Clearing The Air: Understanding Vapor Intrusion And Vapor Intrusion Mitigation Systems – Part 2

In Part 1, we discussed what vapor intrusion is, it’s impacts to indoor air, and how to identify it. But how do we prevent or mitigate the impacts of vapor intrusion? This article will help you to understand vapor intrusion mitigation systems, and how they can help you reduce your risk.

Vapor Intrusion Mitigation Systems: The Solution

In our previous blog, we discussed vapor intrusion and the risks it can pose to occupants and future uses of buildings and properties. Fortunately, some steps can be taken to mitigate the impact of vapor intrusion. One of the most effective methods is to install a vapor intrusion mitigation system. A vapor intrusion mitigation system (VIMS) creates a barrier between contaminated soil or groundwater and the indoor air, preventing the contaminated vapors from entering the building and ensuring the indoor air quality remains safe for occupants. A VIMS does not remediate the source of the contamination.

There are two types of vapor intrusion mitigation systems: active and passive. The main difference between the two is the way they operate to reduce the concentration of volatile VOCs in indoor air spaces. Active mitigation systems are designed to actively remove or divert contaminated air from indoor spaces and vent it to the outside through the use of mechanical equipment such as fans. Examples of active mitigation systems include:

• Sub-slab depressurization systems: These systems create a negative pressure under the foundation to prevent the contaminated vapors from entering the building. This is the most common mitigation system.
• Sub-membrane depressurization systems: These systems operate similarly to a sub-slab depressurization system. Instead of drawing air from beneath the building's foundation, it draws air from beneath a membrane or vapor barrier that has been installed over the soil or building foundation.
• Ventilation systems: Ventilation systems are designed to remove contaminated air from the building and replace it with fresh air. This can be accomplished by either using the existing heating, ventilation, and air conditioning (HVAC) system or installing a dedicated mechanical ventilation system.

Passive mitigation systems, on the other hand, do not require the use of mechanical equipment to remove contaminated air from indoor spaces. Instead, these systems rely on natural air movement and pressure differences to reduce VOC concentration in indoor air spaces. Some of the most common systems include:

• Vapor Barriers: A vapor barrier is a physical barrier, such as a plastic sheet, installed beneath the building foundation to prevent VOC migration from the soil or groundwater into the building. The vapor barrier acts as a barrier to airflow and vapors.
• Sub-slab Venting: This type of passive mitigation system involves the installation of vents in the foundation or slab of a building to allow air to move beneath the slab. The vents allow air to move freely beneath the slab, reducing the pressure differential that can cause vapor intrusion.
• Natural Ventilation: Natural ventilation involves using windows, doors, and other openings in a building to allow fresh air to enter and contaminated air to exit. This can be an effective passive mitigation strategy in situations with low VOC concentration.

Selecting the Right Vapor Intrusion Mitigation System

The type of vapor intrusion mitigation system appropriate for a particular building will depend on several factors, including the type and source of the contamination, the concentration of the contaminant, the building design and construction, local climate and weather conditions, and the specific needs of the occupants. For further details about the design and cost factors to consider, look at this article by BBJ Group’s John Tanaka, P.E.

While vapor intrusion poses a risk and potential threat to indoor air quality, using a VIMS is a critical component of indoor air quality management, especially in areas where the soil or groundwater is contaminated with volatile chemicals. By preventing the migration of volatile chemicals into indoor air spaces, a VIMS helps to protect the health and safety of building occupants, allowing everyone to breathe easily.