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Why Monitor Differential Pressure for Vapor Intrusion?

By: Craig A. Cox, CPG

Published in the June 2020 VAPOR PIN® Newsletter

USEPA and others in the vapor intrusion field have been evaluating a variety of Indicators, Tracers, and Surrogates (ITS) to assess their use in predicting the best  time to collect representative indoor air samples for vapor intrusion studies (Schuver, et. al., 2018).  The idea is that if a predictive combination of easily obtainable, low cost ITS can be identified, they could be used to improve the collection of actionable analytical data at a lower cost.

For vapor intrusion assessments, typical indicators include seasons of the year, wind speed, the difference between the indoor and outdoor temperatures (differential temperature), barometric trends, and the difference between sub-slab and indoor air pressure (differential pressure).  In this article, I will focus on differential pressure.

Differential pressure is a useful indicator for a number of reasons and is “baked in the cake” when it comes to designing and assessing the performance and long-term monitoring of sub-slab depressurization systems.

If the pressure beneath the slab is greater than the indoor air space, there will be a tendency for sub-slab air to migrate into the indoor air space through advection.  The reverse situation should also occur if the pressure beneath the slab is less than the pressure of the indoor air space.  This is why vapor intrusion and radon mitigation can be accomplished through the use of sub-slab depressurization systems.  In fact, the differential pressure required is surprisingly low.  In Ohio, the standard differential pressure for radon mitigation systems is only -0.020 inches of water.

Differential pressure is generally measured with a hand-held manometer.  Recently, however, sensitive differential pressure sensors have become available as part of the “Internet of Things” (IOT) revolution.  These sensors can be connected to permanent sub-slab monitoring points, such as the Vapor Pin®, to collect and transmit differential pressure readings to the web at preset intervals.  They can also be used to set alarm point that will notify users of system faults or other unacceptable conditions.

I have been experimenting with one of the sensors at my home, which has a radon mitigation system.  The graph below plots a segment of the collected data (differential pressure measured in inches of water).  The graph clearly demonstrates that my radon mitigation system is producing a differential pressure of approximately -0.050 inches of water, which exceeds the minimum standard of -0.020 inches of water.  The graph also indicates minimal day-to-day fluctuation with the system running over a period of approximately 10 days. To test how fast the system responds to a system fault, I turned off the radon system for a period of approximately 24 hours.  As you can see differential pressure responded very quickly after the system was turned off and again when it was turned back on.

The sensors connect wirelessly to a data hub that feeds data through the internet to a website.  The cost of a sensor and a data hub is approximately $500.  Data housing and reporting plans through the website can be very inexpensive.  The plan I have been using costs less than $40 per year.

I believe that over the next year or so, sensors like these will become common place and used by field crews to monitor conditions leading up to and throughout sub-slab and indoor air sampling campaigns.  They will also be used to collect and record the results of pressure field testing, whole building pressurization studies, long-term sub-slab depressurization system monitoring, and during high volume sub-slab sampling tests.

Differential pressure is a very powerful and easily obtainable indicator of conditions that drive vapor intrusion.  However, what underlying factors cause the pressure differential. In the next article, I will review two of these potential drivers – temperature and barometric pressure?


Schuver, H, Lutes, C, Kurtz, J, Holton, C, Truesdale, RS. Chlorinated vapor intrusion indicators, tracers, and surrogates (ITS): Supplemental measurements for minimizing the number of chemical indoor air samples—Part 1: Vapor intrusion driving forces and related environmental factors. Remediation. 2018; 28: 7– 31.

Craig Cox is a principal and co-founder of Cox-Colvin & Associates, Inc., and holds degrees in geology and mineralogy from the Ohio State University and hydrogeology from the Colorado School of Mines. Mr. Cox has over 30 years of experience managing large environmental project implemented under CERCLA and state voluntary action programs. Mr. Cox is the inventor of the Vapor Pin® and has developed a variety of software products including Data Inspector, an internet-enabled environmental database application. Mr. Cox is a Certified Professional Geologist (CPG) with AIPG and is a Certified Professional (CP) under Ohio EPA’s Voluntary Action Program.