A radon mitigation system prevents radon from building up inside a home by drawing soil gas from beneath the foundation and safely exhausting it outdoors.
The most common and reliable method is called active sub-slab depressurization, sometimes shortened to SSD. A quiet, continuously operating radon fan creates lower pressure beneath the basement floor than inside the home. Instead of being drawn through cracks, sumps, joints, and other openings, the radon is captured below the slab and directed outside through a sealed pipe.
Health Canada identifies active sub-slab depressurization as the most effective and reliable radon-reduction technique and the method most commonly used by C-NRPP-certified professionals.
Why does radon enter a basement?
The air pressure inside a home is often lower than the pressure in the soil surrounding its foundation. During colder weather, warm air rises through the home and escapes through the upper levels. This can increase the suction at the basement level—an effect commonly called the stack effect.
Radon-containing soil gas can then enter through openings such as:
- Cracks in the basement floor
- The joint between the foundation wall and floor
- Open or poorly sealed sump pits
- Floor drains and service covers
- Gaps around plumbing and utility penetrations
- Accessible openings beneath tubs or showers
- Exposed soil or gravel in crawlspaces
Because radon can pass through very small and sometimes hidden openings, sealing alone usually cannot control a significantly elevated level. An active system addresses the pressure difference that is continually drawing soil gas toward the home.
How does sub-slab depressurization change that pressure?
A suction pipe is connected to the material beneath the concrete slab. A radon fan runs continuously and pulls air from beneath the foundation, creating a controlled area of negative pressure.
That changes the direction in which the soil gas moves. Rather than entering the basement, it travels into the suction system and is discharged outdoors, where it quickly dilutes.
The system is not simply “ventilating the basement.” It is intercepting radon beneath the home before it enters the living space.
How does ATMOS determine what system a home needs?
Every foundation behaves differently. Soil type, slab levels, footings, additions, foundation size, rough-ins, sumps, and crawlspaces can all affect how air moves beneath a home.
ATMOS uses Pressure Field Extension testing, also called communication testing, to evaluate that movement. Small temporary test holes are made at selected locations in the slab. Diagnostic suction is applied at one area while pressure is measured elsewhere beneath the floor.
The results help determine:
- Whether suction can communicate across the basement footprint
- Where an effective suction point can be located
- Whether one or more suction points may be required
- Whether the soil requires greater airflow or greater suction
- Which radon fan is appropriate for the home
The small diagnostic openings are sealed afterward. In some finished areas, testing can be completed beneath a carefully lifted edge of carpet and restored when finished.
This diagnostic process means the system is selected using measurements from the home—not by assuming that the same fan and design will work everywhere. Health Canada specifically identifies Pressure Field Extension testing as a method used to estimate the number of suction points and fan performance needed for an active system.
What happens during a typical installation?
ATMOS begins by protecting the floors and organizing the work area. If the home has a suitable radon rough-in, it is inspected and tested to determine whether it can be used. If the rough-in is blocked, poorly positioned, or unable to create adequate pressure beneath the slab, a new suction point is installed in a more effective location.
The radon pipe and fan are then installed and routed to an appropriate exterior discharge location. In Calgary homes, this often means keeping the fan and most of the system indoors and exiting through the rim-joist area. This arrangement helps protect the fan and piping from cold-weather condensation and icing.
The exterior discharge is positioned with the required clearances from windows, doors, air intakes, and utility equipment. The system is securely supported, sealed, powered, and tested before cleanup.
A standard installation generally takes approximately five to eight hours, depending on the foundation, access, sealing requirements, and pipe route.
How does ATMOS choose the suction point and pipe route?
The mechanical room is usually preferred because it keeps the equipment away from the main living area and often provides a practical route to the exterior. When that is not possible, ATMOS may recommend a closet or another low-use area of the basement.
The final design balances several considerations:
- Effective pressure beneath the entire slab
- A clean and direct pipe route
- Required exterior clearances
- Low sound and vibration
- Secure long-term support
- Access for future inspection or fan replacement
- Minimal impact on the home’s appearance and usable space
Performance comes first, but the system should also look deliberate. ATMOS secures and aligns the piping carefully and places the equipment where it will be as unobtrusive as the home allows.
Why does sealing matter?
Sealing accessible cracks, floor-to-wall joints, sump covers, and penetrations supports the depressurization system in two ways.
