by James P. Cinelli, P.E., P.G., BCEE, Liberty Environmental

When designing sub-slab depressurization systems (SSDSs) for vapor intrusion mitigation, several key steps in the evaluation and design process are critical to avoid over- or under-designing the system. A few of these are outlined below.

Seal Off Entry Points

After confirming that vapor intrusion is a concern—and before beginning system design—identify and seal all potential vapor entry points, then retest indoor air. Eliminating preferential pathways may be sufficient to meet indoor air quality goals without additional mitigation.

If further mitigation is required, sealing entry points also helps prevent system failures caused by short-circuiting.

Perform Pressure Field Extension (PFE) Testing on Existing Buildings

In older buildings, the gravel layer beneath slabs and foundations may be thin or absent, limiting how far a vacuum can extend from a depressurization point.

Pressure Field Extension (PFE) testing—also known as communication testing—involves applying vacuums at varying strengths and locations to determine:

• the radius of influence, and
• the vacuum required for effective depressurization

Designers use these data to determine fan sizing and riser placement, making PFE testing a critical step in the design process.

Design New Buildings According to ANSI/AARST Standards

New construction typically includes a porous gravel layer (the “soil gas collection plenum”) beneath slabs and foundations. ANSI/AARST standards guide the design by specifying:

• appropriate gravel types (e.g., AASHTO #5, #7, #57),
• gravel thickness (e.g., 4", 6"), and
• pipe diameters (e.g., 3", 4", 6")

The number of extraction points (risers) is determined using standard guidelines based on floor area, rather than field testing. Factors influencing the number of risers include:

• Riser diameter
• Presence of a vapor barrier
• Thickness of the gas-permeable layer
• Ability to inspect the plenum

Inspect System Installation During Construction

ANSI/AARST standards allow for fewer extraction points when inspections are performed during installation of the soil gas collection plenum. This is because improper material selection or placement can significantly reduce system performance.

Additionally, foundation configurations often change during construction. For these reasons, frequent inspections during plenum installation and foundation work are strongly recommended.

Conclusion

Effective vapor intrusion mitigation depends on a thoughtful, data-driven approach that integrates site conditions, sound design principles, and careful construction oversight. By sealing entry points, conducting appropriate testing, adhering to established standards, and verifying installation quality, practitioners can develop systems that are both efficient and reliable. Taking these steps not only improves system performance but also reduces the risk of long-term operational issues, helping ensure that indoor air quality objectives are consistently achieved.

The Author:

James P. Cinelli is an environmental engineer and the president of Liberty Environmental, Inc., an environmental consulting firm which he co-founded in 2004. Mr. Cinelli has over 30 years of experience in environmental consulting with an emphasis in the areas of soil & groundwater remediation, brownfield redevelopment, and water resource engineering. He has provided expert witness testimony and has spoken at state and national conferences on topics such as stormwater management, erosion control, site remediation, and spill prevention.