How Sub-Slab Depressurization Works

Sub-slab depressurization (often shortened to SSD) is the method used in the large majority of residential radon mitigation systems. It works with the physics of the problem rather than against it: instead of trying to seal every possible crack a gas molecule could sneak through, it creates a zone of lower air pressure under the foundation than inside the house. Radon gas naturally moves toward that lower-pressure zone and gets vented outside before it ever reaches your living space.

The basic components

A typical SSD system has four main parts:

• A suction point. The contractor drills a hole through the concrete slab, usually a few inches wide, and digs out a small pit in the soil or gravel beneath it.
• A vent pipe. Rigid PVC pipe connects to the suction point and runs up through the house, typically through a closet, utility room, or garage, and continues above the roofline.
• An inline fan. A radon-rated fan, usually installed in the attic or outside the house, runs continuously to pull air (and the radon in it) up through the pipe and out above the roof, where it disperses harmlessly.
• A pressure gauge. A small manometer, usually a U-shaped tube with liquid inside, is mounted somewhere visible so the homeowner can do a quick visual check that the fan is still creating suction.

Step by step: how installation works

1. Diagnostic testing

Before drilling anything, an experienced contractor evaluates your foundation, checking for sub-slab gravel or open soil, the layout of interior footings, and where a sump pit or floor drain might already provide a natural connection to the soil below.

2. Drilling the suction point

The contractor drills a hole through the slab, often near the center of the house or in a spot that gives the best access to the sub-slab area. A small cavity is dug out beneath the hole to give the vacuum room to pull air from a wider area rather than just the immediate hole.

3. Running the vent pipe

PVC pipe is connected to the suction point and routed up through the structure. Contractors try to keep the run as vertical and direct as possible, since a straighter path performs better and looks cleaner. The pipe exits above the roofline, similar to a plumbing vent stack, so exhausted air disperses well above where people are breathing.

4. Installing the fan

The inline fan is usually mounted in the attic, in a garage, or on an exterior wall, positioned so that any minor noise or vibration doesn't carry into living spaces. The fan runs continuously, 24 hours a day, creating steady negative pressure under the slab.

5. Sealing entry points

Once the main vent path is doing the heavy lifting, the contractor seals other obvious entry points: the sump pit lid, major slab cracks, and gaps around utility penetrations. This isn't the primary fix, but it helps the system work more efficiently by reducing how much conditioned indoor air the fan has to pull through unintended gaps.

6. Testing the result

After the system has been running for at least 24 hours, a follow-up radon test confirms the level has dropped below the EPA's 4.0 pCi/L action level. If it hasn't, the contractor may add a second suction point or adjust the system.

Why one suction point is often enough

It might seem like a single hole in a large foundation couldn't possibly depressurize the whole area underneath it, but in most homes it works because of what's directly under the slab. New construction and many older homes have a layer of gravel or crushed stone under the concrete, which is porous and lets air move sideways fairly easily. That means suction applied at one point can pull air (and pressure change) across a surprisingly wide area. Homes with dense soil, multiple separate foundation sections, or a finished basement with interior walls poured after the original slab sometimes need a second suction point to cover the whole footprint.

Crawl spaces work a little differently

In a home with a crawl space instead of a slab, the contractor typically installs a heavy plastic vapor barrier across the dirt floor, seals its seams and edges, and then applies the same vacuum principle underneath that barrier instead of underneath concrete. This is sometimes called sub-membrane depressurization, and it follows the same core idea: pull soil gas out before it reaches living space above.

Why the fan has to run continuously

Depressurization only works while the fan is running. If it's ever turned off or loses power for an extended period, radon can start building back up under the slab and eventually work its way back into the house. That's why the pressure gauge matters: it's a simple way to notice if the fan has stopped so you can call for a repair before radon levels start creeping up again. Most fans are designed to run for years without much attention, but like any continuously operating appliance, they eventually wear out. Our radon fan replacement guide covers the warning signs.

Is SSD always the right approach?

Sub-slab depressurization is the standard, EPA-recommended approach for the vast majority of homes because it's effective, relatively low-maintenance, and doesn't require major structural changes. A licensed or certified contractor will evaluate your specific foundation and may recommend a variation on this design, but the underlying principle, creating negative pressure under the home to intercept radon before it enters, stays the same across almost every residential system installed today.