Sub-Slab Depressurization Explained: The Most Common Radon Fix

If you have researched radon mitigation for more than a few minutes you have almost certainly encountered the term sub-slab depressurization. It appears in EPA guidance documents, on mitigation company websites, and in the reports that certified radon professionals provide after testing. It sounds technical, and it is — but the underlying principle is straightforward enough that any homeowner can understand it fully with a clear explanation. More importantly, understanding sub-slab depressurization helps you understand why it works so reliably and what makes a well-designed installation different from a mediocre one.

Our post on how mitigation systems work introduced sub-slab depressurization as the standard approach to radon mitigation. This post goes deeper — into the mechanics of how the method achieves its results, what happens beneath your slab when the system is operating, and what the key variables are that determine how effectively a given installation performs.

Starting With the Soil

Beneath the concrete slab of a typical basement floor there is a layer of material — usually crushed stone, gravel, or coarser aggregate — that was placed there during construction to provide drainage and a stable base for the concrete pour. This aggregate layer is the operational environment for sub-slab depressurization. Its presence, depth, and composition are among the most important variables in predicting how well an SSD system will perform.

Radon moves through this aggregate layer as it travels from the surrounding soil upward toward the home. The aggregate's porosity — how easily air and gas can move through it — determines how effectively the suction created by a mitigation system can extend across the entire footprint of the foundation. High porosity aggregate allows suction to reach broadly from a single point. Dense, fine, or poorly graded material resists suction extension and may require multiple suction points to achieve full coverage.

This is why a diagnostic assessment before system design matters. Knowing what is beneath your slab — and how well it communicates across the sub-slab space — is foundational to designing a system that actually works rather than one that simply exists.

The Suction Point: Where It All Starts

The entry point of a sub-slab depressurization system is a small hole — typically three to four inches in diameter — core drilled through the concrete floor. The location of this hole is one of the most important decisions in system design, and it is not arbitrary.

The ideal suction point is positioned to maximize the pressure field extension — the reach of the suction — across as much of the sub-slab area as possible. In a home with good sub-slab communication, a single centrally located suction point may achieve adequate coverage across the entire footprint. In a home with interior footings, multiple foundation sections, or poor aggregate communication, the optimal suction point location requires more analysis and sometimes multiple points to achieve full coverage.

A diagnostic step called a communication test — sometimes called a suction pit test — helps determine this. After the suction hole is drilled, a technician uses a vacuum to pull air from beneath the slab and measures how the suction pressure distributes across the sub-slab space. Pressure gauges or smoke sticks placed at other locations around the basement reveal whether suction is reaching those areas effectively. This test tells the installer whether a single suction point will suffice or whether additional points are needed before the permanent system is designed.

Skipping this diagnostic step and installing a single suction point without verifying coverage is one of the most common shortcuts in substandard mitigation work. A system with inadequate sub-slab coverage may produce an acceptable post-installation result initially — because the suction is controlling radon entry in the areas it reaches — while leaving portions of the sub-slab unaddressed that can allow radon entry to continue or resume over time.

Creating the Pressure Field

Once the suction point is established and the pipe and fan are connected and operating, the system creates what is called a pressure field beneath the slab. This is the zone of negative pressure — lower pressure than the air inside the home — that extends through the aggregate layer from the suction point outward.

The pressure field is the mechanism by which the system intercepts radon. Radon moving upward through the soil toward the home encounters this zone of negative pressure before it reaches the slab. Rather than continuing toward the foundation, it follows the pressure gradient toward the suction point — the path of least resistance — and is drawn up through the pipe and discharged above the roofline into the outdoor air.

The size and uniformity of the pressure field determine how comprehensively the system addresses radon entry across the full foundation footprint. A well-designed system with adequate fan capacity and properly placed suction points creates a pressure field that extends to the perimeter of the sub-slab space, intercepting radon across the entire area beneath the home. A system with insufficient fan capacity or poorly placed suction points may create an adequate pressure field near the suction point while leaving peripheral areas with insufficient coverage.