First, it closes direct pathways through which radon-containing soil gas can enter. Second, it reduces the amount of conditioned basement air that can be pulled downward through the openings and into the system. That helps the fan create a stronger and more efficient pressure field beneath the slab while limiting unnecessary heating or cooling loss.
Health Canada describes sealing as an important part of radon reduction but not, by itself, a dependable standalone method. It is difficult to locate and permanently seal every possible opening, especially in a finished basement.
What if the home already has a radon rough-in?
Many newer Calgary homes have a capped radon rough-in beneath the slab. When the pipe is clear, properly located, and able to communicate with the sub-slab material, it can become the suction point for the active system. This avoids creating a new opening through the basement floor and can reduce the installation cost.
However, a rough-in is not automatically usable. Some are blocked, embedded in poor sub-slab material, or positioned where they cannot establish adequate pressure across the foundation. ATMOS tests the rough-in before designing around it. If it does not perform properly, a new suction point is created in a more suitable location.
When might a home need a different system design?
Multiple suction points
A second suction point may be required when the basement has more than one slab level, a very large footprint, separated foundation sections, obstructing footings, or sub-slab soil through which air does not move easily.
A different fan
Radon fans are designed for different combinations of airflow and suction. Free-moving gravel may call for a different fan than compact soil in an older home. A larger fan is not automatically better; the fan must match the resistance and airflow measured beneath the foundation.
Sump-hole depressurization
In an appropriate home, a sump can be fitted with an airtight, serviceable cover and used as the suction location. Health Canada recognizes sump-hole depressurization as a variation of active sub-slab depressurization. ATMOS considers it when the sump and foundation design make it a practical option.
Crawlspace encapsulation
A crawlspace with exposed soil or gravel has no concrete slab beneath which to create suction. Instead, a durable membrane is installed over the exposed ground and sealed to the walls, seams, columns, and penetrations. Suction is then applied beneath the membrane.
This is called active sub-membrane depressurization. It is effective but considerably more labour- and material-intensive than a standard sub-slab installation and may take two or three days to complete.
What will the homeowner see after installation?
A completed system normally includes:
- A sealed suction point or connection to a usable rough-in
- Radon piping routed through the basement
- A continuously operating radon fan
- A manometer mounted on the pipe
- A labelled exterior discharge
The manometer shows that the fan is creating suction. It does not measure the home’s radon level. Homeowners should occasionally look at the manometer and confirm that the fan is operating, then use follow-up radon testing to verify the actual reduction.
Are radon systems loud or intrusive?
A properly selected and installed system should be quiet. The fans ATMOS commonly installs operate at approximately 25 to 30 decibels, although the sound heard in the home depends on fan selection, pipe routing, mounting, and the surrounding room.
The physical footprint is also small. The system generally occupies a narrow vertical area in a mechanical room, closet, or another low-use location. Careful routing and secure supports keep the pipe orderly and prevent avoidable vibration.
How much maintenance does the system require?
Radon systems are low-maintenance. The fan is designed to run continuously—24 hours a day, 365 days a year—and uses relatively little electricity.
Homeowners should:
- Check the manometer occasionally to confirm that the system is creating suction
- Listen for any unusual change in fan sound
- Contact ATMOS if the fan stops or the piping or seals appear damaged
- Continue testing the home to confirm long-term performance
ATMOS provides a five-year manufacturer warranty on the fan. The piping and installation are protected by a 10-year workmanship warranty, excluding damage caused by outside forces such as impact, unauthorized alteration, or structural movement of the home.
Health Canada recommends an initial follow-up test after activation and continued long-term testing. Homes with active mitigation systems should be retested periodically to confirm that the system continues to perform.
A proven way to reduce radon
Radon mitigation systems are not unusually loud, bulky, or experimental. Active soil depressurization has been installed in homes across Canada and North America for many years and is the most established way to achieve a large, reliable reduction.
The essential difference is proper design. The system should be based on diagnostic testing, matched to the foundation, installed with appropriate materials and clearances, sealed carefully, and verified when complete.
ATMOS guarantees that the systems we install will reduce the home’s radon level below 75 Bq/m³.
Sources
- Health Canada: Radon Reduction Guide for Canadians
- Health Canada: Guide for Radon Measurements in Homes