Generally, there are two approaches to the construction of a vapor mitigation system. One option consists of the installation of a polyethylene or polyolefin tarp product. Another, more advanced construction product, uses a spray-applied composite membrane in addition to specified tarp materials to provide the spray material a substrate on which to adhere. The polyethylene/polyolefin barrier materials are often recycled from post-consumer products, making them less expensive. However, in the absence of a spray-applied composite membrane, these barriers are often difficult to seal around penetrating uprights and slab perimeters.
Such mitigation systems work well for new structures, but what if an unprotected building is already standing in an area where vapor intrusion is a potential risk? When a foundation is already laid, is there any way to retrofit a building so a chemical vapor barrier can be applied after it has been constructed? The short answer is ‘yes.’
According to EPA, recognition of soil vapor intrusion to buildings and other enclosed spaces only occurred in the 1980s, sparked by concerns over radon intrusion. (Visit www.epa.gov/vaporintrusion/what-vapor-intrusion for more information on vapor intrusion.) Hence, buildings constructed before 1980 and for some time after had no specifications to reduce risk of vapor intrusion.
With the passage of time, awareness of vapor intrusion from manmade chemicals contaminating soil, groundwater, and soil vapor has grown. Products specifically designed to retrofit an existing structure in order to mitigate vapor intrusion risk are available. These sealant materials are applied on top of the slab as opposed to a barrier installed below. They are distinguished from the aforementioned spray-applied coatings because they are rolled onto the sub-slab similarly to concrete paint or sealer.
Depending on the layout, these materials can be applied to both the basement floor and foundation walls. Many industrial building owners find this method attractive as it can act as a chemical vapor barrier as well as a finished floor surface.
So what steps should a building owner or facility manager take if their particular structure is, in fact, at risk for vapor intrusion?
Factors to consider
To develop an effective and efficient vapor mitigation plan, a few factors need to be taken into account. The first is location—where is the structure in relation to the contaminated vapor? The vapor plume must be characterized to take effective action—are there pressure differentials drawing contaminants from
the soil gas into the structure? Are there noticeable pathways in the form of cracks or utility penetrations allowing more access? Understanding the answers to these questions is imperative to assembling a good vapor mitigation strategy.
Before establishing a vapor mitigation strategy, a qualified professional should be contracted to assess the potential for vapor intrusion and sample as needed. It should be noted assessment of IAQ can be complicated by products (e.g. nail polish, paint, lubricants, inks, and dyes) routinely used inside the building. Understanding the types of volatile chemicals present will better assist the implementation of a vapor mitigation system.
HVAC systems on many buildings can potentially result in negative pressure on a building’s interior. When these negative pressures are present inside of a building, it is more likely at risk for vapor intrusion. This negative air pressure can draw soil vapor through the semi-permeable building slab, as well as through cracks and unsealed uprights into the breathable space.
Temperature differentials should also be factored into the equation. When there is a substantial temperature gradient between the insides of a structure and the subsurface below, the resulting negative air pressure can bring vapors into a building’s interior from below via the stack effect. Finally, wind blowing over a structure, along with stack effect, can create the negative pressure gradient necessary to permit vapor intrusion.
Another factor is the number of openings present in the slab concrete of a given structure. Openings such as cracks, open seams, and utility penetrations can be transmission conduits through which chemical vapors in the subsurface soil gas can pass. These vapors are much more likely to find their way into a building when their conduits are left open after installation of a vapor barrier mitigation system.