Sub-slab depressurization (SSD) is the radon mitigation technique recommended by the US Environmental Protection Agency for the vast majority of slab-on-grade and basement homes: a sealed PVC pipe is run through the concrete slab into the loose aggregate beneath it, an inline fan pulls soil gas up the pipe, and the gas is exhausted above the roofline. Per EPA-402-K-10-005 (the Consumer's Guide to Radon Reduction), a properly designed SSD system typically reduces indoor radon by 50 to 99 percent and should bring most homes below the EPA action level of 4 pCi/L.
What sub-slab depressurization actually is
A sub-slab depressurization system has four physical parts and one operating principle. The parts are:
- A suction point. A 4-to-6-inch hole is cored through the basement or slab-on-grade floor, and a small pit (sometimes called a "suction pit") is excavated in the gravel or aggregate underneath. This pit is the void from which the system draws.
- The vent pipe. A continuous run of 3-inch or 4-inch schedule-40 PVC pipe is sealed into the slab penetration with polyurethane caulk, then routed vertically through the building. The pipe must terminate above the roofline.
- The radon fan. An inline centrifugal fan — rated for continuous duty and outdoor temperatures — is installed in the pipe run, always in non-occupied space (attic, garage, exterior wall). Per ANSI/AARST SGM-SF-2017, the fan must be on the suction side of the building envelope so that any pipe leak vents out, not in.
- A monitor. A U-tube manometer (typically a Dwyer Mark II or similar) is plumbed into the pipe below the fan and mounted in plain sight. It reads the static pressure the fan is pulling and is the homeowner's at-a-glance proof that the system is still running correctly.
The operating principle is pressure, not airflow. The fan does not need to move large volumes of air; it needs to maintain a slight negative pressure under the slab relative to the inside of the house. When that pressure gradient is established, soil gas — which carries the radon — flows toward the suction point and up the stack instead of leaking through floor cracks, the slab-wall joint, sump pits, and utility penetrations.
Why pressure, not ventilation
Homeowners sometimes ask why mitigators don't simply install a larger HVAC fan or open windows to dilute indoor radon. The reason is the stack effect. Warm air rising inside a building creates negative pressure at the lowest level. That negative pressure pulls soil gas in through every crack, gap, and unsealed penetration in the foundation. Diluting indoor air with ventilation can mask the problem temporarily, but the foundation is still acting as a vacuum on the soil. The moment ventilation drops — closed windows, winter, sleeping with the HVAC off — radon rebuilds.
Sub-slab depressurization inverts the stack effect at its source. By making the sub-slab area more negative than the basement, the foundation stops acting like a vacuum and starts acting like a seal. The gas that would have been pulled into the house is pulled into the pipe instead. This is why SSD is permanent in a way that ventilation is not, and why the EPA names it as the preferred technique in EPA-402-K-10-005.
Mitigation is not the same as testing
You should never install a mitigation system without first confirming an elevated reading on a valid radon test. If you have not yet tested, read our DIY radon testing guide and our companion piece on short-term vs. long-term testing before you call a mitigator.
What the ANSI/AARST standard requires
The governing document for residential radon mitigation in the United States is ANSI/AARST SGM-SF-2017, the Soil Gas Mitigation Standard for Existing Single-Family Buildings. It is the standard cited by state radon programs, by NRPP (National Radon Proficiency Program), and by NRSB (National Radon Safety Board). Any certified mitigator works to this standard. Key requirements include:
Suction-point placement and the PFE test
Before a system is designed, AARST requires a pressure field extension (PFE) test — sometimes called a communication test. The mitigator drills a small test hole in the slab, applies suction with a portable vacuum, and measures whether the negative pressure "communicates" across the entire sub-slab area through other small test holes drilled elsewhere. If the sub-slab aggregate is loose and continuous, one suction point may serve the whole house. If the aggregate is fine, compacted, or interrupted by internal footings, multiple suction points may be required.
Pipe diameter and routing
SGM-SF-2017 specifies minimum pipe diameters based on fan size and static pressure. Three-inch PVC is the common minimum; four-inch is used for higher-flow systems or longer runs. The pipe must be sloped continuously back toward the suction point so that any condensation drains down the stack rather than pooling in the fan. Horizontal runs through living space are minimized and insulated where they cross unconditioned areas.
