Radon Testing, Mitigation Systems, and Sub-Slab Depressurization

How Radon Enters Your Home

Radon gas migrates through soil and enters buildings through any opening that contacts the ground: cracks in foundation slabs, gaps around pipes and wires, construction joints between the floor and wall, sump pump openings, and porous concrete block walls. The driving force is air pressure — your home typically has slightly lower air pressure than the surrounding soil (from exhaust fans, furnace combustion, the stack effect), which draws soil gas inward.

Every home is different. Your neighbor's radon level tells you nothing about yours. Geology, soil permeability, foundation construction, and air handling characteristics all affect the level. The only way to know is to test your specific house.

Testing

Short-term tests (2 to 7 days) use activated charcoal canisters or alpha-track detectors placed in the lowest livable area of the home (usually the basement). They give a snapshot of radon levels during the test period. Hardware store kits cost $15 to $30 and include lab analysis. Place the test device according to the instructions — closed-house conditions (windows and doors closed except for normal entry and exit) for at least 12 hours before and during the test.

Long-term tests (90+ days) give a more accurate picture because radon levels fluctuate with weather, season, and soil conditions. An alpha-track detector left in place for 3 to 12 months averages out these variations. If your short-term test is between 2 and 8 pCi/L, a long-term test helps you decide whether mitigation is warranted.

Continuous radon monitors (electronic devices that log hourly readings) are the most informative option. They cost $100 to $200 and give you real-time data plus long-term trends. They're also useful for verifying that a mitigation system is working and for detecting seasonal variations.

If your test result is 4 pCi/L or higher, the EPA recommends mitigation. Between 2 and 4, consider mitigation — there's no safe level of radon exposure, and mitigation costs are modest relative to the health risk.

Sub-Slab Depressurization

The most common and effective mitigation method is active sub-slab depressurization (ASD). A pipe is inserted through the basement slab into the gravel bed underneath. A continuously running fan pulls radon-laden air from under the slab and exhausts it above the roofline, where it disperses harmlessly into the atmosphere.

The pipe is typically 3 or 4 inch PVC. It runs from the sub-slab suction point up through the house (inside a closet, utility chase, or along the exterior wall) and out through the roof or a sidewall above the eave line. The fan mounts in the attic, on the exterior, or in the pipe run — anywhere above the living space so that any pipe leaks on the pressure side don't release radon into the home.

A single suction point is sufficient for most homes. The sub-slab gravel bed (required by code in most construction since the 1970s) provides an air communication path under the entire slab. The fan creates a negative pressure zone under the slab that extends well beyond the immediate suction point, drawing radon from the entire footprint. Diagnostic testing (drilling small test holes and measuring pressure field extension with a micro-manometer) confirms whether one suction point provides adequate coverage.

A properly installed ASD system typically reduces radon levels by 80% to 99%. A home with a pre-mitigation level of 12 pCi/L usually drops to 1 to 2 pCi/L. System cost ranges from $800 to $2,500 installed professionally, depending on house layout and pipe routing complexity.

Other Mitigation Approaches

Sealing cracks and openings in the foundation helps but is rarely sufficient as a standalone fix. Concrete cracks, gaps around pipes, and sump lids should be sealed as part of any mitigation effort, but sealing alone typically reduces radon by only 10% to 30%. The gas finds other paths through porous concrete and construction joints.

Passive sub-slab systems (same pipe configuration, no fan) rely on natural convection to draw soil gas upward. They're less effective than active systems (typically 30% to 50% reduction vs. 80% to 99%) but cost less to operate since there's no fan electricity. Many new homes are built with passive radon pipes roughed in. Adding a fan later converts a passive system to an active one at minimal cost.

For crawl spaces, sub-membrane depressurization works on the same principle. A heavy polyethylene sheet seals the crawl space floor, and a suction pipe and fan pull radon from under the membrane. The membrane seal must be complete — any gaps allow soil gas to bypass the system.

DIY vs. Professional Installation

DIY installation is feasible for a handy homeowner. The components are PVC pipe, fittings, a radon fan ($150 to $300), and concrete/sealant for the slab penetration. The biggest challenge is the pipe routing — finding a path from the basement slab to the roof that doesn't require cutting through structural members or running through finished living space.

Professional installation adds the diagnostic testing (pressure field extension mapping) and guarantee of results. Many states require radon mitigators to be certified or licensed. A professional will also handle the aesthetics — routing the pipe inconspicuously and matching exterior pipe to existing vent stacks. For most homeowners, the $800 to $1,500 professional installation cost is worth the confidence in the result.

Whether DIY or professional, always test after installation to verify the system is working. Place a test device in the same location as the pre-mitigation test and run it for the same duration. Retest every 2 years, or install a continuous monitor.

Frequently Asked Questions