Radon Reduction System: SSD, SMD, and Design Guide
A radon reduction system is the engineered mitigation that pulls radon-laden soil gas from beneath a building and exhausts it above the roofline before it can accumulate indoors. The dominant 2026 residential design is active sub-slab depressurization (SSD), with sub-membrane depressurization (SMD) used for crawlspaces and passive stack ventilation occasionally specified for new construction. EPA’s Radon Mitigation Standards for Low-rise Residential Buildings (RMS-LL) and the ASTM E2121 standard govern design and installation. This guide covers the system types, design parameters, installation cost, and what to look for in a qualified mitigation contractor. Information here summarizes EPA RMS-LL, ASTM E2121, ASHRAE, and NRPP certification guidance current as of 2026.
What a radon reduction system does
A radon reduction system creates a pressure differential between the soil beneath the foundation and the outdoor atmosphere, drawing radon-bearing soil gas out of the sub-foundation zone and venting it where it disperses harmlessly. The four common system architectures are:
- Active sub-slab depressurization (SSD): fan-powered suction through a pipe extending into gravel or porous fill beneath a poured slab foundation.
- Active sub-membrane depressurization (SMD): fan-powered suction beneath a polyethylene vapor barrier installed over crawlspace soil.
- Passive stack ventilation: a vent pipe relying on natural stack effect (typically used in new construction with appropriate sub-slab gas-permeable layer).
- Block-wall depressurization (BWD): fan-powered suction inside hollow block-foundation walls; less common, used when slab-suction is difficult.
The EPA action level is 4 pCi/L; a properly designed system typically brings post-mitigation readings to 0.5 to 2 pCi/L. Homeowners researching the broader Colorado radon testing landscape often arrive here after a test returns above 4 pCi/L and they need to understand what mitigation involves.
Active sub-slab depressurization (SSD): the residential standard
SSD is the standard residential radon-reduction system in 2026. Components:
Suction pit
A cavity excavated into the gravel or fill beneath the basement or slab-on-grade floor, typically 5 to 10 gallons in volume. Larger pits provide more uniform suction across the slab. A single suction point usually services 1,500 to 2,500 square feet of slab for typical soil conditions.
Vent pipe
Schedule-40 PVC pipe (typically 3-inch or 4-inch diameter) extends from the suction pit through the slab, then routes vertically through the building, terminating at least twelve inches above the roofline and at least ten feet from any window, door, or other opening per EPA RMS-LL.
Inline fan
A radon-rated continuous-duty fan (typically 90 to 250 cubic feet per minute capacity) is installed in the vent pipe in an unconditioned space (attic, garage, or exterior wall mount). Common 2026 fan models include the RadonAway RP-series and the Festa Eagle series. Fans are rated for continuous operation and typically last seven to ten years.
U-tube manometer
A small fluid-filled gauge mounted on the vent pipe near the basement provides a continuous visual indicator of suction. Homeowners can verify system operation by checking the manometer monthly.
Sealing
All foundation cracks, slab penetrations (plumbing, electrical, sump-pit lids), and floor-wall expansion joints are sealed with polyurethane caulk or backer rod-and-caulk during installation. Sealing prevents short-circuiting of the system and improves performance.
Sub-membrane depressurization (SMD): the crawlspace solution
For homes with vented crawlspaces or earthen-floor crawlspaces, SSD does not apply because there is no slab to draw through. The SMD approach:
- Install a 6-mil or 10-mil polyethylene vapor barrier across the entire crawlspace floor.
- Seal the barrier seams with vapor-barrier tape and run the barrier up the foundation walls to seal at the rim.
- Excavate a small suction pit beneath the membrane at a strategic location.
- Install vent pipe, inline fan, and roofline exhaust per RMS-LL.
SMD installation runs 20 to 40 percent higher than SSD for equivalent footprint because of the labor to install and seal the membrane. SMD also doubles as a moisture-management measure for crawlspaces, reducing humidity and supporting overall building envelope health.
