How to Vent Radon Gas From Basement
Venting radon gas from a basement is accomplished through a well-established methodology called sub-slab depressurization, or SSD. The system uses a suction point installed below the basement slab, a sealed PVC pipe that routes the suction to a continuously running fan, and a vent stack that discharges the soil gas above the roofline of the home. The design is described in EPA’s Radon Mitigation Standards (EPA RMS-LL) and the ANSI/AARST mitigation standard SGM-SF. This guide explains how the system works, where the components go, what a homeowner should expect during installation, and why DIY versions of the system rarely match a professionally installed SSD in measured performance. This summary reflects EPA, CDPHE, and AARST guidance current as of 2026 — consult a certified mitigator for specific design decisions.
Part of our Radon Levels guide — start there for the full picture →
What sub-slab depressurization actually does
The basic physical principle of SSD is to create a pressure differential between the soil under the basement slab and the air inside the basement. Without mitigation, soil gas — which contains radon — flows upward into the basement through cracks in the slab, joints around plumbing penetrations, and gaps around the perimeter. The pressure gradient driving this upward flow is small but persistent.
SSD reverses the pressure gradient. The fan continuously draws air from the soil below the slab, exhausting it above the roofline. Because the sub-slab area is now at slightly lower pressure than the basement, soil gas no longer migrates upward into the home. Instead, any soil gas in the area below the slab is pulled into the suction point and discharged outside before it can enter the basement.
A properly designed SSD system reduces indoor radon by 50 to 99 percent in most homes. Reductions to below 2 pCi/L are common in well-designed installations. The system is the EPA-recommended primary mitigation method for slab-on-grade and basement foundations.
The five system components
An SSD system has five core components, each with specific design and installation requirements.
The suction point
The suction point is a hole cut through the basement slab, typically four to six inches in diameter, leading into a void area excavated under the slab. The void is typically 18 inches across by 12 inches deep, sized to allow effective gas flow from the surrounding sub-slab area. The hole is sealed where it meets the PVC pipe so that the only air path is through the pipe. The location of the suction point is chosen to maximize the area of slab that the system effectively depressurizes — typically near the center of the largest contiguous slab area.
The piping
The pipe is typically 3-inch or 4-inch schedule 40 PVC, sized to handle the airflow at the design fan capacity without excessive pressure loss. The pipe routes from the suction point through the basement, up an interior chase or exterior wall, through the attic or along the exterior of the home, and terminates above the roofline. The pipe is sealed at every joint with PVC primer and cement. Horizontal runs slope back toward the suction point so that any condensation drains back into the soil rather than collecting in low spots.
The fan
The fan is the active component that creates the pressure differential. Common residential mitigation fans operate at 50 to 150 watts and develop 0.5 to 2.0 inches of water column suction. The fan is installed in the piping run, typically in the attic or on the exterior wall, never in a conditioned space below the slab. This placement requirement prevents the fan, which is under negative pressure on the inlet side, from drawing radon-laden air into the home if the pipe were to develop a leak downstream of the fan.
The vent stack and discharge
The pipe terminates above the roofline at a height and offset specified by EPA RMS-LL and the AARST standard. The discharge must be at least 10 feet above grade, at least 10 feet from any operable window or air intake, and at least 2 feet above the roof surface. These distances ensure that exhausted soil gas disperses in the outdoor air rather than re-entering the home through other openings.
The U-tube manometer
A small U-shaped tube partially filled with colored liquid is mounted on the PVC pipe inside the basement. The manometer indicates whether the fan is running by showing a differential between the two legs of the U-tube. When the fan stops, the liquid levels equalize. This is the homeowner’s primary indicator of system function. Visual inspection of the manometer once or twice a year confirms the fan continues to operate.
How the suction point location is chosen
The suction point location is the most consequential design decision. A poorly located suction point may depressurize only a fraction of the slab, leaving large areas where soil gas continues to migrate upward through joints and cracks. A well-located suction point creates depressurization across the entire slab area.
