Sub-Slab Depressurization in Champaign, IL

The Physics, in Plain Terms

Under most concrete slabs sits a layer of gravel or crushed aggregate, placed there during construction to help with drainage and to give the concrete a stable base. That layer is naturally porous, which means air moves through it fairly easily. Radon gas generated in the soil below collects in that aggregate layer and, without any interference, finds its way up through hairline cracks, control joints, and gaps around plumbing penetrations into the house above.

A sub-slab depressurization system exploits that same porous layer in the opposite direction. A radon fan draws air out of the aggregate through a suction pipe, which lowers the pressure under the slab relative to the air pressure inside the house. Once that pressure difference is established, radon gas in the aggregate layer gets pulled toward the suction point and up through the vent pipe instead of migrating upward into the home. It is a passive redirection of gas that is already there, powered by a fan that runs continuously.

Suction Point Placement and Pipe Routing

Where the suction point goes matters more than most homeowners expect. We look for a location, often a utility room, closet floor, or garage-adjacent slab area, where a core hole can be drilled with minimal disruption and where the aggregate beneath is likely to be well connected across the slab. Before finalizing placement, we consider the layout of interior walls, since footings and thickened slab edges under bearing walls can block airflow and effectively divide a slab into separate zones that a single suction point cannot reach.

From the suction point, PVC pipe runs vertically, typically routed through a closet, a utility chase, or up an exterior wall, to a termination point above the roofline, at least ten feet from any window or opening per standard practice, so exhausted soil gas disperses outdoors without any chance of re-entering the house. The inline fan sits somewhere along that run, most often in the attic where it is out of sight and where sound transmission into living space is lowest.

When One Suction Point Is Not Enough

Some slabs depressurize evenly from a single suction point. Others do not, and that comes down to how the aggregate beneath the slab is structured and how communicative it is across the whole footprint. Larger homes, homes with additions poured at different times, homes with multiple separate slab sections, or slabs where interior footings interrupt the gravel base often need two or more suction points tied into the same fan, or in some cases separate fans, to achieve consistent negative pressure everywhere.

We determine this partly through visual inspection and partly through a diagnostic test during installation, sometimes called a communication test, where we check whether suction applied at one point registers as a measurable pressure change at other points around the slab. If it does not, that tells us the slab is compartmentalized and we design accordingly rather than installing a single point and hoping it works.

Confirming the System Works

Every SSD system we install includes a U-tube manometer, a small liquid-filled gauge mounted somewhere visible, usually near the vent pipe in a basement or utility area. When the fan is running and pulling proper suction, the liquid column sits at a visible offset. It is a permanent, at-a-glance way for you to confirm the system is active without needing any tools or expertise. A flat reading means the fan has stopped and the system needs a look, which is covered in more detail on our system repair and fan replacement page.

After installation, the real test is a post-mitigation retest, ideally between 24 hours and 30 days after the system goes active, to confirm the specific number your home is now reading. A properly designed and installed SSD system typically brings even moderately elevated homes well under the EPA's 4.0 pCi/L action level, and we stand by the design enough to recommend that retest rather than avoid it.

Slab Condition and Why It Changes the Approach

Not every slab is the same underneath, and slab condition affects how a system is designed as much as the layout above it does. Older homes sometimes have a poorly graded or uneven aggregate base, sections poured without a continuous gravel layer, or a slab that was patched or repaired at some point in a way that interrupted the sub-slab material's connectivity. We factor this in during the assessment, since a slab with inconsistent aggregate underneath is part of why a communication test matters more in some homes than others.

Cracks and control joints in the slab itself also play a role. A visible crack is not automatically a problem to seal separately, since the mitigation system's negative pressure often addresses gas movement through minor cracks on its own by reversing the direction gas would otherwise travel through them. Larger or structurally significant cracks get evaluated on their own merits, sometimes sealed as a supplementary step alongside the main suction system rather than relied on as the primary fix.

Frequently Asked Questions