Mitigation · 7 min read
Radon Pipe Systems
How radon system piping is sized, routed, sloped, and terminated, and why those details determine noise, condensate behavior, and long-term performance.
The pipe is the least glamorous part of a mitigation system and one of the most consequential. Sizing, slope, routing, and discharge location each affect performance and longevity.
A well-piped system is quiet, drains itself, and lasts as long as the building.
Sizing
Three and four inch Schedule 40 PVC are the standard choices. Larger pipe reduces air velocity, which reduces noise and friction losses, allowing a smaller fan to achieve the same vacuum.
Undersized pipe is a common source of noise complaints. It also forces the fan further along its curve, wasting energy and shortening life.
Routing options
Interior routing runs the pipe through a closet, garage, or utility chase up into the attic, with the fan in the attic and the discharge through the roof. It keeps the exterior clean, which matters on street-facing elevations and architecturally sensitive homes.
Exterior routing runs the pipe up an outside wall with a wall-mounted fan. It is faster to install, easier to service, and keeps the fan out of hot attic conditions, at the cost of visible pipe.
Slope and condensate
Soil gas is humid, and water condenses inside the pipe as it cools. All horizontal runs must slope continuously back toward the suction pit so condensate drains away.
A sagging horizontal run creates a low spot where water collects, restricting airflow and sometimes blocking it entirely. Audible sloshing or gurgling in a pipe is the diagnostic sign, and the fix is proper support rather than a larger fan.
Discharge location
The termination point has firm requirements for good reason.
- Above the roof edge, so gas disperses rather than pooling
- At least ten feet from any window, door, or air intake
- Away from adjacent structures and neighboring openings
- Clear of rooftop mechanical equipment intakes
- Positioned so prevailing wind carries discharge away from the building
Sealing and support
Joints are solvent welded, and the connection at the slab is sealed with flexible polyurethane so the system draws from soil rather than the room.
Pipe is supported at regular intervals to prevent sag, with corrosion-resistant hardware on exterior runs. In coastal exposure, stainless fasteners and UV-stable treatment on the pipe measurably extend service life.
Labeling
Systems should be labeled so a future occupant, inspector, or contractor understands what the pipe is and does not mistake it for plumbing or cut into it during a remodel.
A simple label at the fan and at any visible pipe section, along with the manometer, is enough to make the system self-explanatory years later.
Key takeaways
- Larger pipe means quieter operation and lower fan energy.
- Continuous slope back to the pit prevents condensate blockage.
- Discharge must clear the roof and stay well away from openings.
- Labeling protects the system from future remodeling mistakes.