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What is a radon mitigation system?

By Radon411 Editorial TeamUpdated How we research

Short answer

A radon mitigation system is a permanent installation, usually a vent pipe and a fan that runs all the time, that pulls radon-laden soil gas from beneath a house and releases it outdoors above the roof before it can get in. It is designed for the building's foundation, and a follow-up test shows whether it lowered the level.

Next step: a certified professional designs and installs the fix. Find a mitigation professional

The most common system is a vent pipe and fan that pull radon from beneath the house and vent it outside. In a home with a basement or a slab, the pipe connects to a suction point through the floor or a sealed sump; over a crawlspace, it draws from under a plastic sheet laid over the earth. Cracks and openings are sealed so the system pulls from the soil.

The fan is the only moving part, and it runs continuously. A warning device, usually a U-tube manometer, shows whether it is working, and a label marks the system so nobody changes it by accident. New homes can be built with a passive version of the same pipe, ready for a fan if a test after move-in shows one is needed.

What to do next: if your home tested high, confirm the result, then get proposals from certified professionals. After installation, test again to confirm the level came down, and leave the fan running.

The parts of a radon mitigation system

Most systems are built from the same handful of parts. The fan is the only moving one, and it runs all the time.

Cross-section of a house with a sub-slab depressurization system: soil gas drawn from beneath the slab into a suction point, up a vent pipe, through a fan outside the living space and out above the roofline.
Read this figure as text
Sub-slab depressurization system in a house cross-sectionGround levelLiving spaceBasement12345
  1. Soil gas drawn from beneath the slab
  2. Suction point through the slab
  3. Vent pipe
  4. Fan, outside the living space
  5. Discharge above the roofline
A subslab depressurization system, the most common kind, with its parts labelled.
What each part does
PartWhat it is and does
Suction pointA hole through the basement floor, often about five inches across with a cavity dug out beneath it, or a sealed sump pit, where soil air is drawn from under the slab.
Vent pipeA 3 or 4 inch plastic pipe that carries the soil air from the suction point to the outdoors, inside or outside the house.
Radon fanA fan in unconditioned space, such as an attic, garage or the outside wall, that pulls soil air continuously. It is the only operating component.
ExhaustThe pipe's outlet, above the roof and away from windows, doors and other openings so radon cannot get back in.
SealingCaulk around the pipe and over cracks and openings, so the system pulls from the soil rather than the room.
Warning deviceUsually a U-tube manometer on the pipe, sometimes an alarm, showing whether the fan is working.
LabelMarks the pipe as part of a radon reduction system so nobody changes it by accident.

How the system works

Air pressure inside a home is usually lower than in the soil around its foundation, so the house pulls soil gas, radon included, in through cracks and openings. A system turns that around. The fan creates a negative pressure, a slight vacuum, beneath the slab, so the soil gas flows into the pipe instead of the house, and the fan releases it into the outdoor air above the roof, where it is quickly diluted.

Active and passive systems

A system with a fan is active. A passive system has the same pipe but no fan, relying on natural pressure differences and air currents; it is usually part of the radon-resistant features built into new homes, and it is not as effective against high radon levels.

Active and passive subslab suction
FeatureActivePassive
Moves soil gas byA radon vent fanNatural pressure differences and air currents
Typical reduction50 to 99 percent30 to 70 percent
Where it is foundFitted to existing homesBuilt into new radon-resistant homes
Upgrade pathAlready activeA vent fan can be added to make it active if a test shows a need

A radon-resistant new home includes a gas-permeable layer under the slab, plastic sheeting, sealed openings, a vent pipe to the roof and junction boxes for a future fan and alarm. Every new home should still be tested after move-in. See radon-resistant new construction and the passive radon system guide.

System types by foundation

The suction point changes with the foundation, which is why each system is designed for its building.

Typical radon reduction, EPA

Basement and slab-on-grade homes

  • Subslab suction (subslab depressurization)50 to 99 percentWorks best if air can move easily in material under slab.
  • Passive subslab suction30 to 70 percentMay be more effective in cold climates; not as effective as active subslab suction.
  • Drain tile suction50 to 99 percentCan work with either partial or complete drain tile loops.
  • Block-wall suction50 to 99 percentOnly in homes with hollow block-walls; requires sealing of major openings.
  • Sump-hole suction50 to 99 percentWorks best if air moves easily to sump from under the slab.

Crawlspace homes

  • Submembrane depressurization in a crawlspace50 to 99 percentLess heat loss than natural ventilation in cold winter climates.
Typical radon reduction for each kind of soil suction system.
  • Subslab: suction directly beneath the foundation; the most common and usually the most reliable.
  • Drain tile: the pipe taps into drain tiles that carry water away from the foundation.
  • Sump hole: a capped sump keeps draining water and doubles as the suction point.
  • Block wall: depressurizes hollow block foundation walls, often with subslab suction.
  • Submembrane: in a crawlspace, a plastic sheet covers the dirt floor, runs up the wall and is sealed, and the pipe pulls from beneath it.

Living with a system

Upkeep is light, much like a furnace or chimney. The fan should never be turned off, because it must run continuously for the system to work. Glance at the warning device on a regular basis; if it shows a problem, call a contractor.

Timeline after a radon mitigation system is installed: the fan runs continuously, test no sooner than 24 hours and within 30 days, then retest at least every two years.
Read this figure as text

  1. Day of installation

    The system goes in

    Keep the fan running at all times: it must run continuously for the system to work correctly.

  2. No sooner than 24 hours

    Earliest post-mitigation test

    Test no sooner than 24 hours after your system is in operation with the fan on, if it has one.

  3. Within 30 days

    Post-mitigation test done

    A two- to seven-day measurement is recommended, and EPA recommends an independent follow-up radon measurement as well as any test the contractor runs.

  4. At least every two years

    Retest

    Retest your home at least every two years to be sure radon levels remain low.

Test conditions: Windows and doors must be closed 12 hours before and during the test, except for normal entry and exit. Not drawn to scale.

After a remodel. After you remodel, retest in the lowest lived-in area to make sure the construction did not reduce the effectiveness of the radon reduction system.

After a system is installed: a follow-up test within 30 days, then a retest at least every two years and after any remodel.

If you sell the home, tell buyers about the system and give them its operating and maintenance information. Buying a home that already has one is covered on should I buy a home with a radon mitigation system.

Questions about radon mitigation systems

Can I turn off my radon system?

No. The fan must run continuously for the system to work. See can I turn off my radon system.

What does the U-tube on a radon pipe show?

It is a manometer, the usual warning device: a visual sign of whether the fan is pulling. Other warning devices include a sound alarm, a light or a dial gauge. If it shows the system is not working properly, call a contractor.

Where does a radon system vent?

Outdoors, above the roof edge, at least 10 feet above the ground and at least 10 feet from windows, doors and other openings unless it vents at least 2 feet above them.

Does a radon system use a lot of electricity?

It adds a little: fan systems slightly increase the electric bill, and most systems cause some loss of heated or cooled air. How much depends on the climate, the system and the house.

How long does a radon fan last?

Fans may last five years or more, and manufacturer warranties tend not to exceed five years. See the radon fan guide.

Do new homes come with radon systems?

Some do, as passive radon-resistant features, and building codes in some states or localities require them. A vent fan can be added if a test after move-in shows a need.

Fix

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