St. Louis Metro · Missouri & Illinois
Radon Mitigation System Diagram
A radon mitigation system diagram shows six working parts: a suction point cored through the slab, a sealed vent pipe, an inline fan, a roofline discharge, a manometer, and sealed openings such as the sump lid.
Together those parts form an active sub-slab depressurization system, which lowers the air pressure beneath the foundation so soil gas is pulled out of the ground before it can enter the house.
Active systems reduce indoor radon by as much as 99 percent, and the EPA action level that triggers the work is 4.0 pCi/L.
What the System Targets
Systems are engineered to bring your home below the EPA action level of 4.0 pCi/L.
Six working partsThe Diagram
What does a radon mitigation system diagram show?
A radon mitigation system diagram shows how soil gas travels from beneath the foundation to above the roofline, and where each component sits along that path. The numbered callouts below match the components explained in the rest of this page.
The logic is simple. The fan lowers pressure inside the pipe, the pipe lowers pressure in the pit beneath the slab, and that pit becomes a lower-pressure zone than the basement. Soil gas takes the easiest path, which is now the pipe rather than the cracks in your floor.
Component 1
What is the suction point in a radon mitigation system?
The suction point is a hole cored through the concrete slab with a pit excavated in the aggregate beneath it, and it is the part of the system that determines whether the whole thing works.
Its placement is decided by a pressure-field extension test, which measures how far a vacuum applied at one spot actually travels under the slab. Loose gravel under a slab can allow one suction point to depressurize an entire footprint. Compacted clay may need two or three.
This is the single most common failure in an underperforming system. A fan running hard on a badly placed suction point will show good pressure at the manometer while leaving half the slab untreated.
Component 2
What does the vent pipe in a radon mitigation system do?
The vent pipe carries soil gas from the suction point to the discharge above the roofline, and it must be sealed along its whole length so the fan pulls from the ground rather than from inside the house.
- Material: schedule 40 PVC in most residential installations
- Diameter: commonly 3 or 4 inches, sized against the system’s airflow and resistance
- Routing: either through the interior of the house or up an exterior wall, each with trade-offs in appearance and freeze risk
- Slope: pitched so condensate drains back down to the suction point rather than pooling in the pipe
Pipe sizing is not cosmetic. An undersized pipe adds resistance that the fan has to overcome, which costs airflow at exactly the point the system needs it. There is more on this in our page covering sub-slab depressurization.
Component 3
Where does the fan go in a radon mitigation system?
The radon fan goes in unconditioned space, either outside the house or in an attic or garage, and never inside the living area or basement. That rule exists because the pipe on the discharge side of the fan is under positive pressure, so any leak there would push radon-laden air into the house rather than out of it.
Fans run continuously. Sizing depends on how much resistance the soil and piping present, and a fan matched to loose aggregate will not perform the same on compacted soil. Most fans last several years before needing radon fan replacement.
Component 4
Where does a radon mitigation system discharge?
A radon mitigation system discharges above the roofline, pointing straight up, so the gas disperses into open air instead of being drawn back into the house.
The standard clearances are:
- At least 12 inches above the roof surface
- At least 10 feet from any window, door, operable skylight, or air intake
- At least 10 feet above ground level
- No flat rain cap that would deflect the exhaust downward
Outdoor radon sits around 0.4 pCi/L, so gas released above the roof dilutes to background almost immediately. Released at ground level next to a window, it does not.
Components 5 and 6
What are the manometer and sealed sump lid for?
The manometer is a U-shaped tube on the vent pipe that shows whether the fan is pulling, and the sealed sump lid closes off what is often the largest single opening between the soil and the basement.
- Manometer. When the system is working, the fluid sits at different heights in the two legs. When both legs are level, the system is not moving air, and a failed fan is the usual reason. Checking it takes about ten seconds.
- Sealed sump lid. An open sump pit is a direct opening to the soil. Sealing it, along with the floor-wall joint and slab cracks, lets the fan develop suction under the slab instead of pulling conditioned air out of the room.
One important limit: the manometer measures pressure in the pipe, not radon in the air. A moving manometer confirms the fan runs. Only a radon test confirms the house is below the EPA action level of 4.0 pCi/L.
Local Conditions
How St. Louis foundations change radon mitigation system design
Radon mitigation system design changes house to house across the St. Louis metro, even though the same radon mitigation system diagram describes every installation. What changes is the number of suction points, the fan size, and how much sealing the foundation needs first.
Older homes in St. Louis City and inner-ring suburbs frequently have stone or block foundations, dirt-floor sections, and floor-wall joints that have opened over decades, so more sealing work is needed before the fan can build suction. Newer slab-on-grade construction in St. Charles and Jefferson counties often depressurizes cleanly from a single suction point. Homes with a basement plus a crawl space, common throughout the metro, usually need two approaches combined, including crawl space encapsulation.
Air Sense Environmental designs each system against the actual foundation. See radon mitigation for the full service, or our service area for coverage across Missouri and Illinois.
Common Questions
Frequently Asked Questions About Radon Mitigation System Diagrams
How does a radon mitigation system work?
A radon mitigation system works by lowering air pressure beneath the foundation so soil gas is drawn out before it enters the house. A fan pulls a vacuum on a sealed pipe connected to a pit under the slab, which makes the pipe the easiest escape route for radon instead of the cracks in the floor. Active systems of this kind reduce indoor radon by as much as 99 percent.
What are the parts of a radon mitigation system?
A radon mitigation system has six main parts: a suction point cored through the slab, a sealed PVC vent pipe, an inline fan mounted in unconditioned space, a discharge above the roofline, a manometer showing system pressure, and sealed openings including the sump lid and floor-wall joint. Some homes also need a sub-membrane liner over an exposed crawl space.
Does a radon mitigation system pipe go inside or outside the house?
Either. An interior run hides the pipe and keeps it warm, which avoids condensate freezing, but it takes up closet or utility space. An exterior run is simpler to install and easier to service, but the pipe is visible on the side of the house. Both meet code if the discharge clearances above the roofline are met.
Why is the radon fan in the attic instead of the basement?
The radon fan sits in unconditioned space because the pipe downstream of the fan is under positive pressure. If that section leaked inside a living area or basement, it would push radon-laden air into the house rather than out of it. Mounting the fan outdoors, in an attic, or in a garage keeps the pressurized section outside the occupied envelope.
Can I see a radon mitigation system diagram for my own house?
Yes. Any reputable installer should walk through the proposed layout before work begins, including where the suction point goes, how the pipe will be routed, where the fan will sit, and where the discharge exits. Air Sense Environmental reviews that layout with the homeowner as part of a free in-home estimate.
References: EPA radon action-level guidance and A Citizen’s Guide to Radon.
See It On Your Home
Get the layout for your foundation
A diagram explains the parts. A walkthrough shows where they go on your house, and what the foundation actually needs.