Curtainwalls' appearance and durability can define buildings with their clean aesthetics and encompassing interior views. Architects have long desired to connect interior building space to the exterior-and curtainwalls, with their ability to incorporate large expanses of glass-let them do that.
Maximizing the sun with curtainwalls creates a healthy and energy-efficient building
A curtainwall’s ability to optimize natural daylight and reap its various benefits is becoming one of its strongest merits. And, today’s curtainwalls have become increasingly efficient at doing that with multi-use glass, specialty glass coatings, advanced thermal breaks and technology that enhances the architect’s understanding of the interaction between curtainwalls and the environment around them.
Orientation and Positioning
Building orientation plays a huge role in the curtainwall efficiency, and designers and architects must consider building location and positioning. Southernfacing facades (in the northern hemisphere) receive the most direct sun exposure throughout the year. “An architect/designer must design a building with this in mind to minimize the solar heat gain, but maximize the visible light,” says Chris Lipp, PE, protective glazing engineer, Kawneer Co. Inc., Norcross, Ga.
Sunlight has both direct and diffuse components. “Obviously, the latitude of the project site and orientation of the façades dictate the hourto- hour mix of these components, as well as the incident angle of incoming solar radiation,” says Steve Fronek, PE, vice president technical services, Wausau Window and Wall Systems, Wausau, Wis. “In higher latitudes, the sun stays lower in the sky, but can rise and set north of due east and west, causing early morning and evening glare issues. In lower latitudes, solar altitude is greater. There is no single configuration that is suitable for all locations in the U.S.”
When designing curtainwalls to optimize the sun, consider obstruction from adjacent buildings, vegetation and self-obstruction due to inside corners in the floor plan. “This may lead to somewhat different configurations at ground level, or on different building elevations,” says Fronek. “In other cases, high-ground reflectance can create glare problems. Adjacent buildings can have reflective surfaces, such as metal panels or glass façades that can significantly modify shading strategies, especially for north-facing windows where glare would normally be a secondary issue.
“Regardless of the use of innovative strategies such as light shelves, vertical façades are limited in their ability to provide natural daylight deep into perimeter spaces, especially under cloudy sky conditions where diffuse light dominates. Only by using atria and courtyards can the ‘daylit zone’ be expanded to include ‘bi-lateral’ daylighting into floor plans.”
Neighboring building positioning has become such an issue, there is even an ominous expression associated with it: urban thermal warfare. “This is when you’re building design and construction infringes upon the performance of another building or the environment in an adverse way,” says Jeff A. Benson, vice president of project management and field operations at Haley-Greer Inc., Dallas. “The responsibilities of an architect and designer do not end once the building has been designed in accordance to the owners’ needs for energy consumption and/or interior comfort for clients and the employees. I submit that the architect and designer should have an outward focus as well. The question should be asked, ‘How will this building affect other buildings that are nearby as well as the environment in the immediate vicinity?'”
Benson cites the Frank Gehry-designed Walt Disney Concert Hall, a structure “USA Today” wrote in 2004, “the glare off the shimmering stainless steel curves is so bad, it’s heating up nearby condos at least 15 degrees and forcing owners to crank up their air conditioners.”
Coatings
Today’s high-performance curtainwall glass provides maximum protection against ultra-violet light and solar heat gain, while allowing a high degree of visible light transmission, without the high reflectivity or glare entering the building. To obtain a low solar heat gain coefficient (SHGC) and thus reduce cooling loads, low-emissivity (low-E) coatings and reflective coatings can be used on curtainwalls. “These coatings continue to improve, with the development of spectrally selective low-E coatings that provide very low SHGC while maximizing visible transmittance (VT),” says Dean Lewis, educational and technical information manager at American Architectural Manufacturers Association (AAMA), Schaumburg, Ill. “In addition to SHGC, the light-to-solar gain (LSG) index is emerging as an important gauge of the efficiency of a glass product in transmitting daylight while blocking solar heat gain. LSG is the ratio between VT and SHGC (LSG=VT/SHGC). The higher the LSG is, the more energy efficient the glass product.” Glass companies continue to refine low-E coatings to improve U-factors while providing coatings that are neutral in appearance. “Coatings can be adjusted to create combinations that meet U-factor, SHGC and condensation resistance requirements at the best price point,” says Greg Galloway, product marketing manager, YKK AP America Inc., Austell, Ga.
Tinting
Some older technologies involve tinted/reflective curtainwall glass to reduce the solar heat gain; however, these often block a large amount of visible light desired by occupants. Dynamic glazing is a new, more expensive technology that can control the amount of visible light and solar heat gain by varying the tint within the glass. Some dynamic glazing systems can be easily switched on and off to accommodate the occupants’ comfort. “Exotic glazing offerings can be quite costly,” warns Galloway. “Energy analysis should always include the curtainwall system and the glazing to find the combination that achieves the desired performance at the lowest cost.” Electrochromic glass (ECM) in curtainwalls allow for light control and energy production. ECM glass options help save energy by tying into the building management system and ‘tinting’ when the system acknowledges a specific set of parameters. “ECM glass often times is more cost effective than shades and can be energy neutral, despite its electric consumption, as it saves on heating and cooling costs for the location,” says Greg Header, president/CEO, Solar Innovations Inc., Pine Grove, Pa. “ECM also provides a desirable aesthetic when compared to traditional blinds, and requires less cleaning and maintenance.”
