Why Sub-Slab Depressurization Is the Gold Standard in Radon Mitigation

When I first started testing homes for radon in Jefferson County, Missouri, I quickly learned that no two crawl spaces or basements are built alike. Some houses sit on sandy loam, others on clay. Some have a thick concrete slab poured over a layer of gravel; others have a thin slab that cracked within the first year. And every single one of them can pull soil gas indoors if the pressure differences are right. Over the years, one method has proven itself again and again: sub-slab depressurization. It is not flashy, but it works. And when it is done right, it keeps radon levels low for the life of the home.

How Soil Gas Gets Inside

Radon comes from the natural decay of uranium in the ground. It moves up through the soil and, because the air inside a house is usually at a slightly lower pressure than the soil beneath it, that gas gets sucked through any opening it can find. Cracks in the concrete slab, gaps around plumbing pipes, and even tiny pores in the concrete itself can serve as entry points. In homes built on a slab foundation, the floor is the main barrier between the living space and the soil. If that barrier is not airtight, radon will find a way in.

The Core Idea Behind Sub-Slab Depressurization

Sub-slab depressurization is a straightforward concept: create a vacuum under the concrete slab so that soil gas is pulled away from the house rather than pushed into it. Instead of trying to seal every crack and hoping for the best, you actively reverse the pressure gradient. A radon fan, typically mounted on the exterior of the house or in an attic, pulls air from beneath the slab through one or more vacuum points. That air is then vented safely above the roofline, where it disperses.

What makes this approach so effective is that it does not rely on perfect air sealing. Even if the slab has cracks or the walls have gaps, the negative pressure under the slab keeps soil gas from migrating upward. In practice, I have seen homes with a dozen visible cracks in the slab that still tested below 2 picocuries per liter after a properly designed sub-slab depressurization system was installed. That is a fraction of the EPA action level of 4 pCi/L.

Sub-Slab Suction vs. Other Methods

Some mitigation techniques rely on sealing alone. You can caulk every crack, patch every hole, and install a thick radon barrier under a new slab. But sealing degrades over time. Concrete shrinks, the ground shifts, and new cracks appear. Sub-slab suction, which is essentially the same concept as sub-slab depressurization, adds a mechanical element that compensates for those imperfections. It is a more robust solution for existing homes, and it is the method recommended by the EPA for most slab-on-grade houses.

There is also a passive version called passive sub-slab depressurization, which relies on natural stack effect and wind to create a slight vacuum. It works reasonably well in cold climates where the temperature difference between inside and outside is large, but in a moderate climate like Missouri, passive systems often do not generate enough suction to keep radon levels low year-round. That is why active systems, with a dedicated radon fan, are the standard in this area.

What a Typical Installation Looks Like

In a retrofit situation, the first step is to locate the best vacuum point. That usually means drilling a hole through the concrete slab, often near a wall or in a utility closet, and checking the soil beneath. If the soil is compacted clay with low permeability, a single vacuum point may not be enough. In those cases, we install multiple vacuum points connected by a network of perforated pipe laid in a bed of gravel. The pipe collects soil gas from under the entire slab and directs it to the fan.

Polyethylene sheeting is often laid over the soil before the slab is poured in new construction, but in retrofits, we have to work with what is already there. If there is no gravel layer under the slab, we may need to create a small void by excavating under the point where the pipe enters. The pipe itself is usually three or four inches in diameter and made of PVC, with slots cut into it to allow gas to enter. It runs from the vacuum point to the radon fan, and then from the fan to a discharge point above the roofline.

The fan is typically a RadonAway model, chosen for its reliability and low noise. It runs continuously, drawing a small amount of electricity. A manometer, often a simple U-tube gauge, is installed on the pipe to show that the fan is maintaining proper suction. If the water column in the gauge drops to zero, it means the fan has failed or there is a blockage, and the homeowner knows to call for service.

Monitoring and Maintenance

After installation, the real test is to measure the indoor radon levels with a radon test kit. I always recommend a long-term test, at least 90 days, because radon levels fluctuate with weather and seasons. A short-term test gives you a snapshot, but a long-term test tells you the average exposure. If the system is working, the results should be well below the EPA action level.

Maintenance is minimal. The fan should be checked annually to make sure it is running, and the manometer should be read at the same time. If the water in the U-tube gauge is level on both sides, the fan is off. If one side is higher, the fan is pulling. That is all there is to it. The system does not need filters, does not produce waste, and does not require chemicals.

When Sub-Slab Depressurization Falls Short

No system is perfect for every home. If the soil under the slab is extremely tight clay with no gravel layer, it can be hard to create a large enough zone of influence with a single vacuum point. In those cases, we may need to install multiple points or combine sub-slab depressurization with other techniques like block wall suction for homes with hollow concrete block foundations. Sometimes the slab is too thin or too deteriorated to support the drilling without cracking further. In those rare cases, we might recommend a different approach, such as sealing the slab and installing a heat recovery ventilator to bring in fresh air.

But those are exceptions. For the vast majority of homes in Jefferson County, including Arnold, Festus, High Ridge, and Hillsboro, sub-slab depressurization is the most effective and cost-efficient way to reduce radon. It addresses the root cause, which is the pressure difference between the soil and the house, rather than just treating the symptoms.

Why Professional Installation Matters

I have seen homeowners try to install their own systems after watching a few videos. They drill a hole, drop in a pipe, and attach a fan. Sometimes it works, but often it does not. The fan may be undersized, the pipe may not be slotted properly, or the vacuum point may be in the wrong location. A professional radon mitigation contractor like Air Sense Environmental knows how to design the system for the specific soil conditions and foundation type. They also know how to seal the pipe penetration through the slab so that the system does not become a pathway for other soil gases or moisture.

In Missouri, radon is a real concern. The EPA has mapped much of the state as Zone 1, meaning predicted average indoor radon levels above 4 pCi/L. Homes in Jefferson County are no exception. Testing is the first step, and mitigation is the second. If the test shows elevated levels, do not wait. Sub-slab depressurization, when done right, is a permanent fix.

Final Thoughts

If you own a home with a concrete slab and you have tested for radon, or plan to, keep this method in mind. It is not a band-aid. It is a well-engineered solution that has been proven in thousands of homes across the country. The components are simple: a perforated pipe, a radon fan, a manometer, and some polyethylene sheeting for sealing. But the design and installation require experience. Find a qualified contractor who understands the local soil and construction practices. Your family will breathe easier, and that is worth the investment.