Radon Mitigation: How Sub-Slab Systems Pull the Gas Out
Radon mitigation is the permanent installed system that lowers the radon concentration inside a home, almost always by capturing the gas in the soil before it can seep through the foundation. If a test came back at or above the EPA action level of 4 picocuries per liter (pCi/L), mitigation is the fix. This page explains what the system actually does, the parts it is built from, how fast it works, and how you confirm it worked.
One point up front: testing and mitigation are two different jobs. A test measures the radon level; mitigation reduces it. You test first, install if the number is high, then test again to prove the reduction. The measurement side is covered in the Front Range radon testing guide and in what radon is and how it gets into homes. This article picks up where a high result leaves off.
How the standard system works
The dominant method in the United States is active sub-slab depressurization (ASD), also written SSD. The principle is simple: a fan keeps the air pressure in the soil under the concrete slab slightly lower than the pressure inside the house. Because air moves from high pressure to low pressure, radon-laden soil gas gets pulled sideways into the system and vented outdoors instead of rising through slab cracks, control joints, and the gap around plumbing penetrations into the living space.
A drilled hole through the slab reaches the gravel or soil beneath. A sealed PVC pipe rises from that suction point, up through a closet or mechanical chase or the garage, and out above the roofline. An inline fan mounted in the unconditioned part of the pipe runs continuously and creates the negative pressure that drives everything. The EPA Consumer’s Guide to Radon Reduction describes ASD as the most common and usually the most reliable residential approach.
The parts, and what each one does
- Suction point — a hole, roughly 4 to 5 inches across, cored through the slab into the gravel or soil below. Most homes need one; larger or poorly connected slabs need two or more.
- Vent pipe — 3-inch or 4-inch PVC (schedule 40 or 3034) running from the suction point up and out above the roof, at least 10 feet from windows, doors, and other openings so exhausted gas does not re-enter.
- Radon fan — a sealed, continuously running fan rated for radon service, typically drawing 40 to 90 watts. It must sit in an unconditioned space (attic, garage, or outside the exterior wall) so that if the pipe ever leaks, it leaks outside the living area, never into it.
- Manometer — a small U-shaped tube of colored liquid mounted on the visible pipe. Unequal liquid levels mean the fan is pulling suction. If the two columns ever sit level, the fan has failed and the system is not protecting you.
- Sealing and labeling — visible slab cracks, the sump lid, and other soil-gas entry routes are sealed so the fan does not waste its pull on open gaps, and the system is labeled per the mitigation standard.
Running full time, the fan uses about 40 to 90 watts, which pencils out to roughly $30 to $90 a year in electricity. Radon fans commonly last 5 to 12 years before they need replacing, and a replacement fan is a modest, quick job.
What standard the work follows
Residential mitigation in the U.S. is now governed by the consensus standard ANSI/AARST RMS-LB (Radon Mitigation Standards for Low-Rise Residential Buildings), which superseded EPA’s older 1990s mitigation protocols and sets the engineering specifics: suction-point placement, pipe sizing, fan location, exhaust setbacks, and labeling. Colorado phased in state licensing of radon professionals through the Colorado Department of Public Health and Environment, and installers also carry national credentials from the National Radon Proficiency Program (NRPP) or the National Radon Safety Board (NRSB). Verify a contractor’s current standing on the CDPHE radon program before hiring.
How fast it lowers the level, and how you prove it
Most systems show a clear drop within 24 to 48 hours of the fan starting, and the level settles to its final reading over the next couple of weeks as the soil under the slab reaches a new equilibrium. Well-designed ASD systems commonly cut indoor radon by 80 to 95 percent, and the practical target is to get as close as the foundation allows toward the roughly 1.3 pCi/L U.S. indoor average, ideally lower.
Do not take the reduction on faith. Run a post-mitigation test no sooner than 24 hours after the system is fully operational, under closed-house conditions, on the lowest lived-in level. A reading below 4 pCi/L confirms the design worked. Because EPA notes there is no fully safe radon level and risk keeps dropping as the number falls, many homeowners push for a result under 2 pCi/L. For context on where the line sits, see what counts as a safe radon level.
After the initial verification, EPA recommends retesting every two years for the life of the system, and again after any work that changes foundation airflow or air pressure — a basement finish, foundation repair, or an HVAC replacement. A five-second glance at the manometer between tests catches a dead fan early.
When plain sub-slab is not enough
ASD fits most slab-on-grade and basement foundations. Other constructions need variations:
- Crawlspaces use sub-membrane depressurization: a sealed plastic membrane is laid over the dirt floor and the fan pulls from beneath it, the crawlspace equivalent of a slab.
- Hollow block-wall foundations sometimes call for block-wall depressurization, which draws from the hollow cores of the block rather than the soil.
- Slabs with little or no gravel, or badly fragmented slabs, may need multiple suction points to spread the pull far enough across the foundation.
There is also a combustion-safety check. In a tightly built home, adding depressurization can, in rare cases, interfere with the draft of an atmospherically vented furnace or water heater. A qualified installer tests appliance draft after installation, which matters most in older homes with natural-draft combustion equipment.
Why it is worth doing
Radon is a naturally occurring radioactive gas and, per the CDC and EPA, the second-leading cause of lung cancer overall and the leading cause among people who have never smoked. The American Lung Association and the National Institute of Environmental Health Sciences back the same finding. A one-time system plus a few dollars a month of electricity buys down that lifetime exposure for everyone who lives in the house, now and after any future sale. Along much of the Colorado Front Range — Douglas, Jefferson, El Paso, and Boulder counties among them — elevated readings are common enough that a working mitigation system has become a routine feature buyers expect and inspect for.
Common questions
This overview reflects EPA, CDC, and CDPHE guidance current as of 2026; consult a CDPHE-listed radon professional for a system designed to your foundation. Front Range homeowners with a recent test above 4 pCi/L can reach out through our contact page for a referral and quote.