How sub-slab depressurization works
Your house is usually at slightly lower air pressure than the soil under it, especially in winter. That draws soil gas, including radon, in through any opening in the foundation. A sub-slab depressurization system uses a fan and pipe to make the soil under the slab lower in pressure than the house, so radon gets pulled into the pipe and exhausted outside.
The EPA says some radon reduction systems can lower levels by up to 99 percent. Results vary by home, which is why every system should be checked with a follow-up test.
The parts of a system
- Suction point: a cored hole through the basement slab with a small pit dug beneath it.
- Pipe: PVC pipe from the suction point up through the house or along the outside.
- Fan: mounted in the attic, garage or outside, never in the living space or basement.
- Sealing: airtight sump lid, sealed cracks and floor-wall gaps where reachable.
- Exhaust: above the roof edge, away from windows and doors.
- Manometer: a simple U-tube gauge so you can see the fan is pulling suction.
Designing for Red River Valley clay
Gravel under a slab lets suction spread easily. Dense lake-bed clay, common around Fargo and Moorhead, does not. A careful installer will check how well suction spreads (pressure field extension testing) before deciding on the design. Options include:
- A larger pit under the suction point
- Two or more suction points
- Using the perimeter drain tile and sump as the suction point
- A higher-suction fan suited to tight soils
Designing for Fargo winters
The air in a radon pipe is warm and humid soil gas. In sub-zero weather, that moisture condenses and can freeze. Common cold-climate practices include:
- Sloping horizontal pipe runs back toward the suction point so condensation drains away rather than collecting
- Routing pipe through the house and attic, or insulating exterior pipe runs
- Choosing fan locations and exhaust details that reduce frost build-up and ice falling from the roof edge
Ask any installer you're considering how they handle winter in their designs.
Crawl spaces and additions
Some Fargo-area homes have a crawl space under an addition or porch. These are usually treated with a sealed plastic membrane over the soil and a suction point beneath it, connected to the main system.
Minnesota requirements
For homes in Moorhead and Dilworth, the work must be done by an MDH-licensed radon mitigation professional, and systems installed since 2019 must have an MDH radon mitigation system tag.
After installation
Run a follow-up test between 24 hours and 30 days after the system starts. Check the manometer occasionally, especially after power outages and in extreme cold. Retest every two years.
Sources: U.S. EPA, Consumer's Guide to Radon Reduction; Minnesota Department of Health radon mitigation licensing; ANSI/AARST soil gas mitigation standards.