Diagnostic testing during installation identifies the optimal location. The mitigator drills small test holes through the slab at candidate locations and measures the pressure differential created by the test fan. The location producing the most uniform sub-slab depressurization is selected for the permanent suction point. In complex foundations with multiple slabs or footings dividing the area, multiple suction points and a manifolded pipe configuration may be required.
Sealing the slab
The SSD system works most efficiently when air leaks through the slab are minimized. Before the suction point is installed, the mitigator identifies and seals visible cracks, gaps around plumbing penetrations, sump pit covers, and the perimeter joint where the slab meets the foundation wall. Polyurethane caulk and concrete patch material are common sealing products. Sealing alone does not constitute mitigation — soil gas continues to find pathways through unsealed micro-cracks — but reducing the leak area improves SSD performance.
Sump pits in particular require sealed lids with a gasketed access cover. Open sump pits are major radon entry points and undermine SSD performance. The sealed lid typically includes a clear inspection port and an electrical pass-through for the sump pump.
Why DIY SSD installations often underperform
SSD looks deceptively simple — a hole in the slab, a pipe, a fan, a roof penetration. DIY installations described in online tutorials often produce real but modest radon reductions, frequently 20 to 50 percent rather than the 80 to 99 percent that a professional installation achieves. The performance gap comes from several factors:
- The suction point is placed without diagnostic testing, often at a convenient location rather than an optimal one.
- The sub-slab void is undersized, limiting effective gas flow.
- The slab is not adequately sealed, leaving major air-leak pathways that bypass the system.
- The fan is undersized or oversized for the specific soil and slab conditions.
- The discharge location does not meet the EPA setback requirements, allowing re-entry of exhausted radon.
- The post-installation testing is incomplete, leaving the homeowner without verification that the system actually works.
Homeowners exploring a deeper explainer on how SSD soil-gas exhaust actually works get a more detailed walkthrough. The bottom line is that professional installation typically costs $1,200 to $2,500 and delivers reliably effective radon reduction. DIY installations save the labor cost but rarely deliver equivalent performance.
What homeowners should expect during installation
A typical professional SSD installation in a Front Range home takes one to two working days. Day one usually covers the suction point, the sub-slab void excavation, the slab sealing, and the interior pipe routing. Day two completes the exterior or attic pipe run, the fan installation, the roof penetration, and the electrical connection.
The household generally remains in the home during installation. The work generates some noise from drilling and concrete cutting and some dust from the slab penetration, but the impact is localized. The mitigator typically sets up a containment around the suction point to control the concrete dust.
Post-installation testing and verification
The mitigator performs a follow-up radon test 24 hours to several days after the fan is energized. A short-term continuous monitor or a 48-hour activated-charcoal kit confirms that indoor radon has dropped to below the EPA action level of 4 pCi/L. A long-term test placed over 90 days produces an annual-average estimate that is the gold standard for verification.
The homeowner should receive a written report showing the post-installation test results, the system design diagram, the fan model and specifications, and any warranty terms. This documentation goes into the home file and supports future real-estate transactions and insurer underwriting. The broader radon testing pillar overview covers post-installation verification practices in more detail.
What about crawlspaces and slab-on-grade configurations?
Crawlspaces use a related methodology called submembrane depressurization (SMD). A heavy plastic membrane is installed over the crawlspace soil, sealed at all edges, and connected to a fan that draws air from below the membrane and exhausts above the roofline. The principle is the same as SSD but applied to a vented crawlspace rather than a sealed basement slab.
Slab-on-grade homes use SSD identical to basement installations. The suction point goes through the living-space slab, which sometimes requires routing the pipe through closets or utility chases to maintain aesthetics.