Passive stack systems
Passive radon mitigation relies on natural stack effect: a vent pipe runs from a gas-permeable sub-slab layer through the building to above the roofline, with no fan. Warm indoor air rising through the pipe creates upward flow that draws soil gas with it. Passive systems are most effective in cold-climate construction with significant stack-effect potential and are commonly installed during new construction as a roughed-in feature that can be activated (add a fan) if post-construction testing shows elevated levels.
Passive systems typically reduce indoor radon by 20 to 50 percent — useful for borderline pre-mitigation levels but insufficient for significantly elevated cases. For confirmed readings above 4 pCi/L, an active fan-powered system is usually required.
Block-wall depressurization
Older homes with hollow concrete-block foundation walls can have radon entry through the block-wall cores in addition to the slab. Block-wall depressurization installs suction into the block-wall cavity, drawing radon out before it enters the basement air. BWD is less common than SSD but is sometimes added as a supplement when slab suction alone does not achieve the target reduction.
System cost in 2026
Typical 2026 installed costs for residential radon reduction systems in Front Range Colorado:
- Standard SSD on slab-on-grade or full basement: $1,200 to $2,500
- SSD with multiple suction points or unusual routing: $2,500 to $4,000
- SMD in crawlspace (typical 1,500 sq ft): $2,000 to $4,000
- Combined SSD + SMD for mixed foundation: $3,000 to $5,500
- BWD supplement to existing SSD: $800 to $1,500
The single largest cost driver is interior versus exterior fan routing (interior routing through closets and conditioned spaces is more expensive due to drywall and finish work). Fan replacement every seven to ten years runs $200 to $450 installed.
How to select a mitigation contractor
Contractor selection criteria:
Certification
The contractor’s mitigation specialist should hold either National Radon Proficiency Program (NRPP) certification or National Radon Safety Board (NRSB) certification. Both organizations test on EPA RMS-LL and require continuing education. Colorado does not separately license radon mitigation contractors, so certification is the primary credential.
Insurance
General liability and workers’ compensation coverage. Specifically ask for a certificate of insurance before the job starts.
Warranty
Most reputable contractors warrant the system to reduce radon below 4 pCi/L for five years. Fan replacement warranties typically run two to five years.
Post-mitigation testing
The contractor should provide or include a post-mitigation test (short-term test 24 hours to 30 days after system activation). Some include a free retest at six months.
References and review history
Request three customer references for similar installations. The NRPP and NRSB websites list certified professionals.
The companion radon mitigation options guide walks through how SSD, SMD, and passive systems compare across different home types.
System diagnostic measurements during commissioning
A properly commissioned active SSD system includes several measurements that the contractor documents at installation. These provide a baseline that supports later diagnostic work if performance changes:
Static pressure (suction) at the slab
Measured in inches of water column or Pascals. A typical residential SSD develops 0.1 to 0.5 inches of water column suction at the slab. The U-tube manometer mounted on the vent pipe provides ongoing visual confirmation of this suction.
Vent pipe airflow
Measured in cubic feet per minute. Residential fan capacities of 90 to 250 CFM typically produce vent-pipe flows of 40 to 150 CFM depending on sub-slab permeability and pipe sizing.
Communication test
Smoke or smoke-pencil test at suspect entry points (foundation cracks, slab penetrations, sump pits) confirms that the sub-slab depressurization is pulling air inward at those locations rather than allowing soil gas to enter.
Sub-slab communication mapping
For homes with complex slabs or unusual foundation footprints, contractors sometimes drill small test holes through the slab away from the suction pit and measure suction at those test holes. The pattern of suction values across the slab confirms whether a single suction point provides adequate communication or whether additional suction points are needed.
Documentation of these baseline measurements lets the contractor or a future diagnostic specialist quickly assess whether performance has degraded over time. A 50 percent reduction in static pressure after several years usually indicates fan degradation or a partial blockage; a smaller reduction indicates normal aging.
EPA RMS-LL design parameters in practice
The EPA Radon Mitigation Standards for Low-rise Residential Buildings (RMS-LL) provide specific design criteria that certified installers follow. Several of the most important practical parameters:
- Vent pipe termination: At least 12 inches above the roofline at the termination point, at least 10 feet from any window, door, or other operable opening, and at least 10 feet from the property line in some jurisdictions.
