Understanding Sub-Slab Depressurization for Radon Mitigation

What Homeowners in West St. Louis County Should Know

If you live in Chesterfield, Ballwin, Wildwood, or Town and Country, you have probably heard about radon. It is a naturally occurring radioactive gas that comes from the soil beneath your home. The health risks are real, and testing is the first step. But once a radon test shows elevated levels, the real question becomes: what do you do about it? The answer for most homes is sub-slab depressurization. It is the most effective radon mitigation method used by professionals today.

Let me walk through how it works, why it matters, and what goes into a proper installation. This is not theory. This is what I see every day in basements and crawl spaces across the county.

How Radon Gets Into Your Home

Radon enters through the foundation slab. It moves through soil gas, which is the air and vapor that travels through the ground. The gas finds every tiny crack, every gap around plumbing pipes, and every spot where the concrete meets the floor drain. It can also seep through the slab itself if the concrete is porous. In homes with a crawl space, radon can come through the dirt floor unless it is sealed. Radon entry routes are not always obvious, which is why a single radon test might not tell the whole story. A continuous radon monitor gives a better picture because it tracks changes over days and weeks.

The Principle Behind Sub-Slab Depressurization

Sub-slab depressurization is a type of Active Soil Depressurization system. The idea is simple: create a vacuum under the slab that pulls soil gas out before it can enter the living space. A PVC pipe is inserted through the slab and connected to a Radon fan mounted outside or in an attic. The fan runs continuously, drawing air from beneath the concrete and venting it safely above the roof. This lowers the pressure under the slab, so the gas naturally flows toward the pipe instead of upward into the house.

It is a quiet system when installed correctly. You might not even know it is running. But the results are dramatic. I have seen homes drop from 10 picocuries per liter down to below 1. That is the difference between a serious health risk and safe indoor air quality.

Key Components of a Good System

A well-built sub-slab depressurization system depends on several parts working together. Here are the main ones you will see in a typical installation:

• Suction pit: A small hole dug under the slab at the pipe location. It creates a cavity where air can collect and be drawn into the pipe.
• Radon sump: Sometimes a plastic sump is placed in the pit to prevent soil from clogging the pipe. It acts like a well point.
• Gravel layer: Most builders in this area pour a 4- to 6-inch gravel layer under the slab. That gravel is critical because it lets air move freely under the concrete. Without it, the system may not work well.
• U-tube manometer: A simple gauge that shows the pressure difference between the pipe and the room. It tells you the fan is working and the system is holding vacuum.

If your home has a crawl space instead of a basement, the same principle applies but the setup changes. Sub-membrane depressurization uses a heavy plastic sheet laid over the dirt floor. The fan pulls air from under the sheet instead of under concrete. It is equally effective for that type of foundation.

What Makes a System Fail

Not every sub-slab depressurization installation works perfectly. The most common problem is a missing or disturbed gravel layer. If the builder poured concrete directly on clay or silty soil, there is no air path. The suction pit cannot pull air from under the whole slab. The system will only affect the area right around the pipe. In those cases, we may need to install multiple suction points or use a different approach altogether.

Another issue is improper fan sizing. A Radon fan that is too small will not pull enough suction. One that is too large can make noise and waste electricity. The right fan depends on the size of the slab, the soil type, and the number of suction pits. This is not a one-size-fits-all situation.

I also see systems where the PVC pipe is not sealed properly at the slab penetration. An air leak there kills the vacuum and lets soil gas escape into the basement. A little caulk or expanding foam can prevent that, but it is often overlooked.

The Role of Radon Resistant Construction

New homes in West St. Louis County are built under a building code that requires radon resistant construction. This includes a gravel layer, a vapor barrier under the slab, and a rough-in for a future radon mitigation system. The rough-in is usually a capped PVC pipe that runs from under the slab up through the attic. If a radon test later shows high levels, the contractor can install a fan and vent without cutting into the concrete.

That is a smart approach. It costs very little during construction but saves thousands later. I have tested homes that were built with radon resistant features, and they often have lower baseline levels even without an active system. But I have also seen homes where the builder skipped the gravel layer or used a thin vapor barrier that tore during pouring. The code is only as good as the inspection.

When Sub-Membrane Depressurization Is the Answer

Homes with crawl spaces present a different challenge. There is no slab to pull air from. Instead, we use sub-membrane depressurization. A thick plastic membrane is laid over the dirt floor and sealed to the foundation walls. A PVC pipe goes through the membrane and connects to a fan that vents outside. The membrane traps the soil gas, and the fan pulls it away. It is similar to sub-slab depressurization in concept but much easier to retrofit because you do not have to cut through concrete.

One thing to watch for: the membrane must be sealed along every seam and around every support post. A single gap can let radon pour in. I always use a continuous radon monitor after installation to confirm the system is working before I leave the site.

Testing and Verification

A radon mitigation system is not finished until you verify it works. I always run a radon test after installation, usually with a continuous radon monitor for at least 48 hours. The monitor gives a detailed picture of how the levels change hour by hour. If the system is working, you should see a steady drop. If levels stay high, I check the U-tube manometer to confirm the fan is running and the suction is holding. Sometimes the problem is as simple as a tripped breaker or a disconnected wire.

Long-term testing is also important. Radon levels can vary with weather, seasons, and changes in the foundation. A yearly test is a good habit. If you ever see a spike, call a professional to check the system.

Cost and Practical Considerations

Sub-slab depressurization systems cost between $1,500 and $3,000 in this area, depending on the complexity. Homes with a gravel layer and a rough-in are cheaper because the work is straightforward. Homes with no gravel may require multiple suction pits, which drives up the price. French drains or other drainage systems can sometimes be used to improve air movement under the slab, but that is a more involved project.

The ongoing cost is low. A Radon fan uses about as much electricity as a 20-watt light bulb. The fan may need replacement after 10 to 15 years, but that is a simple swap. The PVC pipe and manometer require no maintenance. Just make sure the vent pipe is not blocked by snow or debris.

Why It Matters

Radon is the second leading cause of lung cancer after smoking. The health risks are cumulative, meaning long-term exposure is the danger. A properly installed sub-slab depressurization system can reduce your risk to near zero. It is a proven technology, backed by decades of research and real-world use. If your home tests high, do not ignore it. The fix is straightforward, and the peace of mind is worth it.

I have been doing this work for years, and I still get calls from people who put off mitigation because they thought it would be disruptive or expensive. Most of the time, the job takes a day and the homeowner barely notices. The only reminder is the quiet hum of the fan and the steady reading on the manometer. That is a small price for safe indoor air quality.