply.
• Passive-to-active conversion. Newer homes in high-radon regions are often built with a capped passive stack (radon-resistant new construction). Adding a fan to it is the cheapest mitigation there is.

Sealing cracks, sump pits and pipe penetrations is done alongside all of these to make the fan more effective. Sealing alone, or opening windows, is not a reliable fix - the EPA does not recommend either as a stand-alone method.

What it typically costs

Nationally, a standard active sub-slab system typically runs $800-$2,500 installed. That is a typical range, not a quote: prices vary with region and labor market, foundation type (crawlspaces and mixed foundations cost more), the need for a second suction point, a concealed interior pipe route through a finished basement, and whether the exhaust must be routed up through the roof rather than out a wall. Ongoing costs are the fan's electricity (a small continuous load) and fan replacement every 10-15 years or so. Some states offer low-income assistance; ask your state radon program.

Choosing a contractor

• Use a mitigator certified by NRPP or NRSB, and licensed where your state requires it. Start at the EPA find-a-professional page.
• Get two or three bids that specify fan model, number of suction points, pipe route, exhaust location, and a written post-mitigation test.
• Ask how they diagnosed sub-slab communication; a bid without a site visit is a guess.
• Expect a warranty on the fan (often 5 years) and a guarantee to reach a stated level (commonly below 4, often below 2 pCi/L).
• The system should meet the ANSI/AARST mitigation standards your state references; ask.

After installation: verify, then keep verifying

Test again 24 hours to 30 days after the fan starts, under closed-house conditions. Then re-test every two years, after any renovation that touches the foundation or HVAC, and whenever the manometer reads level (which means the fan has failed). A continuous monitor is the easiest way to do this; it also shows if a new crack or a sump-lid seal has opened up. Continuous monitors.

Selling the house later? Keep the installer's paperwork and your post-mitigation test results together; see radon and real estate.

Cost figures are industry-typical ranges, not quotes. Technical description follows EPA's Consumer's Guide to Radon Reduction. Some links are affiliate links.