Breaking Old Rules for Air-Barrier Installation
CASE STUDY: BANKS PHASE II
The performance potential of a self-adhered membrane was demonstrated recently in the Banks Phase II project in Cincinnati, Ohio, where the designers changed from a fluid-applied air barrier to a self-adhered, water-resistive, air-barrier sheet membrane.
Located between Paul Brown Stadium and Great American Ballpark in downtown Cincinnati, The Banks project is a multi-stage development featuring retail, office space, hotel rooms, and residential space. The second of three phases is a 291-apartment mixed-use building that also features 20,000 square feet of retail space.
The senior estimator at the time, Josh Turner, explains why his company switched from fluid-applied to self-adhered barrier.
“The major reasons we switched to the sheet-applied were, number one, the temperature at which we were going to begin installing the air-barrier membrane was going to be too cold for fluid-applied,” he says. “And the other thing was that the building was going together in sections, floor-by-floor. And to have a crew to come in and do the fluid-applied would have been 12 to 15 mobilizations on the project. So what we sold the contractor on was that we could do a product that we could install on smaller sections at a time that would be easier without having all the equipment to set up, and that way we could progress as the building went up.”
Turner added that the self-adhered membrane was also better than fluid-applied because of the wind factor.
“In downtown, there is quite a bit of wind,” he notes. “And that was the other thing we were worried about with fluid-applied was overspray on the windows below.” In some parts of the project, the finished portion was below the point where the membrane would be installed. The cost and trouble of masking off the floor below to protect it from the spray above would have been prohibitive.
Photos courtesy of VaproShield
There was 150,000 square feet of self-adhered, water-resistive, vapor-permeable air-barrier sheet membrane installed on The Banks Phase II mixed-use building (top). Shims used under horizontal cladding attachment components create a ¼-inch rainscreen cavity, allowing for unimpeded vertical drainage of moisture away from the building envelope (bottom).
It is primarily the air-transported moisture that moves through the building’s envelope that is a problem.
When surfaces reach dew point, the vapor in the air or from vapor diffusion condenses, changing into liquid water. If this water is trapped in a wall, floor, or roof cavity and is not allowed to dry, it can, over time, cause problems. It not only degrades the building materials that are food sources for fungus, it also causes oxidations that will rust metal studs and other metallic wall components.
Ideally, a building envelope membrane is resistant to liquid water and air, but allows vapor to enter and exit, within certain bounds. Vapor movement is the rate at which water vapor can diffuse through a material, known as its water vapor transmission rate (WVTR) or water vapor permeance.
The permeance is measured in perms, where one perm is one grain of water per hour through 1 square foot of material, induced by a vapor pressure difference of 1 inch of mercury across the two surfaces of the material. Its driving force is a difference in vapor pressure, and its movement is always high to low. A high vapor pressure means that the number of water vapor molecules in the air is high. Vapor diffusion is different than air transport or convective vapor transport. And it is typically very slow.
Image courtesy of VaproShield
The stack effect brings in outside moisture and pollutants and adds to the work of the HVAC system.
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