Radon Mitigation Methods: ASD, SSD, HRV, and More
Radon mitigation methods are a catalog of engineered approaches to reducing indoor radon concentrations. Most homes need just one method, but the right choice depends on foundation type, basement use, the magnitude of the radon problem, and budget. This guide summarizes EPA’s Radon Mitigation Standards for Low-Rise residential buildings (RMS-LL) current as of 2026 — consult a state-licensed mitigation professional for system design and an EPA-listed measurement professional for testing decisions. The aim here is to walk through each major method, explain what it does, when it is appropriate, and what to expect in cost and effectiveness for a typical Front Range home.
Radon Mitigation Methods: The Catalog
Five primary methods cover the residential mitigation landscape. Most installed systems in the United States are some variant of sub-slab depressurization. The remaining methods serve specific situations where the primary approach is not feasible or not sufficient on its own.
- Active Sub-Slab Depressurization (ASD). The dominant residential method. A fan-driven suction system pulls soil gas from beneath the slab and exhausts it above the roof.
- Sub-Slab Depressurization (SSD) / Sub-Membrane Depressurization (SMD). ASD’s variants for slab and crawl-space foundations respectively.
- Passive Stack. A vertical pipe relying on natural stack-effect airflow without a fan.
- Sealing Only. Closing cracks, joints, and penetrations to reduce entry pathways. Insufficient on its own in most cases.
- Heat Recovery Ventilation (HRV). Whole-house air exchange that dilutes indoor radon with outdoor air. Supplemental rather than primary.
Our broader radon testing guide for Front Range homeowners covers how to decide whether mitigation is needed in the first place.
Active Sub-Slab Depressurization (ASD)
ASD is the most common residential radon mitigation method in the United States and the EPA-preferred approach for slab-on-grade and basement foundations. The system works by creating a low-pressure zone beneath the foundation slab so that soil gas migrates into the suction system rather than into the building interior.
How It Works
A typical ASD system has five components:
- Suction point. A hole drilled through the concrete slab, typically 3 to 6 inches in diameter, with a small pit excavated underneath to allow soil-gas collection.
- PVC piping. Schedule 40 PVC pipe (typically 3 to 4 inches in diameter) running from the suction point up through the building and out through the roof.
- Inline fan. Mounted in the pipe in an unconditioned space (attic or outside wall), typically rated at 50 to 90 watts. Continuous operation.
- Manometer. A U-tube pressure gauge that confirms the fan is running and creating suction. Mounted on the pipe near eye level.
- Roof exhaust. The pipe terminates above the roof line and away from windows and air intakes per EPA RMS-LL siting requirements.
Effectiveness: well-designed ASD systems typically reduce indoor radon by 50% to 99%. Most installed systems achieve below 2 pCi/L from starting levels of 10 to 20 pCi/L. Front Range installation cost: $1,000 to $2,500 for a typical home. Resources from epa.gov/radon list state-licensed professionals.
Sub-Slab vs. Sub-Membrane Depressurization
SSD and SMD are subtypes of ASD that address different foundation types.
SSD: Slab Foundations
SSD is the standard ASD configuration for slab-on-grade or basement foundations with a continuous concrete slab. The suction point penetrates the slab directly into the soil or gravel below.
SMD: Crawl Space Foundations
SMD applies to crawl-space foundations with exposed soil floors. Instead of a concrete slab, a 6-mil polyethylene vapor barrier is installed across the crawl-space floor and sealed at the perimeter. The suction point connects to the air space beneath the membrane. SMD typically installs slightly higher cost than SSD because of the membrane material and labor.
Passive Stack Systems
Passive stack systems use vertical pipe routing without a fan. The natural stack effect of warm air rising through the pipe creates a small suction that pulls soil gas upward. Passive systems are typically installed during new construction as part of radon-resistant building codes, particularly in Colorado where Front Range counties require radon-resistant construction.
Effectiveness is variable. Passive stacks reduce indoor radon by an estimated 30% to 60% in typical installations, less than active systems. Many homes with passive stacks still test above 4 pCi/L and require fan addition (conversion from passive to active) to achieve mitigation targets. The conversion is straightforward: a fan and manometer are added to the existing pipe routing, typically for $500 to $1,000 if the pipe already meets active-system requirements.
Sealing Alone Is Insufficient
Sealing cracks, joints, sump pits, and slab penetrations is part of a complete mitigation package, but sealing alone rarely reduces indoor radon enough to meet the EPA action threshold. The reason: even small unsealed paths can pass substantial soil gas under the stack-effect pressure differential, and finding every entry point is impractical.
EPA’s RMS-LL framework recommends sealing as a supplement to active sub-slab depressurization. Sealing the sump pit cover, sealing slab cracks with elastomeric caulk, and closing floor drains with one-way trap covers all improve overall system performance by reducing depressurization losses through these openings. As a standalone strategy, sealing typically reduces indoor levels by only 10% to 20%.
Heat Recovery Ventilation (HRV)
HRV systems exchange indoor air with outdoor air while recovering heat from the exhaust stream. Indoor radon concentration is reduced through dilution: outdoor air contains roughly 0.4 pCi/L, so adding outdoor air to indoor air lowers the average. HRV is rarely the primary mitigation method but serves as a supplement in homes where ASD alone does not reach target levels.
