Radon Venting System: How SSD Soil-Gas Exhaust Actually Works
A radon venting system is the physical exhaust pathway that takes soil gas out of a home before it can mix with indoor air. The most common design is sub-slab depressurization (SSD): a pipe extends below the foundation slab, a continuously running inline fan creates suction in the soil gas beneath the slab, and the gas is exhausted above the roofline through a vertical pipe. The system’s effectiveness depends on physics — pressure differentials, vent termination height, fan sizing, and air-handling — that EPA’s RMS-LL (“Radon Mitigation Standards — Liaison Letter”) and ANSI/AARST mitigation standards have codified into prescriptive design rules. This guide walks through how the venting actually works, why each design element matters, and how a Colorado Front Range homeowner can evaluate whether their installed or proposed system meets the standard. The information here reflects EPA, ANSI/AARST, and NRPP guidance current as of 2026 and is informational only.
Part of our Radon Mitigation Systems guide — start there for the full picture →
The Basic Design: Sub-Slab Depressurization
SSD is the dominant residential radon mitigation design because it directly intercepts the soil gas before it enters the home. The system has four components:
Sub-slab suction pit. A 4-5 inch hole is cored through the basement floor and a roughly 5-gallon pit is excavated below the slab in the granular fill or soil. The pit creates a low-resistance pathway for the fan to draw air from a broad area of the sub-slab gravel or fill.
Vertical riser pipe. Typically 3-4 inch diameter schedule 40 PVC. The pipe extends from the suction pit, through the basement ceiling, up through the framing or an interior chase, and out above the roofline.
Inline radon fan. A continuous-duty fan installed in the pipe above the highest occupied living level. The fan pulls air from the sub-slab pit and pushes it out the exhaust.
Exhaust termination. The pipe exits above the roofline with the discharge point at or above the eaves, away from operable windows and air intakes.
The physics is straightforward: the fan creates a partial vacuum (typically –0.5 to –1.5 inches of water column) under the slab. That vacuum is lower pressure than the soil-gas pressure on the other side of the slab and lower than the indoor air pressure above the slab. Soil gas migrates toward the suction pit through the granular fill and is exhausted. Because the sub-slab pressure is now lower than indoor air pressure, soil gas no longer leaks upward through slab cracks, control joints, or plumbing penetrations.
Why the Fan Must Be Above the Living Level
EPA’s mitigation standards (and ANSI/AARST consensus standards) require the inline fan to be located above the highest occupied living level of the home — typically in the attic or on the exterior of the home above the eave line. The fan is not installed in the basement, even though that would simplify access.
The reason is failure mode containment. If the fan is in the basement and the pipe develops a crack or leak on the pressure side (downstream of the fan), pressurized radon-laden air is being pushed back into the basement through the leak. The pipe is under positive pressure between the fan and the exhaust point, and any leak in that section vents radon into the occupied space.
By placing the fan above the living level, the entire pipe section in the occupied portion of the home is under negative pressure (suction). Any leak in that section pulls indoor air inward rather than pushing radon outward — a safe failure mode. Only the section between the fan and the exhaust termination is under positive pressure, and that section runs through the attic or along the exterior where any leak vents outdoors.
This is one of the most important — and most commonly cut-corner — design rules in residential radon mitigation. The radon system inspection guide covers this specific design element as a primary verification check.
Vent Termination Height and Location
EPA’s mitigation standards specify where the vent exhaust can and cannot be located:
Termination above the eave. The exhaust point must be at or above the eave line of the roof. This prevents exhausted radon from re-entering the home through ground-level windows or doors below.
Minimum 10 feet from operable windows and air intakes. The exhaust must be at least 10 feet (horizontally) from any window, door, or air intake that opens. If 10 feet isn’t achievable, the exhaust must be at least 2 feet above the top of the nearest such opening.
12 inches above the roof surface where the pipe penetrates. The exhaust point must extend at least 12 inches above the roof surface. This prevents snow accumulation from blocking the exhaust during Colorado winters.
Away from air intakes for neighboring buildings. The standard expands the 10-foot rule to include windows and intakes on adjacent properties.
Capped with a rain cap that doesn’t restrict flow. The cap prevents rain and snow ingress but cannot reduce airflow significantly. A specific radon-rated cap or an open vertical termination is acceptable; a low-flow rain shield is not.
Front Range homeowners should visually verify the exhaust termination meets these rules. A common installation error is routing the exhaust pipe horizontally out a wall and terminating at ground level or below the eaves — this is non-compliant and re-entrains exhausted radon into the home through ground-level openings.
Fan Sizing
The radon fan is sized to match the home’s foundation footprint and the sub-slab permeability. Common fan models in Colorado installations:
RadonAway RP140, RP145, RP260, GP500/501: Different airflow and pressure ratings for small to large foundations.
Festa AMG250: Lower-pressure, high-flow option for permeable sub-slab fill.
