On Spec: 5 Things to Know About Vapor Intrusion

5 Things to Know PhotosWhat is the difference between a vapor and a moisture barrier?

Vapor barriers are often confused with moisture barriers. In actuality, they are two different types of products with two very different purposes. A moisture barrier is designed to keep water in its liquid form from entering a building enclosure. It keeps water from building up behind walls and under slabs to prevent the growth of mold.

A vapor barrier, however, is part of a complete vapor mitigation system in order to prevent vapor intrusion. Vapor intrusion can occur when volatile chemicals are present in soil, soil gas and/or groundwater, and migrate their way through subsurface soils and into the breathing zone of homes and commercial buildings much in the same way as radon gas. A vapor mitigation system often consists of two main parts: a trenched channel to guide vapor to upright flues and out of the building, and a chemical resistant layer (the vapor barrier) that blocks vapors from entering the breathing zone of a building.

One of the biggest mistakes in specifying a vapor barrier is not being clear to the contractor about what type is required.

When is it necessary to recommend a vapor barrier?

The information needed for recommendation of a vapor barrier is often discovered in the due diligence process, as Phase I and/or Phase II activities can uncover potential or actual sources of contaminated vapor on or adjacent to the property. The risk of vapor intrusion occurs when there is the potential of a migration of volatile chemicals, possibly including petroleum products, from contaminated groundwater, soil or a vapor plume into an overlying building. If it is a site that has the potential for these types of environmental issues, a vapor mitigation system is necessary. The wrong barrier may be incorrectly specified if the site’s history is unknown. There are obvious cases when a known operation previously existed, such as gas station or dry cleaner. Now that the industry has a way to address vapor intrusion, architects need to think about this risk in order to prevent potential health concerns and possible future litigation.

What are the different variations of vapor barriers?

Several types of vapor barriers available. There are pros and cons associated with choosing one material over the other, primarily cost versus effectiveness. It is critical that the correct barrier be chosen for the vapor mitigation system. For example, one type of material used is polyethylene or polyolefin, in both a 6-mil or 10-mil variety. These are generally the less expensive options, and are often made from post-consumer recycled materials. However, it is not a guarantee that the material is chemically resistant. Additionally, due to the nature of the material, it is difficult to seal the barrier at walls and utility penetrations. Alternatively, there are barriers that take it a step further with a composite membrane system that combines the advantage of chemically resistant high-density polyethylene and the constructability benefits of spray-applied membranes. This system offers the lowest diffusion rates in the industry, resulting in higher effectiveness for the chemical or chemicals of concern.

Is it necessary at all if designs are properly ventilated?

The answer is yes. Determining whether or not a vapor mitigation system is necessary is not reliant on whether or not the ventilation system is set up properly. There are often times when dangerous chemical vapors are present in the subsurface soil gas of a planned construction site for many reasons. For example, if there had been a dry cleaner operating nearby releasing hazardous chemicals, there is at least a good chance that the soil is contaminated and a vapor barrier is necessary so that these vapors do not make their way into the breathable area of a structure. A building over a vapor plume creates an atmosphere which promotes vapor intrusion. This phenomenon is caused by pressure differentials between the interior of the building and the relatively higher pressure of the gasses in the subsurface soil vapor, which creates a negative pressure gradient that draws vapors into the building.

What is the biggest mistake made in specifying a chemical resistant vapor barrier?

One of the biggest mistakes made in specifying a vapor barrier is not being clear to the contractor about what type of barrier is required. As this is a relatively new concept, contractors may misinterpret specifications for a vapor barrier for a simple water barrier. Therefore, if it is recognized that a vapor barrier is necessary due to the environmental condition of the site, it is paramount that the architect specifies the chemically resistant barrier and/or subsurface vent channels, and exhaust risers, as well as whether the venting system is to be passive or active. If these are not clearly distinguished, there is a chance that the contractor may make the mistake of assuming that a water barrier is being specified not a chemical vapor mitigation barrier.

Published in Architectural Products Magazine