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

In my previous article I discussed the importance of following established design standards to ensure the effectiveness of vapor intrusion mitigation systems. However, the ultimate success of these engineered controls does not rely solely on their design. It also hinges heavily on rigorous construction inspections during installation. Without diligent oversight, even the most advanced systems can fail, leaving future occupants exposed to site contaminants.

The first line of defense against vapor intrusion is the sub-slab vapor barrier or membrane. Vapor intrusion membranes are engineered to resist chemical degradation and diffusion. However, a barrier is only effective if it remains completely contiguous. Movement of heavy machinery, sharp gravel, dropped tools, and worker foot traffic can easily puncture a membrane. Furthermore, sealing penetrations by utilities, columns, and perimeter foundations requires careful workmanship.

Construction inspections ensure that every seam is properly taped according to manufacturer specifications. Inspectors verify that all penetrations are sealed with appropriate boots or mastics, and that any accidental punctures are immediately repaired before the concrete slab is poured. Once concrete covers the barrier, identifying and fixing leaks becomes virtually impossible, making real-time inspection during installation non-negotiable.

Complementing the barrier is the active or passive sub-slab depressurization system (SSDS). An SSDS relies on a network of sub-slab pits or perforated piping in a permeable venting layer beneath the slab. This network creates a continuous low-pressure zone that intercepts vapors and vents them safely above the roofline. For an SSDS to function correctly, the sub-slab dynamics must allow for uniform airflow. Inspectors are responsible for verifying that the aggregate venting layer meets the specified thickness and gradation, ensuring it is free of fines that could clog the system. They also check the slope and placement of the piping network to prevent moisture accumulation or blockages that could choke the airflow. In some jurisdictions, environmental agencies and/or local building departments mandate post-installation testing (typically by measuring vacuum at predetermined monitoring points) to confirm system integrity before issuing certificates of occupancy. Performing frequent inspections to ensure strict adherence to project drawings and specifications helps to eliminate the possibility of disruptive, costly and time-consuming system retrofits after start-up.

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.