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Vapor Mitigation: Challenges, Sub-Slab Solutions, and the Correct Approach for Your Building

Last month, we addressed the potential for vapor intrusion and the investigation and mitigation of vapor intrusion; and the potential for organic vapors to enter residential, commercial, or industrial spaces with adverse human health effects. Remediating the source of those organic vapors or mitigating the exposure potential through control of environmental conditions within or beneath the building indoor space can be an engineering challenge.

The factors that go into the best solution include, but are not limited to:

  • New versus existing construction
  • The actual source and mass of the contaminants
  • The media that one is trying to influence (e.g., soil, groundwater, simply soil vapor)
  • The size of the structure being addressed
  • Age
  • Building construction materials
  • Design and the building HVAC controls

Once the potential for vapor intrusion is established through proper engineering and scientific investigation (as influenced by requirements or guidance where they exist), one must assemble the information on the structure, or portion of the structure to be mitigated, and then design and value-engineer a solution that meets several potentially competing objectives.

Our consultants have been performing this work for over 25 years although many states have not had published guidance until the last decade, if at all. Our work has ranged from large industrial manufacturing facilities, commercial buildings, office towers, strip malls, small one-story office complexes, to multi-family and single-family homes. The potential for and reality of vapor intrusion is not just associated with old industrial facilities.

Mitigation of actual or perceived vapor intrusion (it is not easy to absolutely know that vapors are entering a structure) is not the same as remediation of the source of those vapors. Sometimes it is acceptable, from a regulatory standpoint, to leave the source where it is. This may seem contradictory to environmental laws but is consistent with risk-based site cleanups which are acceptable at the federal and most state levels. Remediation will remove or destroy the source contaminants, where mitigation simply controls the exposure pathway. In the case of vapor intrusion, the main mitigation objective is to keep the vapors from entering an occupied space where inhalation exposure can lead to human health consequences.

If one can reasonably remediate the source of the vapors, or if you are a responsible party and required to do so, then remediation and mitigation may be the same. Although still costly, mitigation is usually less expensive and time-consuming than remediating the source of vapors. Typical capital or up-front costs for vapor mitigation systems can range from $2.00 to $6.00/ft2 of the ground level square footage of the structure. You may be required to have an environmental covenant or property deed restrictions on the property and may have to perform on-going monitoring and reporting.

As we briefly discussed last month, there are well-proven ways to mitigate vapor intrusion. While at first glance it may seem as simple as modifying a building’s HVAC system so that the building is always “pressurized” (and therefore, does not allow the vapors to enter into the building through maintenance of a pressure gradient), this method will likely prove to be impractical and costly.

Sealing a building’s floors, walls, sumps, utility corridors, or other direct pathways from underlying soils into a building also seems like an inexpensive mitigation step but can be hard to accomplish. It is comparable to sealing against water intrusion in basements constructed in saturated soils – a losing battle against Mother Nature.

So, depressurization or venting the “sub-slab” space beneath a building are the predominant choices for vapor mitigation of structures of all kinds. Typical sub-slab depressurization systems (SSDS) are very much like radon systems that you may have in your home. Sub-slab venting systems (SSVS) are a bit different from SSDS in that they typically include the introduction of fresh air into the sub-slab space which can lead to lower energy costs to provide mitigation protection.

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