Multifamily Performance and Value
Opaque Walls and Punched Windows
In spite of the allure of the glass-box look and large windows expanses, many developers are using punched-out windows and more opaque wall surfaces in order to reduce operating costs and—where submetering is used—to reduce utility bills for tenants. “Many of these techniques are geared toward wood construction, which is very common for multifamily developments,” says Cosella-Dörken's Barrett. “They also address cladding materials that can cause moisture issues, such as manufactured stone and conventional stucco.”
According to Barrett, the issue of solar-driven moisture and inward vapor drive has been poorly understood, presenting a common cause of wetting and rot in sheathing behind masonry veneer. Essentially, the sun heating the outside of a wet stucco or manufactured-stone wall forms high vapor pressure that impels moisture further into the wall assembly and prevents it from drying to the outside. “It tends to move from high pressure to areas of low pressure, so the moisture is moving deeper into the wall,” he says.
One solution to the issue has been applying a dimensional, two-sided drainage sheet behind the cladding but exterior to the weather-resistive barrier (WRB). The drainage layer installs easily with standard roofing nails and helps drain more than the requisite 80 percent of moisture entering the wall.
Other enclosure systems are designed to provide drying and drainage, and have been adopted by many architects and their multifamily clients. Ventilated rainscreens, for example, have open-joint claddings designed for drainage and natural ventilation, helping to keep wall assemblies dry. Behind the rainscreen, the insulation and air barriers are protected from weather and solar degradation.
Novel technologies are improving air tightness and moisture control, including self-adhering, vapor-permeable air barrier and WRB materials used over exterior-grade drywall sheathing or concrete masonry. Eliminating leaks at staples, nails, and other fasteners, the self-adhered barriers exceed the relevant requirements of the Air Barrier Association of America (ABAA) and building codes referencing ASTM E2357, Standard Test Method for Determining Air Leakage of Air Barrier Assemblies. Yet the vapor permeable construction also allows moisture within the building enclosure to escape through the membrane via diffusion.
An example of the application of a simple ventilated rainscreen to multifamily developments is seen in projects like the new Bellingrath Town Residences in Atlanta, a grouping of eight unique, luxury town residences on Peachtree in the trendy Buckhead district. Designed by Harrison Design Associates, the linear townhome block is “true to classical forms, [to] make a statement that creates a timeless and lasting impression,” as the architects have stated. Yet the enclosure system uses high-end, modern technologies. About 25,000 square feet of ventilated rainscreen was installed over a polymeric, water-resistive barrier and OSB sheathing behind the limestone and brick façade. The result manages moisture flow in the wall assembly.
A ventilated rainscreen over a polymeric, water-resistive barrier protects the enclosure for Bellingrath Town Residences in Atlanta's Buckhead. Photos courtesy of Cosella-Dörken Products Inc. |
“Air tightness is critical to energy management,” says Barrett, noting that air leakage is responsible for up to one-third of lost heating and cooling energy in some older residential structures. “And moisture is the leading cause of building degradation, so these simple details are shown to make multifamily projects into much better investments.”
The focus on energy costs is reshaping how buildings are designed, leading to more use of high-efficiency design approaches that often exceed the U.S. Green Building Council's LEED requirements. For Museum Tower in the downtown Dallas arts district, for example, the developer's vision was of a classic, modern glass box with a feeling of immediacy and sweeping views of the city. Yet the 42-story, 115-unit luxury high-rise designed by architect Scott Johnson of Johnson Fain Partners, is designed to LEED Gold and uses surprisingly little energy considering its transparent envelope.
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(Pictured: Marselle Condominium, Seattle, WA. PB Architects. Photo by Matt Todd Photography, courtesy of WoodWorks.)
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