The principle: reverse the pressure
Soil gas enters a house because the space under the slab is at slightly higher pressure than the air inside. An active sub-slab depressurization (ASD) system inverts that relationship. A continuously running fan draws air from beneath the slab and discharges it above the roof, so the area under the foundation becomes lower in pressure than the house. Soil gas now takes the pipe instead of the cracks.
The pressure field extension
The measure of a working system is not how much air it moves — it is whether negative pressure extends under the entire footprint of the home. This is called pressure field extension. A suction point creates a vacuum field that spreads through the aggregate or soil below the slab; the design question is how far that field reaches.
| Sub-slab material | Field extension | Design implication |
|---|---|---|
| Clean coarse aggregate | Wide — often 30+ feet | One suction point may cover the home |
| Sandy fill | Moderate | One to two points typical |
| Compacted native soil / caliche | Narrow — sometimes under 10 feet | Multiple points, higher static pressure fan |
Why the fan goes outside living space
Everything downstream of the fan is under positive pressure. If a joint on that section leaks, it discharges radon-laden air. Mounting the fan in an attic, garage attic, or on an exterior wall guarantees any leak on the pressurized side vents outside rather than into the house. This is a protocol requirement, not a preference.
Why sealing matters
An unsealed floor-wall joint or open sump lets the fan pull conditioned indoor air straight down through the slab and out the pipe. That wastes the vacuum, wastes energy, and shrinks the pressure field. Sealing is not what stops radon on its own — it is what lets the fan work on soil gas rather than on your heated air.
The pieces of a complete system
- 1Suction point(s) cut through the slab into an excavated pit.
- 2Sealed PVC piping sized for the airflow the fan will move.
- 3A continuously rated inline fan mounted outside conditioned space.
- 4Roof-level exhaust, above the eave and away from openings.
- 5Sealed cracks, joints, sump lids, and penetrations.
- 6A manometer so you can verify operation at a glance.
- 7System labeling and documented pre/post test results.
What mitigation is not
- It is not an air purifier. HEPA and carbon filters do not address radon gas in any meaningful way.
- It is not just sealing cracks. Sealing alone rarely brings an elevated home under the action level.
- It is not a ventilation fan in a basement window. That may dilute slightly while creating a comfort and energy problem.
- It is not permanent without maintenance — fans wear out and seals age.
Alternative approaches, and when they apply
| Approach | When it is used |
|---|---|
| Active sub-slab depressurization | The default for slab and basement homes |
| Sub-membrane depressurization | Crawlspaces, over a sealed vapor barrier |
| Sump-pit depressurization | Homes with an existing sealed sump serving as the suction point |
| Drain-tile depressurization | Where a perimeter drain loop can distribute vacuum |
| Heat recovery ventilation | Rare; supplemental dilution when ASD is impractical |
Related services
- Radon Mitigation System InstallationPermanently lower radon in an existing home with an active sub-slab system.
- Crawlspace Radon MitigationSeal and depressurize exposed crawlspace soil, the largest radon entry point there is.
- Slab-on-Grade Radon MitigationMitigate the foundation type most Albuquerque homes are built on.
Compare your options
- Passive vs Active Radon SystemsA passive system relies on natural airflow. An active system adds a fan. The distinction determines whether your home is reliably protected or only sometimes.
- Crawlspace Encapsulation vs Sub-Slab DepressurizationEncapsulation is a barrier. Depressurization is a pump. Radon mitigation in a crawlspace needs both working together.