How radon mitigation systems work

If your home tests at or above 4 pCi/L, EPA recommends fixing it. The good news, in EPA’s words: radon reduction systems work, some can cut radon levels by up to 99 percent, and most homes can be fixed for about the same cost as other common home repairs.

The main method: sub-slab depressurization

Most systems in homes with basements or slab foundations use active soil depressurization, often called sub-slab depressurization. A contractor:

1. drills a hole through the floor slab and removes a small pit of fill beneath it,
2. seals a plastic pipe into the hole,
3. runs the pipe to a quiet in-line fan, usually in an attic, garage, or outside the house,
4. vents the exhaust above the roof line.

The fan creates a gentle vacuum under the slab, so soil gas is pulled into the pipe and exhausted outdoors instead of seeping into the house. EPA explains that the discharge must go above the roof, away from windows and other openings, so the concentrated radon disperses and does not re-enter the home.

Other foundation types

• Crawl spaces. A heavy plastic sheet (vapor barrier) is laid over the soil and sealed, and a pipe and fan draw air from beneath it. This is called sub-membrane depressurization.
• Block walls. Hollow concrete block foundation walls may need their cavities depressurized too.
• Sump pits. An existing sump can serve as the suction point if it is sealed with an airtight lid.

Supporting steps

Sealing cracks and openings in the foundation helps a system work better and saves energy, but EPA’s guidance is that sealing alone has not been shown to lower radon reliably. Ventilation approaches, such as heat recovery ventilators, can help in some homes.

Choosing a contractor

EPA recommends using a qualified mitigation contractor. It notes that states receiving federal indoor radon grants keep lists of providers credentialed through a state license or a national proficiency program (NRPP or NRSB). Get more than one estimate. Ask how the contractor will test the soil’s suction (“communication”) under the slab, where the pipe and fan will go, and whether a post-mitigation test is included. Your state radon program, linked on our state pages, can help.

Cost

EPA does not publish a national price. It says costs vary with the size and design of the home and the methods needed, and that most homes can be fixed for about the same cost as other common home repairs. The fan also uses some electricity continuously, and it will eventually need replacing.

Checking that it works

EPA’s guidance for owners:

• Look at the warning device regularly. EPA says radon reduction systems need occasional maintenance and that you should check the system’s warning device on a regular basis. Many systems use a U-tube manometer on the pipe; uneven liquid levels show the fan is pulling a vacuum, and levels that even out suggest the fan has stopped.
• Expect to replace the fan eventually. EPA notes fans may last five years or more (warranties tend not to exceed five years) and then need repair or replacement.
• Retest. Test after installation to confirm the system worked, and retest periodically and after major renovations, as EPA’s testing guidance suggests.

If you are building new

Radon-resistant features are cheaper to install during construction. See radon-resistant new construction.

Why the pipe goes above the roof

EPA explains that radon must be vented into the air above the roof. Discharge concentrations can be extremely high, but radon concentrations fall dramatically with distance and approach background levels within 3 to 4 feet of the discharge point. EPA disallowed ground-level discharge because the gas could be drawn back into the house, and because children might be exposed near the outlet. When you review a contractor's proposal, check where the discharge will be and that it meets the national standard and any state rules.

How well do systems work?

EPA says some radon reduction systems can reduce radon levels by up to 99 percent, and that most homes can be fixed for about the same cost as other common home repairs. Results depend on the foundation, the soil beneath the slab, how well the slab is sealed, and the fan and pipe design. A good contractor runs a quick diagnostic, such as a pressure field extension test that drills small holes in the slab to measure how far suction reaches, to plan suction points.

Installation day: what to expect

1. Assessment. The contractor inspects the foundation, sump, and crawl spaces, and decides on suction point locations and the pipe route.
2. Suction point. A hole is cored through the slab and a small pit is dug out beneath it to improve airflow.
3. Piping. PVC pipe runs from the suction point to the fan, either through the house to the attic or up the exterior.
4. Fan. The fan is mounted in the attic, the garage, or outdoors, never in the living space or a basement, so a leak in the pressurized side can't push radon indoors.
5. Sealing. Accessible cracks, openings, and the sump lid are sealed.
6. Monitor. A U-tube manometer or electronic alarm is installed and marked with the initial reading.
7. Labeling. The system is labeled, with the installer's contact details.

Most single-family installations take a day or less.

After installation: confirm it works

EPA guidance calls for a post-mitigation test, usually a short-term test conducted at least 24 hours after the system starts, and within 30 days of installation. Follow-up testing then continues periodically. EPA's retest guidance suggests testing again after renovations or changes in how you use the home. Many states recommend retesting every two years for homes with mitigation systems.

Operating costs

The fan runs continuously. Typical radon fans draw roughly 20 to 150 watts, depending on the model, so annual electricity cost varies with fan size and local rates. Heating and cooling costs can rise slightly, because the system draws a small amount of conditioned air from the house. Sealing helps reduce this.

Warranties and contracts

Ask for a written proposal with:

• the system design and fan model;
• the post-mitigation result the contractor commits to, often below 4 pCi/L, sometimes lower;
• what happens if the target isn't met;
• the warranty on workmanship and the fan;
• who's responsible for permits;
• the contractor's credentials, such as state license and national certification.

See hiring a radon mitigation contractor for a full checklist.