Radon Mitigation System Inspection Standards of Practice

About Radon and Standards for Inspecting Radon Mitigation Systems

Radon is a radioactive gas that has been found in homes, schools and other buildings around the world. Radon comes from the natural breakdown of uranium in soil and rock, and moves up into the indoor air that people breathe. Radon is the leading cause of lung cancer in non-smokers. Radon mitigation systems reduce radon levels in homes and buildings. Inspection of these systems helps assure that they were installed properly and are performing as designed.

Although this Standard applies to both commercial and residential radon mitigation systems, this standard exceeds the requirements of both InterNACHI’s commercial and residential Standards of Practices.


The purpose of this section is to establish international standards for the inspection of radon mitigation systems. It also provides universal radon mitigation inspection reporting language.

Radon Mitigation System-Specific Definitions

  • active soil-depressurization system: one or more of the following types of radon mitigation system types involving mechanically driven soil depressurization: sub-slab depressurization; sump (pit) depressurization; drain-tile depressurization; sub-membrane depressurization; hollow-block wall depressurization; and crawlspace depressurization.

  • crawlspace depressurization: an active radon-mitigation system that lowers the air pressure inside a crawlspace in relation to the rooms adjacent to or above the crawlspace. A fan draws air directly from the air space of the crawlspace and discharges it outside. This type of system is not the best choice because of the great potential for appliance back-drafting and energy loss.

  • defect: a condition of a radon mitigation system that may have an adverse impact on its performance.

  • depressurization: a negative pressure created in one area compared to an adjacent area.

  • discharge: the end of a vent stack pipe open to outside air.

  • drain-tile depressurization: an active soil-depressurization system whereby a suction point is located at a drain tile.

  • heat-recovery ventilation (HRV) system: a system that lowers radon levels by using outside air to dilute and pressurize the indoor air. HRV systems are considered active radon mitigation systems.

  • hollow-block wall depressurization: an active radon mitigation system that depressurizes the open spaces within concrete block foundation walls.

  • inspection: a non-invasive, visual examination of a radon mitigation system.

  • manifold pipe: a pipe between a vent stack pipe and suction-point pipe with two or more suction points.

  • radon mitigation system: any system designed to reduce the radon concentrations of indoor air.

  • radon system piping: the piping of a passive or active radon mitigation system that is composed of a suction-point pipe, manifold pipe, and vent stack pipe.

  • readily accessible: a system or component that is, in the judgment of the inspector, capable of being safely observed without the removal of obstacles, detachment, or disengagement of connecting or securing devices, or other unsafe or difficult procedures in order to gain access.

  • sub-membrane depressurization: an active radon mitigation system that creates low air pressure under a vapor retarder. A common example is when a vapor retarder (polyethylene plastic sheet) is installed over the exposed dirt floor of a crawlspace.  The radon fan draws air from below the vapor retarder and sends it outside.

  • sub-slab depressurization (active): a radon mitigation system that creates low air pressure under a concrete floor using a fan.

  • sub-slab depressurization (passive): a radon mitigation system that creates low air pressure under a concrete floor without the use of a fan.

  • suction point: the end of a radon mitigation system that penetrates the slab, wall, vapor barrier, sump cover, or drain tile.

  • sump (pit) depressurization system (active): a radon mitigation system that has a suction point installed in the sump (pit).

  • vent stack pipe:  a pipe leading from the suction point (in a system with a single suction point) or the manifold pipe (in a system with more than one suction point) to the outside air. In active radon mitigation systems, the radon fan is installed vertically in the vent stack pipe.
For more terminology commonly found in commercial property inspection reports, visit https://www.nachi.org/comsop.htm#101 

Goal of Inspection

The goal of the inspection is to provide observations that may indicate that a radon mitigation system was installed improperly, is not performing as designed, or is in need of repair.


The inspection is limited to readily accessible and visible portions of the radon mitigation system. The inspection should not be considered all-inclusive or technically exhaustive. It is not a substitute for a radon level measurement.

This standard does not require the inspector to:

  • inspect any portion of the system that is not readily accessible and visible.
  • activate a system that has been turned off, unplugged, or deactivated.
  • measure the radon level.

Optional Add-On Inspection Service

Although InterNACHI's Standards of Practice for Inspecting Commercial Properties and InterNACHI’s Home Inspection Standards of Practice do not require the inspector to perform radon mitigation system inspections, one may be offered in conjunction with a complete commercial or residential property inspection, or as a separate, stand-alone inspection service.


Radon Mitigation System Type

The inspector shall describe the radon mitigation system as one of the following types:

  • active sub-slab depressurization;
  • passive sub-slab depressurization;
  • sump (pit) depressurization;
  • drain-tile depressurization;
  • sub-membrane depressurization;
  • hollow-block wall depressurization;
  • crawlspace depressurization; or
  • heat-recovery ventilation.