Fan placement and exhaust
The fan must be located outside the occupied envelope of the building. The exhaust terminus must be:
- At least 10 feet above grade;
- At least 10 feet from any window, door, or air intake;
- Above the eave line of the roof so exhaust does not re-enter through soffit or attic vents.
These setbacks are why mitigators almost always route the stack up the exterior of the house and through the eave, or interior through a chase and out the roof. They are not aesthetic choices — they are the dispersion distances the standard specifies.
The system label and the manometer
Every AARST-compliant system carries a permanent label at the manometer identifying the installer, install date, NRPP/NRSB certification number, and the design pressure. The manometer itself is the simplest piece of equipment in the system and the single most important diagnostic the homeowner ever uses: if both legs of the U-tube read level, the fan has failed.
How effective is SSD, really?
EPA-402-K-10-005 states that properly installed SSD systems typically reduce indoor radon levels by 50 to 99 percent. AARST goes further: a properly designed system should bring indoor radon below 2 pCi/L wherever practical, well under the 4 pCi/L EPA action level. The wide range reflects starting concentration (a house at 20 pCi/L has more to remove than one at 5), foundation type, and the quality of the sub-slab communication.
The only way to know what your system actually achieved is to test after install. AARST requires a post-mitigation radon test no sooner than 24 hours and no later than 30 days after the system is energized. The post-test should be performed in the lowest livable area, with the home in normal closed-house conditions, exactly like the original diagnostic test. If the post-mitigation result is still above 4 pCi/L, the mitigator is obligated to diagnose and correct — typically by adding a second suction point, upsizing the fan, or sealing additional foundation entry routes.
What does SSD cost?
The EPA cites a typical SSD installation cost of $800 to $2,500 in EPA-402-K-10-005, with regional variation and complexity driving the spread. The drivers are:
- Foundation type. A walk-out basement with exposed slab on one side is the easiest case. A finished basement with multiple footings, a crawlspace, or a slab broken by interior load-bearing walls is harder and may require additional suction points.
- Pipe routing. An exterior stack up a back wall is fastest. A fully interior routing through a chase, then through the roof, costs more in carpentry and roof flashing.
- Fan sizing. A standard residential radon fan (e.g. RadonAway RP145, GP series, or AMG Legend) draws about 70 to 90 watts continuously. Larger fans for tighter sub-slab aggregate cost more up front and slightly more to run.
- Sealing scope. Major slab cracks, large open sump pits, and an unsealed crawlspace floor all add labor.
Operating cost is small: a continuously-running 80-watt fan uses roughly 700 kWh per year, or on the order of $80 to $120 in electricity at typical US residential rates.
Mitigation techniques compared
SSD is the default for slab-on-grade and full-basement homes, but it is not the only technique in the AARST playbook. The right approach depends on what is between the soil and the living space.
| Technique | When it applies | How it works | Notes per AARST / EPA |
|---|---|---|---|
| Sub-slab depressurization (SSD) | Slab-on-grade or full basement with sub-slab aggregate | Fan + pipe creates negative pressure beneath slab | EPA-preferred; 50–99% reduction typical |
| Sub-membrane depressurization (SMD) | Crawlspace with dirt or gravel floor | 6-mil-plus polyethylene membrane sealed to walls; pipe and fan pull from beneath the membrane | AARST sealed-crawl detail; requires sealing all wall penetrations |
| Drain-tile suction | Homes with continuous perimeter drain tile (interior or exterior) | Fan attached to the drain-tile loop pulls soil gas around the entire footprint | Highly effective when drain tile is intact and continuous |
| Block-wall depressurization | Foundations built from hollow CMU (concrete masonry unit) block | Sealed pipe taps into the hollow cores of the wall, pulling gas from inside the blocks | Used alone or in combination with SSD; cores must be sealed at top |
| HRV / ERV (heat- or energy-recovery ventilator) | Supplemental only, in tight homes where SSD alone cannot reach target | Dilutes indoor air with conditioned outside air | EPA notes ventilation alone is rarely sufficient; energy penalty is real |
| Passive radon-resistant new construction (RRNC) | Homes built to EPA RRNC / IRC Appendix F | Same pipe + roof termination as SSD, but no fan unless post-construction test fails | Fan can be added retroactively in roughly an hour |
In practice many mitigators combine techniques. A house with a partial basement and an adjoining crawlspace will typically get SSD on the basement slab and SMD on the crawlspace, with both pipes joined into a single stack and a single fan.
