Mitigation · 10 min read
How Mitigation Systems Work
A clear explanation of active soil depressurization: suction points, pressure fields, fans, pipe routing, sealing, and how performance is verified.
Nearly every effective radon mitigation system in the world uses the same principle: make the space under the building slightly lower in pressure than the space inside it.
Once that relationship is established, soil gas stops flowing into the house. It flows into the pipe instead, and out above the roof.
The core principle
Air moves from higher pressure toward lower pressure. Normally a house is slightly negative relative to the soil, so soil gas moves inward.
A mitigation fan reverses that locally. Applying suction beneath the slab creates a zone of low pressure under the floor. Now the interior of the house is the higher-pressure side, so any air movement through slab cracks goes downward into the pipe rather than upward into the room.
Note what this means: the system does not remove radon from indoor air. It prevents it from arriving.
The suction point and pit
A hole is cored through the slab and material is removed beneath it to create a void, often several gallons in volume. The pit matters because it gives the fan a large surface to draw from rather than a pipe mouth pressed against soil.
A larger, well-formed pit produces better pressure field extension for the same fan, which is why an experienced installer spends time on excavation that a hurried one skips.
Pressure field extension
The measure of a system's reach is how far the vacuum travels beneath the slab. Test holes drilled at distant points reveal whether the pressure field extends to the far corners of the foundation.
In coarse gravel it may extend fifty feet or more from a single point. In tight clay it may fade within ten. This single measurement determines how many suction points a house needs, and it cannot be guessed from square footage.
The fan
Radon fans are sealed inline centrifugal units designed for continuous duty. They are selected from a performance curve that plots airflow against static pressure.
High-suction, low-flow fans suit tight soils. High-flow, low-suction fans suit permeable gravel or alluvium. Choosing the wrong end of the curve produces either a system that cannot reach the far corners or one that roars while achieving little vacuum.
The fan is always mounted outside the conditioned space, in an attic or on an exterior wall, so that any leak in the positive-pressure section downstream of the fan discharges outdoors rather than into the house.
Pipe and discharge
Pipe is typically 3 or 4 inch PVC, sloped so condensate drains back to the suction pit rather than pooling in a low spot.
The discharge terminates above the roof edge, at least ten feet from windows, doors, and air intakes, so the exhausted gas disperses rather than being drawn back inside.
Sealing
Sealing slab cracks, control joints, sump lids, and plumbing penetrations is a supporting measure. It does not stop radon on its own, but it prevents the fan from wasting its capacity pulling conditioned indoor air.
Every cubic foot of indoor air the fan pulls through an unsealed opening is a cubic foot it is not pulling from under the far side of the slab.
Verification
A manometer on the pipe gives permanent visual proof that the system is under vacuum. Offset liquid columns mean it is pulling; level columns mean it is not.
The manometer does not measure radon. Only a post-mitigation test does that, which is why verification testing after 24 hours of operation is a required part of a complete job rather than an optional add-on.
Key takeaways
- Mitigation prevents entry by reversing the pressure relationship, not by filtering air.
- Pressure field extension, measured on site, determines suction point count.
- Fan selection must match the soil's airflow and pressure behavior.
- A manometer proves vacuum; only a test proves radon reduction.