Most homeowners in the Carolinas have heard that radon comes from the ground. Fewer know why it ends up inside the house instead of simply drifting off into the open air. The answer comes down to air pressure. Understanding the pressure relationship between your home and the soil beneath it explains how radon gets in, why a properly designed radon mitigation system works, and why testing is the only way to know what’s actually in the air you breathe.
What Causes Radon to Move From the Soil Into Your Home?
Radon is a radioactive gas that forms as uranium naturally breaks down in soil and rock. Parts of North and South Carolina, especially areas with granite and other uranium-bearing rock, produce enough radon to create elevated levels indoors.
Radon doesn’t push its way into a house on its own. The driving force is a pressure differential. When the air pressure inside your home is even slightly lower than the pressure in the soil around and beneath the foundation, soil gas is drawn toward the house and flows in through any opening it can find. Radon travels along with that soil gas. The bigger and more persistent that pressure difference, the more soil gas your home can pull in.
Why Is the Air Pressure in Your Home Lower Than the Soil?
Homes lose air all the time, and the air that leaves has to be replaced. Some of that replacement air comes from the soil. Several everyday factors contribute to lower pressure at the bottom of a house:
- The stack effect: Warm indoor air rises and escapes through the upper parts of the home, which lowers pressure near the foundation. This is usually strongest in cooler weather when the heat is on.
- Exhaust appliances: Clothes dryers, bathroom fans, kitchen range hoods, and fuel-burning appliances all push air out of the house.
- HVAC leaks: Leaky return ducts, especially in crawl spaces, can pull air from below the living space.
- Wind: Wind blowing against one side of a home can lower pressure on the other side and change how air moves through the building.
Because these factors shift with the seasons, the weather, and how you use your home, radon entry can change over time too. That’s one reason results can vary, which we discussed in our post on whether time of year affects radon testing results.
Can Air Pressure Tell You Where Radon Will Build Up in Your Home?
No. This is one of the most common misunderstandings about radon. Pressure differences explain why soil gas is drawn into a home, but they don’t predict how much radon is present or where it will accumulate. The amount of radon in the soil, the paths it takes through the foundation, and how air moves inside the house all vary from one property to the next.
The only way to know your radon levels is to measure them with proper testing devices. Professional radon testing gives you a reliable result for the space being tested, and continuous radon monitoring shows how levels change over hours, days, and seasons. If you’re deciding between a home test kit and a certified professional, Tennessee Radon Services explains the differences in their post on DIY vs. professional radon testing.
How Does Radon Mitigation Use Pressure to Keep Radon Out?
An active radon mitigation system works by reversing the pressure relationship. Instead of your home being at lower pressure than the soil, the system makes the area beneath your slab, or beneath a sealed membrane in a crawl space, lower in pressure than your living space.
To do that, a radon fan connected to one or more suction points pulls soil gas from beneath the foundation and vents it safely above the roofline. When that low-pressure zone extends across the footprint of the home, soil gas is pulled into the system instead of into your house. Sealing major openings, such as sump pits and large cracks, helps the system hold that pressure field.
Getting this right depends on the building. Soil conditions, foundation type, and how air moves beneath the slab all affect how a system should be designed. That’s why True Air Technologies relies on diagnostics and fact-based design instead of a one-size-fits-all installation, and why we confirm results with testing after the system is running.
Does Air Sealing Your Home Affect Radon Levels?
It can, in either direction. Sealing gaps between the soil and your living space, such as openings around plumbing or in a crawl space, can reduce the pathways radon uses. On the other hand, tightening up a home without addressing radon can sometimes reduce the fresh air that dilutes it, or change pressure relationships inside the house.
That’s why it’s important to test before and after major energy upgrades, and to think about radon whenever you change how your home handles air. Our posts on air sealing a new build and our crawl space encapsulation services explain how sealing and radon control can work together.
Can Opening Windows Lower the Radon Level in Your Home?
Opening windows can temporarily lower radon levels, but it isn’t a reliable long-term solution. Fresh outdoor air dilutes indoor radon, and opening windows on the lowest level may also reduce the pressure difference that draws soil gas into the house. However, the effect stops as soon as the windows are closed, and leaving windows open isn’t practical during hot, humid Carolina summers or chilly winter nights. You should also avoid opening windows during a radon test, since it can make results inaccurate.
Do Crawl Space Homes Have the Same Pressure Issues as Homes With Slabs or Basements?
Yes. Every home that sits on soil can be affected by pressure differences, whether it’s built on a slab, a basement, or a crawl space. In crawl space homes, soil gas can collect in the crawl space and then be drawn into the living area through gaps in the subfloor or leaky ductwork. In many cases, the solution is a sealed membrane over the crawl space floor with active suction beneath it. Testing the living space is still the only way to know whether radon levels are elevated.
Does a Tightly Built New Home Need Radon Protection?
It’s a good idea to plan for it. Tightly built homes use less energy, but they can still have pressure differences that draw soil gas from below. Building radon-resistant features into new construction, such as a gas-permeable layer beneath the slab, a vapor barrier, sealed penetrations, and a vent pipe that can be connected to a fan later, makes it much easier to reduce radon if testing shows it’s needed. Once the home is complete, a radon test confirms whether further action is necessary.
Why Can Radon Levels Change After You Replace Your HVAC System?
A new HVAC system can change the way air moves through your home and, with it, the pressure relationship between your house and the soil. Changes in ductwork, return locations, fresh air intakes, or system size can increase or decrease how much soil gas is drawn indoors. If you’ve recently replaced or modified your heating and cooling system, retesting is the best way to see whether your radon levels have changed. If you already have a mitigation system, retesting also confirms it’s still performing as expected.
How Do You Know if Your Radon Mitigation System Is Still Keeping Radon Out?
Retesting is the best way to know. Most systems have a manometer on the vent pipe that shows the fan is creating suction, which is a helpful quick check, but it doesn’t measure radon. Periodic radon testing or continuous monitoring confirms that levels in your home remain low. If the manometer reading changes, the fan sounds different, or a test result comes back higher than before, schedule a system inspection so the cause can be identified and corrected.
True Air Technologies was founded by Merritt Gantt, who has taught radon mitigation as an instructor at Kansas State University since 2023 and has designed systems for homes across the Carolinas. If you have questions about radon in your home, or an existing system that needs attention through our radon system repair services, we’re here to help. Environmental Doctor also offers a helpful look at why radon testing results vary house by house. Find out whether we serve your community on our service area page, or contact True Air Technologies to get started.