- Fan location: Outside conditioned space (attic, garage, or exterior wall) so that any unlikely fan failure does not vent radon-laden gas into the home.
- Pipe sizing: Typically 3-inch or 4-inch Schedule 40 PVC. Smaller pipe creates higher pressure drop and reduces fan effectiveness.
- Routing: Vertical runs preferred; horizontal runs minimized. Avoid traps that can collect condensate and obstruct flow.
- Labeling: All visible vent pipe inside the home labeled as a radon mitigation system, with the installer name and installation date.
- Sealing: All accessible foundation cracks, slab penetrations, sump pit lids, and floor-wall joints sealed with polyurethane caulk or equivalent during installation.
- Electrical: Fan wired to a dedicated circuit with a switch that allows the fan to be powered off only for service. Indicator showing fan-power status visible to the homeowner.
Installations that skip RMS-LL parameters (vent terminations too low, fans inside conditioned space, undersized pipe, missing sealing) often still reduce indoor radon but may produce sub-optimal results and may not meet warranty requirements with the fan manufacturer.
Ongoing system care
A radon reduction system requires modest ongoing care:
- Check the U-tube manometer monthly to confirm the fan is operating.
- Retest the home every two years to confirm post-mitigation levels remain below 4 pCi/L.
- Replace the fan when it fails (typically seven to ten years).
- Re-seal any new foundation cracks or slab penetrations that appear over time.
Energy and noise considerations
Active radon mitigation systems run continuously, which means homeowners care about energy consumption and noise. Typical 2026 system characteristics:
Energy consumption
Residential radon fans typically consume 60 to 150 watts continuously. At 2026 Front Range electricity rates of approximately $0.13 to $0.16 per kWh, annual electricity cost runs $70 to $200 per year. Energy-efficient fan models (electronically commutated motor designs from manufacturers like RadonAway RP-series) can reduce this by 30 to 50 percent versus older-style PSC motors.
Conditioned-air loss
Properly designed SSD systems pull soil gas, not conditioned indoor air. With adequate slab sealing, the conditioned-air loss is negligible. Poorly sealed installations can pull conditioned air through unsealed slab joints, which both wastes energy and reduces system effectiveness. Smoke testing during commissioning identifies any conditioned-air loss paths.
Noise level
Standard radon fans produce 35 to 55 decibels at one meter distance, comparable to a quiet bathroom exhaust fan. Properly located fans (attic, garage, exterior wall) typically produce minimal audible noise inside living spaces. Interior-routed fans or installations with poor vibration isolation can create more noticeable noise; mitigation includes flexible coupling between fan and pipe, vibration-isolating mounting hardware, and rerouting if the fan must be inside conditioned space.
What can go wrong with a radon reduction system
Several common failure modes affect radon mitigation systems over their service life. Understanding these helps homeowners maintain performance:
Fan failure
The most common eventual failure. Fan bearings wear out after seven to ten years of continuous operation. The U-tube manometer on the vent pipe collapses to zero suction when the fan fails. Replacement fan installation runs $200 to $450.
Pipe disconnection or blockage
Vibration over time can loosen pipe couplings, and condensate freezing in cold attic runs can occasionally block the pipe. Annual visual inspection identifies these. Repair is straightforward.
Suction-point clogging
Sub-slab gravel can settle around the suction pit, reducing suction. Less common in modern installations because the suction pit is designed with adequate void volume.
New foundation cracks
Foundation settlement or seasonal soil expansion can produce new cracks that short-circuit the system. The mitigation contractor seals visible cracks during installation; new cracks appearing later may require sealing or system rebalancing.
Renovation impact
Basement renovations that involve cutting into the slab, modifying the foundation, or relocating HVAC can affect the radon mitigation system. Notify the mitigation contractor before such work and retest after.
References
- EPA Consumer’s Guide to Radon Reduction — U.S. Environmental Protection Agency
- InterNACHI radon mitigation overview — International Association of Certified Home Inspectors
- American Lung Association radon mitigation guidance — American Lung Association
- Colorado radon program contractor resources — Colorado Department of Public Health and Environment