Fan Sizing and System Performance

The fan is what creates and maintains the pressure field, and fan sizing is one of the most technically important decisions in system design. A fan that is undersized for the conditions beneath a particular home — dense aggregate, long pipe runs, multiple suction points — will not create an adequate pressure field regardless of how well the rest of the system is installed. A fan that is significantly oversized for the conditions draws more electricity than necessary and may create a pressure differential that is stronger than needed without providing proportionally better radon reduction.

Proper fan sizing requires knowing the sub-slab communication characteristics, the pipe run length and configuration, and the target pressure differential needed to achieve the desired radon reduction. This is why the diagnostic assessment before installation is not a formality — it is the foundation for a fan selection decision that directly determines how well the system performs.

Fan quality matters alongside fan sizing. As we touched on in our post on how mitigation systems work, radon mitigation fans run continuously around the clock, every day. Over a system lifetime that may span ten to twenty years or more, a quality fan designed for continuous operation in the conditions of a mitigation application outperforms a cheaper alternative significantly. The difference is not visible at installation — it shows up in consistent performance, lower failure rates, and longer service life over years of continuous operation.

What Happens at the Discharge Point

The radon-laden air drawn from beneath the slab travels up through the PVC pipe and exits at the discharge point — above the roofline of the home, positioned so that the discharged air disperses into the outdoor atmosphere rather than re-entering the home through windows, doors, or HVAC intakes.

Discharge point placement follows specific guidelines for this reason. The pipe must terminate above the eave of the roof, at a minimum distance from any window, door, or opening that could allow discharged air to re-enter the living space. In most installations this means the pipe exits through the roof or runs up an exterior wall to a point above the roofline. The specific routing depends on the home's layout and what makes for a clean, code-compliant installation.

The radon concentration in the discharged air is higher than ambient outdoor levels, but because it is released at height and disperses immediately into outdoor air, the health impact at ground level is negligible. This is the same principle that governs industrial exhaust stacks — dilution in the outdoor atmosphere renders the discharge harmless.

When One Suction Point Is Not Enough

Returning to a point introduced earlier: not every home can be adequately mitigated with a single suction point, and recognizing when multiple points are needed is one of the markers of a thorough, well-designed installation.

Homes where multiple suction points are commonly needed include those with interior concrete footings that divide the sub-slab space into separate sections, homes with additions built on separate foundation pours, homes with a mix of basement and crawlspace areas, and homes where the aggregate beneath the slab is dense or inconsistent. In these cases, a single suction point may create an excellent pressure field in one section of the home while leaving another section essentially unaddressed.

A post-installation test that shows adequate radon reduction is the ultimate confirmation that the system's coverage is sufficient. But a rigorous pre-installation communication test and thoughtful suction point placement reduce the likelihood of discovering coverage gaps only after the system is installed.

Why SSD Works So Reliably

Sub-slab depressurization has become the standard radon mitigation approach because it addresses the root cause of radon accumulation rather than trying to manage symptoms. It does not attempt to seal every possible entry point — a strategy that is both difficult to execute completely and subject to failure as homes continue to age and develop new gaps. It does not rely on diluting radon after it has entered the home — a strategy that requires significant and ongoing air exchange that conflicts with modern energy efficiency goals.

Instead it changes the pressure relationship between the home and the soil beneath it, making it physically difficult for radon to move toward the home rather than toward the suction point. As long as the fan runs and the pressure field is maintained, radon follows the system rather than the home. It is a mechanically simple solution to a geological problem, and its reliability over decades of widespread application is the reason it remains the approach of choice.

At All Michigan Mitigation, every system we design starts with a thorough diagnostic assessment and ends with a post-installation test that confirms the system is performing at the level we designed it to achieve. Sub-slab depressurization done well is one of the most reliable home health investments available, and we bring the expertise and care to every installation that the method deserves.

Visit us at allmichiganmitigation.com to learn more about what a mitigation system would involve for your home or to schedule a consultation.

All Michigan Mitigation is a West Michigan radon testing and mitigation company dedicated exclusively to helping homeowners understand and eliminate radon risk in their homes.