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How does a radon mitigation system work?

Radon mitigation system

Let’s say you find out that you have elevated radon levels in your home.  What’s the next step? You will need to get a licensed mitigation specialist to install a system for your home.  That’s great. But how does a mitigation system work? How do you know if it is properly working? Let us examine two common mitigation systems in use today: passive and active.

Passive Radon Mitigation Systems

A passive radon mitigation system is one method to prevent radon gas from entering your home. This is typically installed when a property is being constructed.  Take a look at the illustration above. What generally happens in this system is that PCV piping is installed beneath the concrete slab in a basement or in a sump pit.  This is done to get access to the radon gas in the ground before it enters into your living area. The passive system heavily relies on the upward flow of air (like rising warm air) to send the gas from below the concrete slab through this pipe traveling inside the walls of the home, and out of the building at the roof. When retrofitted onto an existing home, it is common to have the vertical pipe travel on the outside of the home.  The pipe extends past the roof of your home where the gas escapes into the outdoor air.  

Since June of 2013, anyone building a new home in the state of Illinois must have a passive radon mitigation system.  This is part of the Radon Resistant Construction Act and 32 Illinois Administrative Code 422 further sets up regulations and control methods for radon service providers.

At first blush, this seems great!  Now that you have a new house with a passive system, you don’t have to worry about elevated levels of radon!


Just because you have a passive radon system does not guarantee that the levels will be within acceptable ranges.  The only way to know for sure is to have an impartial third party test the radon levels within the home. The requirement in Illinois is that you must have a passive system for a newly constructed home.  Sadly, it doesn’t require that the system actually lowers radon levels below the action level. We have seen radon levels in homes with passive systems that exceed the action level!

Active Radon Mitigation Systems    

Fan for an active mitigation system

Active systems are exactly like passive systems with one very noticeable exception: a fan (see photo above) is installed in the PCV pipe to create suction (or negative pressure) of the radon.  This gas is sucked out from below the concrete floor or crawl space through the venting pipes and is safely delivered outdoors (this is also known as sub-slab depressurization). These fans are generally installed outside of the home or in the attic and are continuously running to pull out the radon gas. That way the PVC pipe running inside your home will be under negative pressure.

If you have an older home that wasn’t built with PVC pipes under your slab, how do you install a radon mitigation system?   This involves creating a small hole that allows a 3 to 4-inch diameter vent pipe to penetrate the concrete slab (See the top illustration).  Once this hole is created, an active system as described above is installed and used to pull the radon gas outdoors.    

Passive systems can easily be converted into active systems with the installation of the fan.  This is relatively inexpensive, with a fan costing anywhere between $300 to $400. 

Active systems are far more effective than passive systems.  However, it is still important to have an independent radon measurement test performed.  This is the only way to know for certain that radon levels are below the 4.0 pCi/L recommended by the EPA.  

How Do I Know If My Radon System Is Working?

The shape of the red dye shows that the fan is operating

After you have your active system installed, you will have a device known as a manometer (See photo above).  This is a gauge that lets you know if the active system is creating the proper negative pressure under the slab.  Generally, you can tell if the fan is working because there is a red dye on the meter that has an inverted “J” shape to show that the fans are working.  When a fan is not working, the dye is “U” shaped and requires repair or replacement.    

Added Benefit of an Active System

The radon fans in the active system are constantly running, which provides the added benefit of removing a lot of moisture,noxious odors, and any other soil gases (such as those that cause vapor intrusion) from beneath the slab or crawl space.  Think of it as getting rid of two health issues for the price of one solution!

Do Active Radon Systems Still Need To Be Monitored?

We can breathe a little easier after an active system is installed, but just because it’s there doesn’t mean that we shouldn’t be vigilant.   The system needs to be monitored by occasionally checking the manometer readings to see if the fan is in operation. The fan system may last for five or more years before it needs to be replaced.  And it can not be overemphasized that testing should still be performed in your home on a regular basis. It is a good idea to retest the property with a radon measurement professional every two years in alternating seasons to be sure radon levels are low.  Then you can breathe easy and enjoy healthy air in your home.

Ian Cull

Ian Cull is a nationally recognized expert in the field of indoor air quality. He is the Chief Science Officer of Indoor Science, a company he started in 2004. He speaks around the world on air quality topics and is a training provider of the Indoor Air Quality Association. Mr. Cull is a Licensed Professional Engineer (PE) and Certified Industrial Hygienist (CIH). His degree is in Environmental Engineering from the University of Illinois - Urbana Champaign. Mr. Cull has developed 50 air quality related courses for the IAQA University and is the author of the book, “Fundamentals of Mold Remediation”. In his words… “Besides being passionate about indoor air quality, I enjoy cycling, music, the Chicago Bulls, and having fun with my three kids.”

4 thoughts on “How does a radon mitigation system work?

    Excellent article, Scott. Thank you for the information. We just had an active system installed this past Fall and it’s working great. However, I noticed the vent stack does not have a hood or rain shield at the top to prevent precipitation from entering the 3-inch pipe. We live in northern Ohio and can receive large amounts of rain in a single storm (earlier this week we received 16 inches of heavy, wet snow over the course of 24 hours). Will the introduction of rain or snow harm the system or decrease its life or effectiveness in any way? Thank you.

    Hello Jeff! Almost every stack that I have seen in my area (Chicagoland) does not have a hood or shield. Because the stack should be above the gutter area, I don’t believe that it should be a problem. However, this is a bit outside my area of expertise. I would contact any mitigation company and ask them if this has ever been a problem.

    Hi Scott,
    My name is Bob Bacon I’m a luxury home designer and would like to layout and specify an active radon mitigation system below the slabs of a home currently on the drawing board. I presume these systems use perforated PVC pipes but don’t know the size and spacing recommendations. I’m also curious about the static pressures encountered when sucking air out of an aggregate base course.
    The soils in North Scottsdale AZ are largely decomposed granite, but I will be over-excavating and bringing in appropriate base course and back- fill gravels. I would appreciate any help you can offer.
    I can be reached at: [email protected] or 602-997-8070. You can see the proposed home at: http://www.thebaconhouse.com
    Bob Bacon

    Unfortunately, I am not familiar with the radon codes for Arizona. I would recommend contacting your state for the best advice.