When it comes to radon mitigation systems, there are at least 10 common system configurations. However, any of these systems can be set up to fit into one of two categories, passive or active. All these systems consist of some sort of depressurization area, whether it is below a basement floor, plastic membrane in a crawlspace, or a sump pit. The difference is in how they operate.
Passive Systems
Passive Systems use a vent pipe that originates below a basement floor or crawlspace vapor barrier. This vent pipe travels through the conditioned space of the home, through the attic, and terminates above the roof. Note the graphic below.
This type of system relies on what is called the stack effect. It works similarly to convection currents. Ever see a hot air balloon? When heated the air inside the balloon becomes less dense than the air outside the balloon which allows it to float upwards. The stack effect in a home isn’t much different really. The higher you move inside of a home, the warmer the air will be. As the vent pipe travels up from below, it passes through warmer areas of the home, in turn warming the air in the pipe. This causes a slight upwards current within the pipe that can generate a negative pressure area where it originates below the house. Radon gas will flow to that area, up the pipe with the current, then out into the atmosphere.
The biggest drawback to a passive system is they aren’t very efficient, especially in northern climates like here in Colorado where it can get quite cold in the winter. Since the vent pipe travels through the attic space before traveling through the roof, the changing of the seasonal temperatures can have a positive or negative effect on the air current within the pipe. In the summer, the attic can get very hot, increasing the stack effect and convection currents. During the middle of the winter however, the attic can be as cold as the outside temperatures, thereby cooling the air in the pipe. When this happens, a phenomenon known as inversion can occur. Normally warm air will rise through the cooler air without a problem, but when the air in the top of the pipe is super cold, it becomes too dense for the warm air below to penetrate, any current in the pipe will stop, or can even reverse, trapping the Radon inside the pipe and below the home.
Active Systems
For the most part, the only difference between a passive system and an active system is active systems have a powered fan. In the United States, the fan must be located in the attic or mounted to the exterior of the home. This fan stays operational 24 hours a day, 365 days a year. Having a fan in the vent pipe creates a steady negative pressure under the home and air current through the pipe, rather than relying on temperature to generate it. In new construction, many builders now include a type of passive system with provisions like a power source in the attic for a fan should the home owner need or want to “upgrade” the system at a later date.
