Written by John Tanaka,P.E., Senior Engineer and BBJ Group's Remediation Practice leader
OK, only the nerdiest engineer can love a vapor mitigation system, but the assessment and mitigation of vapors in buildings is squarely in the bullseye of regulatory agencies, making it an important element of projects with environmental concerns. This article will help you understand some key design and cost factors for sub-slab depressurization (SSD), one of the most common vapor mitigation techniques.
What is Sub-Slab Depressurization?
SSD was first developed in the early 1990s to fix radon gas problems in residences, and many of the original radon design concepts are used for current SSD systems. A typical SSD system uses a fan or blower connected to pipes installed in shallow pits beneath a building floor slab and exhausts the vapors to the atmosphere, thus preventing vapors from entering occupied spaces. The amount of negative pressure required (and hence the size of the blower) depends on two primary site conditions: 1) air flow in the material immediately beneath the slab; and 2) air flow and pressure variations caused by the building heating and air-conditioning systems. An SSD system typically requires relatively low vacuum, so blower sizes tend to be cost-efficient per building area covered.
Once a site has been adequately investigated, a design test should be performed to verify that SSD will be able to extract air from beneath the slab. The design test is a small-scale mock-up of a full-scale system during which small diameter extraction and monitoring points are installed. A portable blower is connected to the extraction point and the amount of negative pressure at the monitoring points (several to 50 feet or more away) is measured using hand-held instruments. Multiple tests may be required in different parts of a building, but field testing can typically be completed in one or two days.
The building sub-slab material directly influences air (and hence vapor) movement, which in turn dictates the number of required extraction points and blower size. For example, sandy material will typically require more SSD extraction points with a blower that has relatively high air flow. Conversely, clayey material will require fewer extraction points (less air flow) with a blower that can generate a higher vacuum. A simplified method to determine the number of extraction points is to determine the “radius of influence” around one extraction point and use this area of coverage to draw overlapping circles over the required mitigation area. Modeling can also be performed using concepts and formulas developed for groundwater pumping tests. More advanced analysis might be appropriate for larger sites or to limit SSD system installation to less than the entire building footprint.
SSD System Installation and Operation Costs
An often-quoted rule of thumb cost for an SSD system is $1 to $5 per square foot of building area (ITRC, 2007), and is a reasonable range for budgetary estimating. Factors that could cause installed costs to be on the higher end of this range include:
- Highly variable subsurface conditions (requiring more or closely spaced extraction points);
- High soil moisture or high groundwater table;
- Vapors that require treatment prior to discharge to the atmosphere (but this is uncommon);
- Special construction for piping systems, such as firestops through fire-rated walls;
- Larger blowers, condensate removal, specialized mechanical systems, upgraded or explosion-proof electrical systems, and more advanced controls; and
- Retro-fit requirements of existing buildings.
SSD systems are designed to prevent migration of vapors into occupied spaces rather than remediate impacted media (e.g., subsurface soil or groundwater) to specific cleanup criteria, so they operate as long as the potential for vapor intrusion exists. In a small commercial building, operation of an SSD may cost $100 to $200 per year or less (mostly electricity), so long-term operation may not be a significant budget concern. However, a large industrial building will likely use more or larger blowers and other equipment requiring routine maintenance, thus increasing maintenance costs.
An SSD system is a common and cost-effective approach to mitigate vapor intrusion, but collection of key data and development of basic design parameters are important steps to ensure successful operation. The chances of love are small, but understanding the basics could at least be the start of a beautiful friendship.
Interstate Technology Regulatory Council, 2007. Vapor Intrusion Pathway: A Practical Guideline. https://www.itrcweb.org/documents/VI-1.pdf