How Sub-Slab Depressurization Works for Radon Mitigation in Jefferson County Homes

Living in Jefferson County, Missouri, means enjoying the rolling hills and quiet communities like Arnold, Festus, High Ridge, and Hillsboro. But it also means living with a hidden risk that comes from the ground beneath your home: radon gas. Over the years, I have walked into dozens of basements in this area, set up test kits, and watched the numbers come back higher than anyone wants to see. The good news is that radon is a problem with a proven solution. The most effective approach for most homes here is a technique called sub-slab depressurization.

I have been doing this work long enough to know that the best mitigation system is the one that fits the house. But when a home has a concrete slab on grade, or a basement slab, sub-slab depressurization is almost always the right starting point. It is not a one-size-fits-all magic fix, but when done right, it reliably drops radon levels below the EPA action guideline. Let me walk you through how it works, what the key pieces are, and what you should look for if you are having a system installed.

The Basic Idea Behind Sub-Slab Depressurization

Radon comes from the natural decay of uranium in the soil and rock. It moves up through the ground, and when it finds a path under your home, it seeps in through cracks in the slab, gaps around pipes, or even through the pores of concrete itself. Sub-slab depressurization creates a low-pressure zone under the slab, so instead of that soil gas being pulled up into your living space, it is drawn toward a vacuum point and vented safely outside.

Think of it like a straw that runs from beneath your basement floor up through your roof. A small radon fan creates continuous suction, pulling the gas from under the slab and discharging it above the roofline, well away from windows and doors. The pressure inside your home stays higher than the pressure under the slab, so the natural movement of air pushes radon away from your family.

Key Components of a Sub-Slab System

Every sub-slab depressurization system shares a few critical parts. When I inspect an existing system or design a new one, I check each of these carefully.

The Vacuum Point and Perforated Pipe

A hole is cut through the concrete slab, typically four inches in diameter. Into that hole, a length of perforated pipe is placed, extending horizontally into the gravel or soil beneath the slab. That pipe collects the soil gas from a wide area. The rest of the pipe running up to the fan is solid PVC, not perforated, because you do not want suction leaking out before it reaches the fan. The location of the vacuum point matters a lot. It needs to be placed where the gravel layer is deepest and most continuous, and where it can pull from the largest footprint of the slab.

Polyethylene Sheeting and Air Sealing

Before the concrete was poured, many homes built after the 1990s had a layer of polyethylene sheeting placed under the slab as a vapor barrier and radon barrier. If that sheeting is intact, it helps the depressurization system work more efficiently by preventing the fan from pulling air from above the slab. But even with a good barrier, air sealing around the edges of the slab and through any sump pump openings is essential. In Jefferson County, I have seen older homes with no barrier at all, and in those cases the system still works, but the fan may need to run a little harder.

The Radon Fan and Vent Pipe

The radon fan is the heart of the system. Brands like RadonAway make reliable units that are designed to run continuously for years. The fan is usually installed in the attic or on the exterior of the home, pulling air from under the slab and pushing it up through a vent pipe that extends above the roofline. The fan must be sized correctly for the square footage of the slab and the resistance of the pipe run. A fan that is too small will not create enough suction. One that is too large can waste energy and create unnecessary noise.

The Manometer or U-Tube Gauge

Every system should have a manometer, often a simple U-tube gauge, installed on the vent pipe near the fan. This little device shows you whether the fan is actually moving air and maintaining suction. When the fan is running, the fluid in the gauge sits at different levels on each side of the U, indicating a pressure difference. If the levels are equal, the fan is either off or the system has a leak. I always show homeowners how to read their manometer so they can check their system at a glance.

Active Versus Passive Sub-Slab Depressurization

There are two main types of sub-slab depressurization: active and passive. In a Passive Sub-Slab Depressurization system, there is no fan. The vent pipe relies on natural stack effect and wind passing over the roof to draw air up. These systems are sometimes installed in new construction to meet code, but they rarely provide enough suction to bring high radon levels down to safe numbers. Most homes in Jefferson County need an Active Soil Depressurization system, which uses a fan to create continuous, reliable suction.

I have tested homes where a passive system was installed during construction, and the radon levels were still above four picocuries per liter. Adding a fan to that existing pipe turned it into an effective active system. If you are building a new home, installing a passive stub with an electrical outlet nearby is a smart move. You can always add the fan later if testing shows you need it.

How the System Is Tested

After a sub-slab depressurization system is installed, the real test begins. I place a radon test kit in the lowest livable level of the home, usually the basement, and run it for at least 48 hours. The EPA recommends action if the long-term average is above four picocuries per liter, and my goal is to get every home below that number, ideally well below it. I have seen systems drop radon levels from over 20 pCi/L down to under one pCi/L. That is the kind of result that makes this work rewarding.

One thing I always tell homeowners is that a single test after installation is not the end of the story. Radon levels can change with the seasons, with weather, and with changes to the home's foundation. I recommend testing every two years, or after any major renovation that involves the slab or foundation. The manometer will tell you if the fan is running, but only a test kit will tell you if the radon is still low.

Common Mistakes and Trade-Offs

Not every sub-slab depressurization system works perfectly on the first try. I have seen systems where the vacuum point was placed too close to the edge of the slab, pulling outside air instead of soil gas. I have seen perforated pipe that was crushed during the concrete pour, rendering the system useless. And I have seen fans that were undersized for the pipe run, creating barely any measurable suction. That is why a good installer does not just bolt on the hardware and leave. They measure the pressure field extension by drilling small test holes and checking how far the suction reaches.

The biggest trade-off is between cost and completeness. A basic system with a single vacuum point and a small fan might cost less upfront, but it may not cover the entire slab if the house is large or the foundation is complicated. Adding a second vacuum point or a larger fan costs more, but it can make the difference between a system that sort of works and one that reliably protects your family. In Jefferson County, where homes vary from 1950s ranches to new custom builds, I always recommend spending the extra time on the diagnostic phase before cutting any concrete.

When Sub-Slab Depressurization Is Not Enough

There are a few situations where sub-slab depressurization alone may not bring radon levels down. If the home has a crawl space instead of a full basement, or if the slab is badly cracked and the gravel layer is missing, you might need to combine it with other measures like sealing the crawl space or installing a membrane. In homes with high radon levels above ten pCi/L, a combination of sub-slab suction and increased air sealing around the rim joist often does the trick. I have also seen homes where the radon source is not just the soil but also the well water, and in those cases a separate water treatment system is needed.

The key is to treat radon mitigation as a process, not a product. You cannot just buy a fan and pipe and expect it to work. You need someone who understands how air moves through soil, how pressure differences drive gas entry, and how to troubleshoot when the numbers do not drop. That is why I always start with a thorough radon test, followed by a visual inspection of the foundation and slab. Only then do I design the sub-slab depressurization system for that specific house.

Finding the Right Help in Jefferson County

If you live in Jefferson County and are concerned about radon, the first step is simple: get a test. You can buy a radon test kit at most hardware stores or order one online. If the result comes back above four pCi/L, call a professional who specializes in radon mitigation. Look for someone who carries liability insurance, uses quality components like RadonAway fans, and offers a post-installation test to verify the work. Companies like Air Sense Environmental have been serving this area for years and know the local soil conditions and construction styles.

Radon is a serious health risk, but it is also a fixable one. Sub-slab depressurization is the most reliable tool we have for homes with concrete slabs, and it has helped thousands of families across Missouri breathe easier. If you take the time to understand how it works and what to look for, you can make an informed decision about protecting your home. That is the kind of knowledge that turns a scary test result into a straightforward project with a measurable outcome.