Radon mitigation system as installed, outside view (C) Daniel Friedman A Homeowner's Guide to Reducing Indoor Radon Gas or Radon in Water

  • RADON REMOVAL INDOORS, HOW-TO - CONTENTS: How to reduce the level of radon indoors in air or water - Radon Mitigation Guide. Radon mitigation system installation advice. Health effects of exposure to radon gas in homes - a consumer summary. Table of lung cancer risk from radon exposure in air or water. An easy guide to Radon Remediation in Homes
  • POST a QUESTION or READ FAQs about the best methods for reducing indoor radon gas hazard levels

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Radon Mitigation Systems: How to Perform Radon Remediation to Remove Harmful Levels of Indoor Radon

Radon mitigation system - US EPA

As reported in Best Practices Guide to Residential Construction:

The EPA and the U.S. Geological Survey have rated every county in the United States as Zone 1 to 3 for radon risk. Links to state maps with county by- county risk levels can be found at www.epa.gov/ radon/zonemap.html.

The EPA recommends that all homes in Zone 1 counties be built with radon-resistant features, which can be easily upgraded to a radon remediation system if needed.

Since homes in Zones 2 and 3 can also have high levels, it is best to check with your state radon office to see if they are aware of any local “hot spots.”

The techniques for radon-resistant building vary for different foundation types and site conditions, but all contain the six basic elements described below.

Following these steps creates a passive soil depressurization system, which sufficiently lowers radon levels in about 50% of homes requiring mitigation.

If radon levels need to be lowered further, the system can be easily converted to an active system by adding an inline fan, which can meet the target levels in nearly all cases (see Figure at above left, showing a typical radon mitigation system installation).

The goal of radon remediation is to lower the average indoor radon gas level to less than 4 pCi/L, and preferably 2 pCi/L.

  • Radon Gas permeable layer. This is usually a 4-inch layer of clean, course gravel installed beneath the slab for drainage, but which also allows the gas to move freely beneath the house. In areas where native soils are sufficiently permeable to build on, a loop of perforated pipe inside the footings is an option, and may also serve as a drain tile.

    The perforated pipe should be about 12 inches in from the foundation wall and 1 inch below the slab, with a minimum diameter of 3 inches for slabs under 2,000 sq ft and 4 inches for slabs up to 4,000 sq ft. Where subgrade soils are compacted or frozen, another option is to use geotextile drainage mats to create a gas-permeable layer on top of the subgrade but beneath the slab.
  • Plastic sheeting to Stop Radon Gas Penetration. Lay minimum 6-mil polyethylene sheeting (or 3-mil cross-laminated sheeting) on top of the gas permeable layer. This helps keep soil gases from entering the home and also keeps concrete from clogging the gravel layer. Overlap seams by at least 12 inches, and repair any punctures or tears with tape or a patch of sheeting material.
Radon manometer (C) Daniel Friedman
  • Radon Gas Vent Pipe Details. Run a 3- or 4-inch PVC pipe from the gas- permeable layer up through the house to the roof to vent soil gases above the house. Where better suction is needed, connect the subslab tee to a minimum 10-foot length of perforated, corrugated pipe run horizontally in the gravel layer.

    The vertical pipe should be as straight as possible and should be located inside the insulated shell of the building to keep it warm, inducing the stack effect.

    Field data has indicated that 4-inch vent pipes work better than 3-inch vent pipes for passive systems. Some builders cap the stub just above the basement slab and connect the riser to the roof only if the house tests high for radon. If so, clearly label the capped pipe so no one mistakes it for a plumbing drain in the future.

    Our photo (above left) shows an air pressure gauge or manometer that is usually installed on a professionally-installed radon vent pipe. This device simply measures the pressure difference between the room interior where the radon suction point is installed through the floor slab and the pressure in the vent pipe interior.