Drain-Tile Depressurization Systems

The inspector should inspect drainpipes that extend to daylight for missing devices, such as one-way flow valves, or water traps that prevent outdoor air from entering the sub-slab area.

Sub-Membrane Depressurization Systems

The inspector should inspect the vapor retarder used for sub-membrane depressurization systems (passive or active) for seams that are lapped less than 12 inches, and edges that are not sealed to the walls, posts, or other penetrations. 

Hollow-Block Wall Depressurization Systems

The inspector should inspect hollow-block walls for cracks, openings, and open top-courses.

Crawlspace Depressurization Systems

The inspector should inspect the crawlspace for the presence of asbestos-like material and combustible fuel-served appliances located within the crawlspace or in spaces adjacent to the crawlspace.

Heat-Recovery Ventilation (HRV) Systems

The inspector should inspect the area around the HRV system for the presence of asbestos-like material.

Piping and Fittings

The inspector should inspect for:

  • penetrations of pipes or ducts that penetrate a firewall or other fire resistance-rated wall or floor not protected in accordance with applicable building, mechanical, fire, or electrical codes;
  • submersible pumps not used in systems that use sump pits as the suction point for active soil depressurization if sump pumps are needed;
  • joints and connections that are not permanently sealed with adhesives;
  • joints and connections that are not airtight;
  • attic and external runs subject to sub-freezing that are not protected to prevent the risk of vent pipe freeze-up;
  • piping that is not PVC, ABS, or downspout (outside);
  • piping subjected to weather or physical damage that is not Schedule 40;
  • pipe and fitting connections of different materials;
  • piping that isn’t solid and rigid;
  • reducers that are installed in the direction of air flow;
  • radon vent pipes blocking access to any areas requiring maintenance or inspection;
  • radon vent pipes not designed with removable or flexible couplings to facilitate removal of the sump pit cover for sump pump maintenance;
  • radon vent pipes not installed in a configuration that ensures that any rain water or condensation within the pipes drains downward into the ground beneath the slab or soil-gas retarder membrane; and
  • a missing one-way flow valve, water trap, or other control device installed in or on the discharge line to prevent outside air from entering the system while allowing water to flow out of the system when a radon mitigation system is designed to draw soil gas from a perimeter drain tile loop that discharges water through a drain line to daylight or to a soak-away.

Piping Supports

The inspector shall inspect for:

  • hangers, strapping, or other supports that inadequately secure the vent material;
  • existing plumbing pipes, ducts, or mechanical equipment used to support or secure a radon vent pipe;
  • supports installed more than 6 feet apart on horizontal runs;
  • supports installed more than 8 feet apart on vertical runs; and
  • pipes not supported or not secured in a permanent manner so as to prevent their downward movement to the bottom of suction pits or sump pits, or into the soil beneath an aggregate layer under a slab to prevent blockage of air flow into the bottom of the radon vent pipes.


The inspector shall inspect for:

  • vent pipes not made of Schedule 20 PVC, ABS, or equivalent;
  • vent pipes used in garages and in other internal and external locations subject to weathering or physical damage not made of Schedule 40 piping or its equivalent;
  • vent pipe fittings in a mitigation system that are not of the same material as the vent pipes;
  • cleaning solvents and adhesives to join plastic pipes and fittings that are not recommended by the manufacturers of the pipe material;
  • improperly used caulks and sealants at any cracks in slabs or other openings around penetrations of the slab and foundation walls;
  • non-shrink mortar, grout, or expanding foam not being used when sealing holes for plumbing rough-in or other large openings in slabs and foundation walls that are below the ground surface;
  • sump pit covers not made of durable plastic;
  • sump pit covers not providing an airtight seal;
  • penetrations of sump covers that are not airtight;
  • plastic sheeting installed in crawlspaces as soil-gas retarders that are not a minimum of 6-mil polyethylene or equivalent material; and
  • any wood used in attaching soil-gas retarder membranes to walls or piers not pressure-treated or naturally resistant to decay and termites.

Point of Discharge for Fan-Powered Soil Depressurization and Block-Wall Depressurization Systems

The inspector should inspect for:  

  • the point of discharge being below the eaves of the roof;
  • the point of discharge being less than 10 feet above ground level;
  • the point of discharge being less than 10 feet away from any window, door, or other opening into conditioned spaces of the structure that is less than 2 feet below the exhaust point; and
  • the point of discharge being less than 10 feet away from any opening into an adjacent building.

