Sub Slab Depressurization System (SSDS) Services

What is a Sub Slab Depressurization (SSD) System or SSDS?

Conceptually, a sub slab depressurization or mitigation system is quite simple.

A pipe, typically PVC, is installed through the building’s concrete floor slab (or basement floor) to create a “vapor extraction point”. The vapor extraction point is then vented outside through the wall or roof. A blower or fan is then connected to the piping, which generates suction, creating a vacuum beneath the building. The vacuum draws in contaminated vapors from the subsurface soil and sends them up the pipe to the atmosphere, where they dissipate. Collectively, this is called a sub-slab depressurization system. The system needs to be designed to ensure that the vacuum is strong enough to prevent vapors from entering the building other than at the extraction point. In certain scenarios, the system can operate passively (without the blower or fan), as a sub slab ventilation system.

SSDS Schematic

Source: New Jersey Department of Environmental Protection
Depending on the chemical, the concentrations, and the flow rate, the air being vented from the system may require treatment (typically granular activated carbon) prior to discharge.

A site investigation and engineering analysis must be performed as part of the vapor intrusion solution and be designed by competent experts. Subsurface gas transport is highly variable and can be strongly affected by the building’s construction, the presence of subsurface barriers (e.g., old foundation walls), mechanical ventilation, cooling and heating systems in the building, the condition of the floor slab, and other variables.

The first step is to determine the location and concentrations of the chemicals in the subsurface. Then a pilot study or pressure field extension test using Pressure Points will determine how many extraction points are required, where they should be located, and what flow rate and vacuum is needed to ensure full protection. When properly designed, installed, and operated, these vapor intrusion systems are fully capable of rendering indoor air concentrations of the contaminants to below detectable levels. Cooperstown has scores of successful projects where our services have secured the safety of building occupants.