Radon mitigation systems — how they work, what to expect, who does the work

Sub-slab depressurization (SSD) is the standard mitigation approach for nearly all US homes with basement or slab-on-grade construction. The diagnostic concept is simple: radon enters the home from soil gas through the basement slab, sump pits, drain tiles, crawl spaces, and any below-grade openings. SSD reverses the pressure differential that pulls soil gas in by creating negative pressure beneath the slab, redirecting soil gas through a sealed pipe that exhausts above the roofline rather than into the home.

The core components: a 3-4" PVC pipe penetrating the slab (usually in a closet, mechanical room, or utility area to minimize visual impact), a sealed connection at the slab penetration to prevent re-entry, the pipe routing through the home (commonly through an interior wall or attic) to above the roofline, an in-line radon fan rated for the soil-gas volume, and a U-tube manometer or digital pressure gauge that lets the homeowner verify the system is operating at appropriate negative pressure.

The system runs continuously. Modern radon fans are rated for 24/7 operation and typically last 10-15 years. The U-tube manometer is the homeowner-readable verification: when the system is operating correctly, the two columns of liquid show a clear pressure differential. If the columns equalize, the fan has failed and needs replacement.

The right system specification depends on multiple variables that a licensed mitigator evaluates during the on-site assessment:

Number of suction points. Most homes need one suction point in the basement slab. Larger homes, compartmentalized basements, and homes with attached additions may need two or three points connected to a single fan via a manifold. Walkout basements and split-level homes often need multiple points to maintain consistent negative pressure across the foundation footprint.

Fan size. Radon fans are sized by static pressure capacity (inches of water column) and airflow (CFM). Soil permeability under the home is the dominant factor: clay-rich soils allow less airflow but require more static pressure; sandy soils allow more airflow at lower pressure. A licensed mitigator does a permeability test before specifying the fan.

Pipe routing. The vent pipe must terminate above the roofline (above eaves) and at least 10 feet from any window, door, or air intake per EPA Radon Mitigation Standards. Interior routing through closets, mechanical rooms, or attics is generally preferred over exterior routing because exterior pipes face freeze-thaw cycles and ice loading in cold-climate markets.

Sealing scope. The slab-wall joint, sump pit covers, drain-tile loops, and any visible cracks in the slab need to be sealed during install. Soil gas leaks through these openings short-circuit the system and reduce its effectiveness. A thorough mitigator includes sealing in the install scope; a less-experienced contractor may skip steps that matter for long-term performance.

Post-mitigation testing is essential. Even a properly-designed system can underperform due to subtle factors (an unsealed sump pit, a hidden crack, a foundation pathway nobody noticed). The right verification protocol:

Short-term test 24-72 hours after fan activation. Closed-house conditions, basement or lowest-occupied level, away from windows. Properly-installed systems should produce results well below 2 pCi/L. If the result is still above 4 pCi/L, the licensed mitigator returns to diagnose — typically a sealing issue or a missed suction point.

Confirmation test at 30 days. Reduces the noise of one-time test variability and confirms the system performs consistently.

Long-term verification. After confirmation, retest every 2-5 years to verify ongoing performance. Replace the radon fan when the U-tube manometer readings indicate failure (columns equalize, indicating no pressure differential). Modern fans last 10-15 years; pipe and slab penetrations are essentially permanent.

Real-estate transactions. Most US states require radon disclosure on residential transactions. A documented mitigation system from a licensed contractor with post-install test records is what buyers and inspectors look for. Unlicensed installs create real-estate-disclosure problems at closing — a real cost that often exceeds the savings of cutting licensing corners.

A few scenarios call for design variations beyond standard SSD:

Crawl-space-only homes. Pure crawl-space homes need crawl-space encapsulation plus a depressurization system designed for the crawl. The encapsulation (continuous vapor barrier on floor + walls, sealed vents, dehumidifier) is a separate scope and a major component of the project. Hybrid homes with both basement and crawl-space sections need both treatments coordinated.

New construction with passive RRNC stacks. Many states require radon-resistant new construction (RRNC) since around 2010 — a passive vent stack pre-installed during construction. Passive stacks reduce radon but don't guarantee levels below 4 pCi/L. Test every new home; if results exceed the action level, a licensed mitigator activates the passive stack with an in-line fan rather than installing a full new system. Activation is faster and less expensive than standard SSD.

Walkout and split-level basements. Partial below-grade exposure changes the pressure dynamics and often requires multiple suction points. Mitigators experienced with the local housing stock specify accordingly.

Water intrusion combined with radon. SSD installed over an unaddressed water problem produces inconsistent results. The diagnostic order: address visible water sources (gutters, grading, perimeter drains) first, then mitigate. A licensed mitigator with experience can advise on whether your specific water issue blocks mitigation.

Elevated radon at upper floors. Rare but real. Indicates significant air movement from basement to upper floors that mitigation alone may not fully resolve. A combined approach (mitigation + air-sealing on upper floors) produces better outcomes.