Layered Vapor Intrusion Mitigation: Field Insights on Integrating SSD Systems and Vapor Barriers
Robert C. Carvalho, P.G. CPG-10588
The Importance of Vapor Intrusion Strategies Vapor intrusion (VI) is a critical consideration in the redevelopment
of brownfields, the reuse of landfills, and the reuse and construction of contaminated sites. As expectations for long-term indoor air quality protections rise—both from regulators and the public— relying on a single mitigation method isn’t sufficient. Today’s best practices call for layered systems that combine sub-slab depressurization (SSD), passive or active venting, and vapor barriers to block and remove harmful vapors before they enter occupied structures.
According to the EPA’s Community Guide to Vapor Intrusion
Mitigation (
https://semspub.epa.gov/work/HQ/401625.pdf), several mitigation options are available for both new and existing buildings. These include:
• Installing vapor barriers,such as geomembranes or sprayed-on materials, beneath the slab, to prevent vapor entry.
• Passive venting systems, which use subsurface venting layers in combination with vapor barriers to enable vapors to dissipate into the outdoor air.
• Active sub-slab depressurization (SSD),inwhich ablowerand suction pit system pulls vapors from beneath the building and vents them safely outside.
Integrating these system components
creates a robust, multi-barrier defense against vapor intrusion. This layered methodology ensures durability, performance, and redundancy across a wide range of soil types, contamination levels, and structural footprints. This approach not only aligns with evolving EPA guidance but also reflects a growing industry standard that prioritizes long-term protection and adaptability in real-world conditions.
Start with the Site: Evaluating Risk and Selecting SSD Systems Every site requires a tailored strategy. Factors such as vapor
source type, soil permeability, building footprint, and occupancy type influence the selection of the system.
Passive SSD systems (SSDS) may be sufficient for small, stable
retail projects. But for high-volume footprints like warehouses, 42 TPG •
Jan.Feb.Mar 2026
Figure 1. Layered vapor intrusion mitigation system with GeoVent low-profile venting beneath a spray- applied vapor barrier. Gases are routed through vent risers for active SSD. Photo courtesy of CETCO.
that would be required for standard sub-slab pipe installations. These systems are highly compatible with vapor barriers and can be activated post-construction if necessary, offering flexibility as site conditions evolve.
Integrating Vapor Barriers and SSDS
The EPA’s Citizens’ Guide clearly outlines the role of vapor barriers as a primary line of defense. Spray-applied systems or sheet-based geomembranes help physically block the vapor intrusion pathway.
The correct installation sequence is critical: • Place venting layer and header piping • Install reinforced base fabric (for spray-applied membranes) • Apply vapor barrier: spray-applied or sheet layer
www.aipg.org
schools, or institutional buildings, active SSDS are typically required to establish and maintain sub-slab negative pressure. EPA guidance affirms this trend, recommending active systems for buildings where long-term reliability is essential.
Vapor mitigation system design begins early in the development
process to ensure the system can manage site-specific vapor levels and protect occupant safety. Our typical approach combines subsurface venting infrastructure with a robust vapor barrier, addressing both vapor transport and intrusion pathways as part of a unified mitigation system tailored to each project’s risk profile.
Low-Profile Venting: Efficient and Scalable On large projects, minimizing trenching saves time and cost.
Low-profile venting systems allow gas collection across the entire slab footprint without the need for deep or disruptive excavations
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