HRV installation costs typically range from $1,500 to $4,000 depending on system capacity and duct integration. Effectiveness as a radon mitigation tool is moderate — typical radon reduction is 25% to 50%. The system has additional benefits beyond radon: it reduces indoor humidity, removes other indoor air pollutants, and improves overall air quality. Our companion piece on radon mitigation options covers HRV’s role in supplemental mitigation.
Choosing the Right Method for Your Home
EPA’s RMS-LL framework provides a decision tree based on foundation type and initial radon level.
Slab or Basement Foundation
Active sub-slab depressurization (SSD configuration). Standard for the majority of Front Range homes. Add sealing of cracks, sump cover, and floor drains as part of installation.
Crawl Space Foundation
Sub-membrane depressurization (SMD configuration). Install vapor barrier first, then suction system. Especially effective because crawl-space soil contact area is typically larger than basement slab area.
Mixed Foundation
Many homes have both slab and crawl-space sections. Multi-point systems with separate suction points for each foundation section are typical. A single fan can serve multiple suction points if the runs are configured appropriately.
Very High Initial Radon
For homes testing above 20 pCi/L, ASD plus supplemental HRV may be needed to reach the EPA target. Confirmation testing after installation determines whether the additional supplement is required.
Code Requirements for New Construction
Many Colorado Front Range counties — including Boulder, Larimer, and Adams — require radon-resistant new construction (RRNC) under their adopted building codes. RRNC includes:
- Gas-permeable layer beneath the slab (gravel or perforated drainage matting).
- Vapor barrier (6-mil polyethylene) over the gas-permeable layer.
- Vent pipe (passive stack) from the gas-permeable layer to the roof.
- Electrical junction box in the attic for future fan addition if the home tests above the action threshold after construction.
RRNC does not guarantee that the home will test below 4 pCi/L; it makes the conversion to an active system inexpensive if needed. Post-construction testing is recommended regardless.
Maintenance Requirements
An installed mitigation system requires minimal but regular attention:
- Check the manometer monthly. A glance confirms the fan is running and creating suction. If the gauge reads zero, the fan has failed and needs replacement.
- Replace the fan every 10 to 20 years. Fan failure is the most common system issue. Replacement cost is typically $250 to $500.
- Re-test radon every 2 to 5 years. A short-term test confirms the system continues to perform.
- Re-test after any foundation work or major renovation. Modifications can change the system’s depressurization profile.
Continuous radon monitors (CRMs) are increasingly popular for ongoing assurance. A consumer-grade CRM costs $150 to $300 and provides real-time readings indefinitely. Our piece on radon reduction system design covers ongoing monitoring options.
Common Installation Mistakes That Reduce Effectiveness
Not every installed mitigation system performs as designed. Several recurring installation errors reduce effectiveness or create new problems. Awareness of these failure modes helps homeowners evaluate the work after completion.
Fan Mounted in Conditioned Space
EPA RMS-LL requires the fan to be in unconditioned space (attic, garage, or exterior wall). Some installers cut corners by mounting the fan in a basement utility room or interior closet. This is non-compliant and creates a safety risk: if the fan housing develops a leak, radon-laden air enters living space instead of being contained in the unconditioned area.
Inadequate Suction Pit
The excavated pit beneath the slab at the suction point must be large enough to allow soil-gas collection across a meaningful footprint. A pit that is too small limits the system to drawing only from the small area immediately around the pipe end. Installations that skip the pit or make it too shallow often achieve reduced performance.
Pipe Sloped Incorrectly
The vent pipe should slope back toward the suction point so that any condensation drains back into the soil. Pipes installed with the wrong slope can accumulate water at low points, which restricts airflow and can eventually clog the system. Outdoor pipe runs are particularly susceptible during cold months when condensation freezes.
Roof Termination Too Close to Windows
EPA siting requirements call for the roof exhaust to terminate at least 10 feet from any window, door, or air intake. Installations that violate this requirement can cause exhaust radon to re-enter the building through nearby openings — defeating the purpose of the mitigation.
No Post-Installation Test
EPA recommends a post-installation radon test to confirm the system has brought indoor levels below the action threshold. Some installers skip this step or rely on the homeowner to test independently. Without confirmation, the system may not be performing as designed and the problem can go undetected for years.
Choosing a Mitigation Professional
State licensing requirements for radon mitigation professionals vary, but the National Radon Proficiency Program (NRPP) and the National Radon Safety Board (NRSB) maintain national certification programs that most states recognize. Reputable contractors hold current certification from at least one of these programs, carry general liability and pollution liability insurance, and provide written warranties on their installations (typically 5 years on workmanship, 1 year on the fan).
Front Range homeowners can verify Colorado-licensed mitigation professionals through the Colorado Department of Public Health and Environment radon program. The state maintains a public lookup tool that confirms certification status and shows any disciplinary actions. Selecting a contractor from the verified list is the simplest way to ensure professional standards.
References
- EPA Consumer’s Guide to Radon Reduction — U.S. Environmental Protection Agency
- Colorado Radon Program — Colorado Department of Public Health and Environment
- American Lung Association Radon Resources — American Lung Association
- DOE Whole-House Ventilation Guidance — U.S. Department of Energy
Front Range homeowners considering mitigation can reach out to a vetted local inspector to coordinate testing and discuss the right system design for the home’s foundation type.