Radon Mitigation and Soil Gas: Diagnostics, SSD Design, and Verification
Mitigation and soil gas questions cover diagnostics, sub-slab depressurization (SSD) design, sealing, vent termination, and post-mitigation verification under ANSI/AARST SGM-SF.
Diagnostics and design
- Run a pressure field extension (PFE) or communication test with a micromanometer and temporary test holes. Adequate extension is typically about 10 ft from the suction pit; if suction does not reach, add a suction point or improve the pit.
- SSD uses a fan to draw soil gas from beneath the slab and exhaust it outdoors. Seal slab cracks, control joints, sump lids, and pipe penetrations so the system pulls soil gas, not room air.
- Size the fan from measured pit vacuum and flow; use Schedule 40 PVC and slope horizontal runs to drain back to the pit.
Venting and verification
- Terminate the vent above the roofline/eave and away from windows, doors, and air intakes; follow manufacturer and local code.
- Label the system, provide an owner's manual, and verify with an independent post-mitigation test no sooner than 24 hours after start-up and within 30 days.
Worked example: A 1,500 ft² slab-on-grade home has one suction pit. PFE testing at 0.5 in. w.c. shows suction only 3 ft from the pit. That is inadequate for the slab, so the installer adds a second suction point about 12 ft away, seals the new penetration, and re-tests. Post-mitigation radon is 1.8 pCi/L.
Traps: assuming a fan alone is mitigation without sealing; testing immediately after start-up; terminating the vent under a window; skipping PFE; failing to label.