Radon Fan

The inspector should inspect for:

  • radon fans not designed or not sealed to reduce the potential for leakage of soil gas from the fan housing;
  • radon fans not sized to provide the pressure difference and air flow characteristics necessary to achieve the radon reduction goals established;
  • radon fans installed in the conditioned space of a building, in any basement, crawlspace, or other interior location directly beneath the conditioned spaces of a building;
  • radon fans installed in attics that are suitable for occupancy;
  • radon fans installed in attached garages beneath conditioned spaces;
  • radon fans installed underground; 
  • radon fans installed in a configuration that allows condensation to build up in the fan housing;
  • radon fans that are not mounted vertically;
  • radon fans mounted on the exterior of buildings that are not rated for outdoor use or not installed in a watertight protective housing;
  • radon fans that are not mounted and secured in a manner that minimizes transfer of vibration to the structural framing of the building;
  • radon fans that are not installed using removable couplings or flexible connections to facilitate maintenance and future replacement; and
  • radon fans used in crawlspace pressurization or building pressurization that do not have removable screens or filters on the fan intakes to prevent ingestion of debris or personal injury.

Suction Pit for Sub-Slab Depressurization Systems

The inspector should inspect for:

  • an inadequate amount of excavated material from the area immediately below the slab penetration point of the system’s vent pipes.


The inspector should inspect for:

  • uncovered or unsealed sump pits that permit entry of soil gas or that would allow conditioned air to be drawn into a sub-slab depressurization system;
  • openings around radon vent pipe penetrations of the slab, foundation wall, or crawlspace soil-gas retarder membrane that are not cleaned, prepared and sealed in a permanent, airtight manner;
  • open or unsealed openings in the tops of walls and all accessible openings or cracks in the interior surfaces of the walls, where a block-wall depressurization system is used to mitigate radon;
  • openings, perimeter channel drains, or cracks where the slab meets the foundation wall that are not sealed;
  • seams and joints in the baseboard of baseboard-type suction systems that are not joined and not sealed;
  • seams in soil-gas retarder membranes used in sub-membrane depressurization system that are not overlapped at least 12 inches and not sealed;
  • open and unsealed access doors and other openings between the basement and the adjacent crawlspace where the crawlspace has been confirmed as a source of radon entry; and
  • open and unsealed openings and cracks in floors above the crawlspace that would permit conditioned air to pass out of the living spaces when crawlspace depressurization is used.


The inspector should inspect for:

  • wiring that does not conform to provisions of the National Electrical Code (NEC) and local building codes;
  • wiring located in or chased through ducting;
  • cord and plug assemblies supplying power to radon fans that are more than 6 feet in length;
  • cord and plug assemblies supplying power to radon fans that pass through walls, floors or ceilings, or that are concealed within building components;
  • radon fans installed on the exterior of the building that are not hard-wired into an electrical circuit;
  • radon fans used outdoors that are plugged;
  • a missing electrical disconnect switch or a missing circuit breaker for radon mitigation system fan circuits;
  • a means of disconnect not in sight of its radon fan;
  • missing grounded receptacles (required within 6 feet of radon fans installed under roofs);
  • missing GFCI receptacles (required within 6 feet of radon fans installed above roofs); and
  • missing electrical junction boxes (required within 6 feet of radon fan locations of both active and  passive systems).


The inspector should inspect for:

  • condensate drainpipes that are not directed into condensate pumps, not directed into trapped floor drains, or do not have 6-inch or greater standing water-trap seals;
  • unsealed perimeter (channel or French) drains; and
  • a sump pit (used for protection or relief from excess surface water) that has a cover not recessed and not fitted with a trapped drain.


The inspector should inspect for:

  • modifications to an existing HVAC system proposed to mitigate elevated levels of radon that are reviewed and approved by a qualified contractor;
  • foundation vents (used to reduce indoor radon levels by increasing natural ventilation) that are closable;
  • heat-recovery ventilation (HRV) systems that are installed in rooms that contain friable asbestos;
  • supply and exhaust ports of heat-recovery ventilation systems installed less than 12 feet apart; and
  • confirmation by the contractor that the incoming and outgoing flow from heat-recovery ventilation systems are balanced.

Monitoring and Labeling

The inspector should inspect for:

  • a missing mechanism to monitor performance of an active soil depressurization system and block-wall depressurization system and to warn of system failure;
  • electrical radon mitigation system monitors installed on switched circuits;
  • electrical radon mitigation system monitors not designed to re-set automatically when power is restored after failure;
  • manometer-type pressure gauges not clearly marked to indicate the range of pressure readings that existed when the system was initially activated;
  • a missing system description label placed on the mitigation system;
  • a missing system description label placed on the electric service entrance panel;
  • a system description label that is not legible from at least 3 feet away;
  • a system description label that does not include all of the following information:
    • “Radon Reduction System”;
    •  the installer’s name and contact information;
    •  the date of the installation; and 
    • an advisory that the building should be tested for radon at least every two years;
  • a missing system description label on each floor level of all exposed and visible interior radon mitigation system vent-pipe sections reading “Radon Reduction System”;
  • missing identification of the circuit breaker controlling the circuit on which the radon vent fan and system-failure warning devices operate; and
  • missing labels on the plastic vapor barrier (if installed).

Sample inspection report for radon mitigation systems

Other inspection checklists.