Fan placement and access considerations
The mitigation fan is the only mechanical component in the system, and its placement affects both performance and serviceability. EPA standards require the fan to be located such that any leak in the piping between the fan and the discharge point cannot draw radon-laden air back into the conditioned space. In practice this means the fan must be either in the attic, in an unconditioned garage, or on the exterior of the home. The fan cannot be installed in a basement or living space below the slab.
Attic installations are common in Colorado because the attic space is convenient for routing pipes and the fan noise is dampened by the ceiling assembly below. Exterior installations are more common in homes without attic access, on slab-on-grade construction, or where the homeowner prefers to avoid attic penetrations. Fan service access is a practical consideration — fans last 8 to 12 years and need to be replaceable without major disassembly of finished spaces.
Noise expectations
A properly installed mitigation fan produces audible but not disruptive noise. Inside the home, the dominant sound is air flow through the PVC pipe, typically in the range of 25 to 35 decibels at the manometer location. The fan itself, when mounted in the attic, is usually inaudible from finished living spaces below. Exterior fan installations produce slightly more audible noise outside the home, comparable to a small bathroom exhaust fan running continuously.
Homeowners sensitive to mechanical noise should ask the mitigator about ultra-quiet fan models. Several manufacturers offer fans rated below 30 decibels at standard residential operating conditions. The premium for ultra-quiet models is typically $100 to $200 over standard fans.
Electrical and code considerations
The mitigation fan requires a dedicated electrical circuit, typically a 15-amp 120-volt branch. Most installations include a switch and indicator light in a convenient location so the homeowner can confirm power to the fan. The fan should be on a circuit that is not switched off when the home is unoccupied — radon migration continues whether the home is occupied or not, and the system should run continuously.
Permit requirements vary by jurisdiction. Some Front Range jurisdictions require a mechanical or electrical permit for mitigation installations; others do not. The NRPP-certified mitigator handles permitting as part of the project scope. Permit fees, when required, typically run $50 to $200 and are included in the contractor’s bid.
References
- EPA Consumer Guide to Radon Reduction — U.S. Environmental Protection Agency
- CDPHE radon program guidance — Colorado Department of Public Health and Environment
- InterNACHI radon mitigation reference — International Association of Certified Home Inspectors
Frequently asked questions
What method does EPA recommend for venting radon from a basement?
Sub-slab depressurization, or SSD. The system uses a suction point below the basement slab, a sealed PVC pipe, a continuously running fan, and a discharge stack above the roofline. SSD reduces indoor radon by 50 to 99 percent in most homes.
Where does the radon discharge from the system?
Above the roofline of the home, at least 10 feet above grade, at least 10 feet from any operable window or air intake, and at least 2 feet above the roof surface. These setback distances ensure exhausted soil gas disperses in outdoor air rather than re-entering.
Can a homeowner install SSD themselves?
Possible but typically underperforms. DIY installations often achieve 20 to 50 percent radon reduction versus 80 to 99 percent for professional installations. The performance gap comes from suboptimal suction-point placement, undersized sub-slab voids, incomplete slab sealing, and lack of diagnostic testing.
How does the homeowner know the system is working?
A U-tube manometer mounted on the PVC pipe shows a liquid-level differential when the fan is running. Post-installation radon testing confirms indoor concentrations dropped below the EPA action level of 4 pCi/L. Follow-up testing every two years confirms continued effectiveness.
Front Range homeowners considering radon mitigation can reach out through our contact page for a referral to an NRPP-certified mitigation contractor.
Test your home for radon
Radon is invisible and odorless, so a test is the only way to know your level. A short-term kit gives a first reading in days; a digital monitor tracks it over time.
| Product | Why | Buy |
|---|---|---|
 AirChek Charcoal Radon Test Kit | Lab fee usually included; EPA short-term protocol. | Amazon — $21.95 |
 First Alert RD1 Radon Test Kit | Widely stocked; 48-hour charcoal test. | Amazon — $17.95 |
 Airthings Corentium Home | Digital monitor with a long-term average. | Amazon — $142.49 |