Tjernlund radon fans and Fantech RN series: Alternative manufacturers with similar specifications.
The contractor selects the fan based on a pre-installation pressure field extension test (PFE) — a small test fan applied to a temporary suction point that measures how widely the suction propagates through the sub-slab fill. The PFE result determines whether one suction point is sufficient or multiple are needed, and what fan capacity matches the foundation.
An undersized fan fails to reduce radon below 4 pCi/L. An oversized fan wastes electricity and increases heat loss without proportional benefit. The right-sized fan operates near its design point with margin for fouling and motor wear over time.
The System Pressure Monitor
Every compliant radon venting system includes a pressure monitor that confirms the fan is operating. The two common types:
U-tube manometer. A simple plastic tube containing colored fluid. When the fan is running, the suction pulls one side of the fluid down and pushes the other up. The pressure differential between the two sides reads in inches of water column. Standard operating range is –0.5 to –1.5 inches of water.
Digital pressure gauge. An electronic gauge that displays current suction pressure and may include alarm contacts.
The monitor is mounted on the pipe near the fan (or in a basement-visible location with a connecting line) so the homeowner can verify at a glance that the system is running. If the monitor shows zero pressure differential, the fan has failed and radon levels in the home will rise back to pre-mitigation levels within hours.
Homeowners should check the monitor monthly. A diminishing pressure reading over time can indicate fan motor wear and is the leading indicator that fan replacement is needed.
Why Not Just Open Windows?
A common homeowner question is whether passive ventilation (opening windows, running exhaust fans) can substitute for a venting system. The answer is no, for two reasons:
Inconsistent and uncontrolled. Window-based ventilation depends on weather, temperature, and human behavior. A home that’s ventilated in summer reverts to elevated radon every winter when windows close. The cumulative exposure isn’t reduced reliably.
Doesn’t address the source. SSD reduces the soil-gas inflow at its source — the sub-slab area. Window ventilation only dilutes radon after it has entered the indoor air. The fundamental pressure differential driving soil gas into the home remains; in fact, exhaust fans without make-up air can increase the depressurization of the lower level and pull more soil gas inward.
Heat recovery ventilators (HRVs) and energy recovery ventilators (ERVs) can supplement SSD by maintaining controlled fresh-air exchange, but they don’t replace the sub-slab depressurization. EPA does recognize sub-membrane depressurization for crawlspaces, sub-slab suction for slabs, and basement air pressurization as valid mitigation approaches — but window opening is not on the list.
For the broader Front Range context including how venting fits into the mitigation cost picture, see the Colorado mitigation cost guide.
Verification After Installation
EPA, ANSI/AARST, and NRPP all require a post-mitigation radon test 24-48 hours after the system is operating. The test verifies the venting system reduced indoor radon below the 4 pCi/L action level. Most properly designed Front Range SSD systems achieve 1-2 pCi/L post-mitigation.
Ongoing verification includes monthly visual check of the pressure monitor, annual visual inspection of pipe joints and the exhaust termination for damage or weather impact, and biennial radon retesting to confirm continued system performance. The Colorado radon testing pillar covers the broader monitoring framework.
References
- EPA Consumer’s Guide to Radon Reduction — U.S. Environmental Protection Agency
- CDPHE Colorado radon mitigation system standards — Colorado Department of Public Health and Environment
- InterNACHI radon mitigation system inspection — International Association of Certified Home Inspectors
- American Lung Association radon information — American Lung Association
Frequently asked questions
Why is the radon fan installed above the living level?
EPA mitigation standards require the fan above the highest occupied living level so the entire pipe section through the home is under negative pressure (suction). Any leak in that section pulls air inward rather than pushing radon outward — a safe failure mode. The fan is typically in the attic or on the exterior above the eaves.
How high does the radon exhaust pipe need to extend?
EPA standards require the exhaust point at or above the eave line, at least 10 feet from operable windows and air intakes (or 2 feet above them), and at least 12 inches above the roof surface where the pipe penetrates. The cap must not restrict flow significantly.
How do I know my radon venting system is working?
A pressure monitor (U-tube manometer or digital gauge) installed on the pipe shows the fan is producing suction. Standard operating pressure is –0.5 to –1.5 inches of water column. A zero reading indicates fan failure. Verify monthly and retest radon every 2 years.
Can I just open windows instead of installing a radon mitigation system?
No. Window ventilation is inconsistent and weather-dependent, and exhaust fans without make-up air can actually increase soil-gas inflow through depressurization. EPA recognizes sub-slab depressurization, sub-membrane depressurization, and basement air pressurization as valid mitigation — not passive ventilation.
Front Range homeowners verifying a new or existing venting system, or comparing mitigation contractor proposals, can reach our team through the contact page for a referral to NRPP-certified Colorado professionals.
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.
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 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 — $16.98 |
 Airthings Corentium Home | Digital monitor with a long-term average. | Amazon — $147.14 |