    As long as the room air pressure is slightly higher than the slab or suction pipe air pressure radon gases will be more inclined to pass up the vent pipe than enter the room - that is, the room is at "positive pressure" with regard to the air pressure under the floor (the most likely entry path and source of radon gas).
  • Sealing and caulking to Stop Radon Gas Entry. Seal all cracks, perimeter joints, control joints, and other openings in the foundation floor with long-lasting materials to reduce soil gas entry. Seal large openings with expanding foam or nonshrink mortar or grout.

    Seal smaller holes with a high-grade elastomeric sealant conforming to ASTM C920-87. If the home has a sump, it should have an airtight cover and, if needed, can have a floor drain with a trap (filled with oil so it will not evaporate).

    If the sump is not connected to the drain tile loop, it can be vented into the radon system with a 3-inch pipe connected to a special sump cover available from suppliers of radon mitigation products. Also seal and caulk the rest of building envelop to reduce the stack effect in the home.

    The tighter the home, the less the building will draw radon out of the soil. Also tightly seal any return air ducts that pass through basements or crawlspaces.
  • Seal ducts and air-handling units Where Radon Gas is Present. Placing any return-air ductwork under the concrete slab is not recommended, since this will tend to draw radon into the ductwork and distribute it around the house. If supply ductwork must pass through a subslab space, it should be seamless or sealed airtight with durable aluminum tape or duct mastic.

    Also tightly seal any air-handling units or ductwork passing through basements, crawlspaces, or any areas in contact with the slab. In addition to saving energy, this will prevent the HVAC system from drawing radon out of the soil.
  • SLAB DUCTWORK - catalogs the functional and environmental problems found when HVAC air ducts are routed in or below floor slabs
  • Junction box to power radon mitigation system fan. Install an un-switched junction box in the attic or attached garage within about 6 feet of the vent pipe. A dedicated circuit is not needed. In the event that the passive system is not enough to keep radon levels below 6 Pci/L, then an inline fan will need to be added and run continuously.

    The fan should be located so that all positively pressurized sections of the system (from the fan to roof outlet) are located outside of habitable space. An active vent system should also have a visible or audible alarm to alert the occupants in the event of a loss of pressure or airflow in the vent pipe.

A post mitigation radon test of 2 to 7 days should be done within 30 days of system installation. For an accurate reading, all windows and doors must be closed 12 hours before and during the test, except for normal use for entry and exit.

-- Adapted with permission from Best Practices Guide to Residential Construction.

Watch out: home ventilation systems, particularly powerful exhaust fans, can subvert a typical sub-slab suction type radion mitigation system by creating negative air pressures within the building. The radon system needs to be able to handle these pressure variations in the home.

See VENTILATION, BALANCED for an optimum approach to bringing in fresh outdoor air without increasing heating or cooling costs and without risking subverting the radon mitigation system.

Also see BACKDRAFTING HEATING EQUIPMENT for warnings about potentially fatal carbon monoxide hazards if exhaust fans cause back-drafting of heating appliance exhaust into the building.

US EPA Radon Zone Map For a Thorough Background in Radon Hazards, Radon Mitigation, & the History of Radon Concerns in the U.S. also see these articles reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.


  • "Defeating Radon" part 1 - [PDF] Terry Brennan, Bill Turner, Solar Age Magazine - How does radon get into buildings, how do I know if a building has a radon gas problem, how can I solve radon problems in existing homes, and what can I do to prevent radon from entering new homes. Part 1: where Radon comes from, how to diagnose radon
  • "Defeating Radon" part 2 - [PDF] Key spots to seal, to stop radon gas leaks into buildings
  • "Defeating Radon" part 4 - [PDF] Data on radon levels in buildings before & after radon mitigation treatment
  • "Defeating Radon" part 5 - [PDF] Air filtering, testing after radon mitigation, where to buy radon tests

Also see "Radon Basics", Q&A article, Solar Age, April 1984, includes advice for radon-resistant construction for an underground house built of concrete

This article series includes excerpts or adaptations from Best Practices Guide to Residential Construction, by Steven Bliss, courtesy of Wiley & Sons.


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