Sub-Slab Depressurization Systems – A Brief Overview

Posted on May 29th, 2014

Existing buildings over groundwater and residual soil contamination can often affect indoor air quality, or a “vapor concern.”  Mitigation of a vapor concern can include excavation and removal of contaminated soil from the site; however, this is often not possible or practical. 

The institution of a sub-slab depressurization system (SSDS) is often a viable solution.  These systems incorporate the use of inline fan blowers to capture the harmful vapors emanating from the soil.  PVC piping is installed to these blowers to a small hole in the sub-slab, suspended just above the contaminated soil.  These vapors are then expelled from a vent (usually placed on top of a roof) and allowed to disperse into the atmosphere.  A sample figure is shown below.

The size of these fan blowers are determined by by measuring the atmospheric pressure beneath the building slab.  This will determine a radius of influence for the blower vents beneath the sub slab. 

Following the installation of the SSDS system, additional indoor air sampling is required to evaluate the effectiveness of the system.  Further testing would be required during heating months along with routine inspections to remain in compliance. 

Compared to a complex soil excavation, an SSDS system is a relatively inexpensive alternative to address harmful vapor intrusion.  Furthermore, the system itself is comprised of 4-inch PVC piping, which is relatively easy to conceal behind and in between drywall and crossbeams within a building. 



 Figure courtesy of Montana Department of Environmental Quality (

Here is a link to the NJDEP Vapor Intrusion Guidance (March 2013):


Nicholas Mazza
Environmental Scientist

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