Post-mitigation testing and ongoing maintenance
The mitigation does not end when the fan turns on. AARST and EPA both treat the post-install test as part of the installation, not an optional follow-up. Beyond that:
- Monthly manometer check. Look at the U-tube. The two columns of liquid should be at different heights, indicating the fan is pulling pressure. Equal columns mean the fan has failed and needs replacement.
- Fan service life. Manufacturers rate residential radon fans for a service life of roughly 5 to 10 years under continuous duty. Most quality fans carry a 5-year manufacturer warranty. When the fan fails, replacement is generally straightforward and inexpensive — typically under $400 including labor.
- Retesting cadence. EPA-402-K-12-002 (the Citizen's Guide to Radon) recommends retesting every two years in mitigated homes, and after any event that could change pressure dynamics in the building.
- Retest after renovation. Foundation work, HVAC replacement, new windows, an addition, or finishing a basement can all change radon behavior. See our dedicated guide on radon retesting after renovation for the cases that warrant a fresh measurement.
Selling a home with a mitigation system
Buyers' agents in radon-aware markets routinely ask for the install paperwork, the AARST/NRPP certification number, the original post-install test, and a fresh confirmation test at inspection. If you are listing, get those documents together early. Our guide on selling a home with high radon walks through the disclosure and pricing implications.
Hiring a mitigator
Radon mitigation is one of the few residential trades where there is a real, enforced national certification regime. The two recognized bodies are:
- NRPP — the National Radon Proficiency Program, administered by AARST (the American Association of Radon Scientists and Technologists).
- NRSB — the National Radon Safety Board.
Many states (Illinois, New Jersey, Pennsylvania, Florida, and others) require state-level licensure on top of NRPP/NRSB certification, and some require a permit for the install itself. A mitigator should be able to produce a certification number on request, name the standard they work to (SGM-SF-2017), and walk you through what a post-install test will look like before they begin.
Reasonable things to ask for in writing before signing:
- The specific suction-point plan and the expected pipe routing;
- The fan model and warranty length (typically 5 years from the manufacturer);
- The mitigator's workmanship warranty (commonly 5 years);
- A performance warranty — a written commitment that post-mitigation radon will be below 4 pCi/L, with the mitigator returning to remedy at no charge if it is not;
- The post-install test plan (who runs it, when, and how the result is reported);
- Any required permits and who pulls them.
RadonZoneReport does not recommend or endorse individual mitigators. Use the NRPP directory at aarst-nrpp.com or the NRSB directory at nrsb.org, cross-checked against your state radon office's licensure list. If you live in a high-radon county, you should also read our companion piece on the highest-radon counties in the US and our explainer on the EPA radon zones map for the geological context your mitigator is working in.
What the dossier covers
If you want the EPA radon-zone designation, the state-survey averages, and the AARST mitigation cost guidance for your specific county in one printable place, our $15 county dossier pulls all of it from primary sources. See the methodology for how the report is built and our sources page for every dataset we cite. To pull your county now, head to the order page.
Sources
- US Environmental Protection Agency. Consumer's Guide to Radon Reduction: How to Fix Your Home. EPA-402-K-10-005, updated 2013. The primary federal document on residential radon mitigation, including SSD effectiveness ranges and cost guidance.
- ANSI/AARST. SGM-SF-2017 — Soil Gas Mitigation Standard for Existing Single-Family Buildings. American Association of Radon Scientists and Technologists. The consensus national standard cited by state radon programs and certification bodies.
- US Environmental Protection Agency. A Citizen's Guide to Radon: The Guide to Protecting Yourself and Your Family from Radon. EPA-402-K-12-002.
- National Radon Proficiency Program directory, aarst-nrpp.com, and National Radon Safety Board directory, nrsb.org — for certification verification of individual mitigators and measurement professionals.
- American Lung Association. Radon health information and indoor-air-quality resources, lung.org.
Related guides
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