A large share of houses across Huntsville, Madison, and Harvest sit on concrete: either slab-on-grade, which is the norm in most subdivisions built since the 1990s, or a basement, which is more common on the slopes of Monte Sano, Green Mountain, and Chapman Mountain. For both foundation types, the standard radon fix is sub-slab depressurization (SSD). Here's how it works and what to expect.
The idea in one paragraph
Radon comes from the soil under your slab. SSD turns the soil and gravel directly under the concrete into a low-pressure zone. A fan pulls on a pipe that enters a hole in the slab, so instead of soil gas drifting up through cracks and into the house, it moves sideways under the slab toward the suction point, up the pipe, and out above the roof. When it's designed correctly, the whole slab area stays under slight negative pressure all the time.
Step by step: a typical SSD installation
- Diagnostics. The installer drills a small test hole and uses a vacuum and a pressure gauge, or a smoke pencil, to see how easily air moves under the slab. This "communication" test decides how many suction points are needed and how strong a fan to use.
- Choosing the suction point. It's usually placed somewhere central, out of the way, and close to a practical path to the attic or an exterior wall: a utility room, garage, or closet.
- Coring the slab. A hole of a few inches is cored through the concrete, and some fill is removed from beneath it to create a small open pit. The pit increases the area the fan can reach.
- Pipe and fan. PVC pipe is sealed into the hole and run to an inline fan located outside the living space, typically in the attic or on an outside wall. From the fan, pipe continues to discharge above the roof.
- Sealing. Accessible cracks, gaps around plumbing, the floor-to-wall joint in a basement, and any sump lid are sealed so the system pulls soil gas rather than conditioned air.
- Commissioning. The installer confirms suction with a manometer, labels the system, and explains how to check it.
- Retest. A post-mitigation radon test, usually 24 hours to 30 days after start-up, confirms the result.
Slab-on-grade homes
Slab-on-grade houses are the simplest case when the slab was poured over a clean gravel layer. One suction point can often cover a whole house. When the slab sits directly on compacted clay, the red clay soils many north Alabama builders work with, air doesn't move as freely, and the installer may need a second suction point or a higher-suction fan.
Many newer Madison County homes have monolithic slabs with post-tension cables. Coring into those requires care: a professional will locate tendons, or choose a spot where cables aren't present, before drilling. Ask your installer how they handle post-tension slabs if your home is one of these.
Basement homes
Basements add a few specifics:
- Sump pits can make excellent suction points if the pit connects to perimeter drain tile. The lid must be sealed airtight, with a clear viewing port if you want to see the pump.
- Block walls. Hollow concrete-block basement walls can carry soil gas up inside the cores. Sealing the top course or adding block-wall suction may be needed when sub-slab suction alone isn't enough.
- Finished basements. Carpet, drywall, and built-ins limit where the slab can be reached. Installers often find space in a mechanical room or under a stair.
- Walk-out basements. Common on Huntsville's hillsides, these have part of the slab at grade and part below. Air movement under the slab can be uneven, so diagnostics matter more.
Where does the pipe go?
This is the question homeowners care about most, because it affects appearance.
- Interior routes run through a closet, chase, or garage corner into the attic, with the fan in the attic and a roof penetration. They're nearly invisible from the street, but they take more labor and need attic space.
- Exterior routes exit the wall near the suction point and run up the outside of the house to above the eave. They're faster and usually cheaper. Paintable or colored pipe and downspout-style covers can reduce the visual impact.
- Garage routes are a common Huntsville compromise: the suction point sits in or near the garage, and the pipe runs up through the garage ceiling.
Wherever the pipe goes, the fan must not be inside the living space, and the discharge must clear the roof and stay away from windows, doors, and neighboring openings.
How much reduction to expect
SSD is the most studied radon fix, and in most slab and basement homes it brings radon well below 4 pCi/L. Results depend on how well air moves under the slab, how many openings are sealed, and whether part of the house sits on a different foundation. A certified installer should tell you up front what reduction they expect and what they'll do if the retest comes back high, which is usually adding a suction point or changing the fan.
Cost context
SSD is the "active suction" system most cost guides price. Angi's 2026 national data puts that type of system at roughly $805 to $1,326 on average across 1,184 projects. A second suction point, a long interior pipe run, or electrical work raises the total. These figures are national averages for context only; your quote will come from the professional who inspects your home.
Living with an SSD system
- Noise. A properly mounted fan in the attic or outdoors makes a low hum. Rattles usually mean a loose mount or a pipe touching framing, both easy fixes.
- Electricity. The fan runs continuously; ask the installer for the wattage of the model they propose so you can estimate the cost.
- Monitoring. Glance at the manometer whenever you change the HVAC filter. If the liquid columns are level, the fan isn't pulling.
- Retesting. Test again every two years or so, and after remodeling, adding on, or replacing the HVAC system.