Published: March 6, 2026 | Home Inspector New Ulm
Minnesota Radon Mitigation System Inspection Guide
Southern Minnesota sits in one of the highest radon zones in the country. The geology beneath New Ulm, Mankato, St. Peter, and surrounding communities produces naturally occurring radon gas that can accumulate in homes to dangerous levels. Many homes in our area already have radon mitigation systems installed, either by previous owners who tested high or as part of new construction requirements. But simply having a system installed does not guarantee your family is protected. The system needs to be properly designed, correctly installed, and actively maintained.
Whether you are buying a home with an existing mitigation system or maintaining one in your current home, understanding how these systems work and how to verify they are performing correctly is essential. During our radon testing and home inspection services, we evaluate radon mitigation systems and test radon levels to ensure your home is safe.
How Radon Mitigation Systems Work
The most common type of radon mitigation system in Minnesota homes is an active sub-slab depressurization system, often abbreviated as ASD. This system consists of a pipe that penetrates the basement floor slab, connects to a continuously running fan, and exhausts above the roofline. The fan creates negative pressure beneath the slab, drawing radon gas from the soil before it can enter the home and venting it safely into the outdoor air where it disperses harmlessly.
The system relies on creating a pressure difference. The area beneath your basement slab must have lower pressure than the air inside your basement. This prevents radon from being drawn into the home through cracks, joints, sump pits, and other openings in the slab. When the system is working properly, it typically reduces indoor radon levels by 80 to 99 percent.
Some homes have passive radon systems, which include the pipe but no fan. These are commonly installed during new construction as a precaution. A passive system relies on natural air convection rather than a fan to draw radon upward through the pipe. Passive systems are less effective than active systems and may not reduce radon levels sufficiently. If radon testing shows levels at or above 4 pCi/L with a passive system, adding a fan to convert it to an active system is straightforward and cost-effective.
Inspecting the System Components
A radon mitigation system has several components that should be checked regularly. The suction point is where the pipe penetrates the basement slab. The area around this penetration should be sealed with caulk or polyurethane sealant to prevent soil gas from entering the basement around the pipe. If the seal has cracked or separated, the system's effectiveness is reduced.
The piping should be PVC schedule 40, typically 3 or 4 inches in diameter. It should run from the sub-slab suction point upward through the home and exit through the roof or an exterior wall. All joints should be properly glued, and the pipe should be supported at appropriate intervals to prevent sagging or separation. The pipe should be labeled as a radon vent pipe so future homeowners and service professionals can identify its purpose.
The radon fan is the heart of the system. It runs continuously, 24 hours a day, 365 days a year. Located in the attic, on the exterior of the home, or in the garage, the fan should never be installed in the living space because a leak at the fan could release concentrated radon into your home. The fan should be wired to a dedicated electrical circuit or plugged into a GFCI-protected outlet. Listen for unusual noise or vibration, which can indicate bearing failure.
Every properly installed system includes a manometer or pressure indicator. This is typically a small U-tube filled with colored fluid mounted on the radon pipe. When the system is operating, the fluid levels in the two sides of the U-tube will be unequal, indicating that the fan is creating suction. If the fluid levels are equal, the fan is not working and the system is not protecting you. Check this indicator monthly as part of your routine home maintenance.
Common Problems We Find
During home inspections in New Ulm and Mankato, we encounter several recurring issues with radon mitigation systems. Failed fans are the most critical. Radon fans have an average lifespan of 5 to 10 years, and when they fail, radon levels can return to pre-mitigation levels within 24 hours. Homeowners who do not regularly check the manometer may not realize their system has stopped working.
Improper discharge locations are another common problem. The exhaust pipe must terminate above the roofline and at least 10 feet from any window, door, or other opening into the home. We sometimes find systems that exhaust at ground level, near basement windows, or into enclosed areas where the radon-laden air can re-enter the home or accumulate in dangerous concentrations.
Unsealed slab penetrations undermine the system's effectiveness. The sub-slab depressurization only works if the area beneath the slab is isolated from the indoor air. Open sump pits, uncapped drain tiles, cracks in the slab, and gaps around plumbing penetrations all allow indoor air to short-circuit the system, reducing the pressure difference that keeps radon out. All openings should be sealed with appropriate materials.
Testing Is Still Essential
Even with a mitigation system in place, regular radon testing is essential. The EPA and the Minnesota Department of Health recommend testing every two years to verify that the system continues to maintain radon levels below 4 pCi/L. Testing should also be done after any system maintenance or modification, after significant home renovations that could affect the building envelope, and after any change in HVAC systems that could alter the home's pressure dynamics.
When buying a home with a mitigation system, do not assume the system is working just because it is installed. We recommend radon testing as part of every home purchase in southern Minnesota, regardless of whether a mitigation system is present. The test confirms actual radon levels in the home under current conditions, which is the only way to know if you are truly protected.
For homeowners in Brown County and the surrounding area, radon awareness and mitigation system maintenance are ongoing responsibilities. The system that was installed five or ten years ago may need a new fan, resealing, or adjustment to continue providing effective protection. A professional evaluation gives you peace of mind that the invisible threat of radon is being properly managed in your home.
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