Vapor Intrusion: How a Building Works and Breathes
Vapor intrusion is a common issue that both environmental consultants and building scientists deal with every day. From an environmental perspective, vapor intrusion can come from old dry-cleaning chemical spills, various solvents, gasoline and diesel materials and much more. As a building scientist, I mainly deal with vapor intrusion in the form of water vapor. The principles are roughly the same, but it’s important to understand how a building lives and breathes throughout its lifetime, to help prevent harmful vapors from entering buildings.
There are some natural occurrences, such as the physics of air movement, height and weather that can cause adverse effects to building, but even the way we structure or mechanically ventilate a building can cause unforeseen effects. Water vapor can enter a building through concrete slab in the basement and have an effect on the flooring adhesives. The moisture finds its way through small vertical pores in the concrete where bleed water comes to the surface during concrete placement. Applying a topical vapor retarder under the concrete slab during construction, while not cheap, is the preferred method of stopping vapors from spreading. Similar to environmental vapors, this would then be taped and sealed.
Vapors can be pulled into buildings via hollow cores in concrete block foundations, or through mechanical systems that exhaust air. This needs to be considered no matter what kind of vapor you are dealing with. Today, internal building systems are asked to do a number of things: heat, cool, ventilate, dehumidify, and also deal with vapor. With the desired improvements in building energy efficiency, we are reducing the amount of outside air that comes into mechanical systems. Typically we like to see buildings maintain a slight positive pressure to the exterior to prevent drawing in cold or hot air during the different seasons. If not controlled, this can seriously increase your energy costs.
Some systems now use special ventilation methods of stack effect, solar vents, carbon dioxide monitoring and active wall opening systems to allow mechanical systems to be shut off for up to 45% of the day. This can greatly change how air enters or exits a building, so the system must be dependable in its operation for the vapor mitigation system to work properly. If the natural, mechanical and mitigation system don’t all work together, then additional engineered systems may be required. For example, we are currently working on a high rise in Chicago. The exterior walls of the building are being subjected to positive and negative pressures of the wind or the suction of the wind, in the range or 120-150 pounds per square foot. Small leaks in the building can now pull air into the structure when the building is operating about neutral, once the wind picks up.
Vapor mitigation systems are not usually thought to address these additional building and weather influences, but deal primarily with creating a barrier in a mitigation system. The more we understand how a building works, however, the better we can address the influence it has on a mitigation system. For more information, contact Steve Flaten, or click here to view a webinar recording on this topic.