Glass Works
A laminate placed in between curtainwall glass allows it to withstand windblown debris projected from natural disasters like hurricanes and twisters. Invisible looking through the glass, these layers allow curtainwall products to be used in areas prone to hazardous weather. “Inert gases-Argon, Xenon and Krypton, a chemical elemental and yes, Superman’s home planet-reduce the conductance between the panes of glass approximately 25 to 30 percent more than traditional air,” says Greg McKenna, PE, chief engineer at Kawneer. Warm edge spacers are another material used to improve curtainwall performance. These nonmetal spacers bond insulating glass units (IGUs), and provide greater internal gas retention and less conduction leading to lower U-factors and better overall glass performance.
High-performance insulated glass can be manufactured with thin layers of metal in a sputter-coat process. “Metals used in this process are: silver, titanium, copper, gold and pewter,” says Benson. “The different metals are designed to provide various performance characteristics and appearance. Hence, different metals create unique colors and provide designers with many options to achieve the aesthetics desired for their buildings.” Glass-reinforced nylon can be manufactured into thermally broken curtainwalls. “Though other materials can be utilized, glass reinforced nylon is commonly used for thermal break construction,” says Benson. “The glass-reinforced nylon is designed to work with the structural properties of the curtainwall framing member. In addition, the design completely separates the exterior metal from the interior metal, thus virtually eliminating heat and cold transfer through the curtainwall framing system. This type of thermal break construction enhances the performance of the curtainwall, but does not affect the aesthetics for the framing system.”
Visual effects of coatings can vary from transparent to reflective when viewed from the exterior. For example, the low-E coating in channel glass walls has a slight iridescent appearance. “Translucent insulation materials are available in some systems to increase the heat transfer resistance of the glass assembly,” says James Donoghue, technical services manager, Bendheim Wall Systems Inc., Passaic, N.J. “Insulation tends to change the visual appearance of curtainwalls. For example, insulated channel glass curtainwalls transmit approximately 23 to 48 percent of total visible light, appearing as nearly opaque white walls.”
Tilt, Tip and Cant
Curtainwall sun control and shading devices- whether cant, tilt or motorized-can dramatically reduce building peak heat gain and cooling requirements, improving the natural lighting quality of building interiors. Benson believes depending on the building orientation, and the amount and location of fenestration, reductions in annual cooling energy cost up to 15 percent savings can be achieved. These types of sun control and shading devices will also improve building occupant comfort by controlling glare. They are an effective tool for building owners, architects and designers when designing a building to achieve LEED certification.
Architects and designers have an array of options available from curtainwall manufacturers with regards to sun control and shading devices. These devices come in many of forms, including vertical and horizontal sunshades as well as internal light shelves. “Exterior shade screens electronically connected to light sensors can extend and rotate automatically in accordance with the incident solar radiation,” says Donoghue. “These screens can be glass or metal, and are available as add-ons to many new and existing curtainwall systems.”
Outrigger systems cantilever out from above the window. “In typical bolt-on aluminum applications, outrigger systems can be up to three feet in depth before requiring diagonal stays,” says Lipp. “In these larger projections, the dead load, wind load and snow/ice loads often create a moment too large to practically be resisted by a moment connection alone.” A single blade system can be installed horizontally or vertically, and is generally much shallower than the outrigger system.
For the single blade option, multiple blades can be used to cover the entire glazed surface. “The latitude of the building and the orientation of the elevation will determine the optimal depth and angle of the shading device,” says McKenna. “Even though there are seasonal changes of sun angle, the angle of tilt is generally set at the time of installation. There are some articulating sunshades on the market but the benefit of this feature may be overshadowed by the additional moving parts and cost.” “Computer renderings can be used to help design the system and determine the best location for shades, and simulate the sun’s location throughout the day and year,” says Header. “With correct placement and selection, projects can benefit from light diffusion and minimizing glare. Some of the available systems even operate like giant operable blinds, controlled by building management systems to maintain light levels.”
Curtainwall sun control and shading device performance is maximized when they are correctly matched to the sun angles for a specific location. “Operating shading systems can be adjusted manually or motorized/programmed for best performance throughout the day and year,” says Galloway. “Total life cycle cost should be carefully considered, as operating shading systems can require increasing maintenance and repair as they get older.” The above factors, as well as an increased awareness of daylighting, heat transfer and safety will become even more important factors in the utilization of glass in building curtainwalls. “New technologies will transform the building industry in the coming years, and make glass both an effective insulator and visually transparent,” forecasts Donoghue. “Nanotechnology products, such as graphene sheets, will have a tremendous effect.”