There was a time when the buildings being built in New York City were largely  constructed of earthy materials—limestone, brownstone, brick, terra cotta—and glass really only figured into their design in the form of windows and  skylights. As times and architectural fashions changed however, glass and steel  gained a foothold among all the stone and marble, until gleaming glass facades  became the norm rather than the exception—particularly in new residential development.  

 The Glass Age

 Glass itself has come a very long way since it was just the stuff windows were  made of. Today, its colors, tints, finishes, and treatments are nearly  limitless—and it's strong enough to be a building material in its own right.  

 “We cannot ascertain that glass has overtaken other building materials,” says Steven Jayson, owner of architectural glass manufacturer S.A. Bendheim,  Ltd., in Passaic, New Jersey. “But it's easy to see the unique benefits, such as daylighting and views, that  glass can bring to areas dominated by tall buildings and skyscrapers.”  

 Jayson says that the evolution of the architectural glass industry can also be  credited with the increased use of glass in buildings.  

 “These developments include the fact that the glass manufacturing process has  been designed to conserve resources and allow for cost-effective production,” he says. “There’s also the development of energy-efficient glass products, including thermal  coatings and insulated glass wall systems; and of course the incredible  decorative glass options that can introduce a unique, customizable aesthetic.”  

 These options include etched patterns, textures, decorative interlayers, colors  and more. Glass can be laminated with other glass types for strength, or to  create a completely new aesthetic. “It can be combined with a decorative interlayer such as a fabric or wood  veneers,” says Jayson. “It can be insulated, tempered, bent, color coated in a standard or custom color,  or acid etched with a decorative pattern. There are hardly any limitations to  customizing architectural glass. Thermal performance coatings such as our  channel glass Low-E, azure, and bronze coatings are interesting because they  simultaneously provide energy efficiency and attractive visual effects.”  

 Windows can also be tinted almost any color, or may have a partially or  completely reflective coating. “Architects sometimes use the tinted coatings to hide irregular interiors  (curtains, blinds, etc.) so that the building exterior can appear more  harmonious,” says Douglas J. Lister, an architect based in New York City. “Solid ceramic or painted coatings can also be applied to glass. These coatings can create patterns on the buildings and give added privacy to  occupants.”  

 Jayson explains that another reason glass is becoming so popular is because of  its daylighting properties—so important in a city where buildings can be mere feet away from each other,  and high-rises loom over shorter buildings, often blocking out the sun.  

 “The see-through quality of glass allows daylight and opens up spaces, reducing  the need for artificial lighting and thus conserving energy,” says Jayson. “Naturally day-lit spaces have also been proven to create a better work and  living environment.”  

 One Example

 The recent technological and aesthetic advancements in architectural glass—as well as its accompanying decline in cost—made it the material of choice in many (if not most) of the new condo  developments that went up during the city's residential construction boom in  the early 2000s, from five-story lofts downtown to towering high-rises in the  revitalized Hell's Kitchen.  

 One example of the latter is the Orion, a 60-story, 650,000-square-foot condo  tower at 350 West 42nd Street designed by Cetra Ruddy Architecture + Interior  Design. The building made use of HYBRID-WALL, a new cladding system developed  by Canadian manufacturer Sota Glazing. According to Keith Goich, Cetra Ruddy's  project executive on the Orion, “The client was thinking of a traditional brick face over concrete masonry unit,” but the benefits—both economic and aesthetic—of the hybrid glass panel system made it the clear choice, literally. “Once we ran all the numbers,” says Goich, “it turned out to be more cost effective than a traditional system.”  

 Using glass also gained precious square footage in the Orion's apartment units.  According to Goich, the Orion’s exterior walls are 8.25 inches thick, as opposed to the 18 inches required for  a traditional cladding system. “This is phenomenal in terms of layout and selling points,” he says, but using glass also enabled the design team to achieve their creative  vision in a way that masonry would have limited. “Usually in a residential building you see different sized windows,” says Goich, “but I didn’t want an odd pattern. I wanted the building to look monolithic.” Working with the greater design possibilities of a glass curtain wall allowed  the architects to realize their monolithic vision, cladding the Orion in  three-foot-one-inch-wide panels that obscured divisions between floors and gave  the building an almost-seamless “skin.”  

 Green is Good

 Glass also lends itself well to energy-efficiency. According to Lister, in  modern “green” buildings, glass is key in controlling the amount of heat and light that passes  into and out of the structure. This permeability is measured by what are  referred to as “R” and “U” values. The u-value measures the heat that is gained or lost due to the  difference in the indoor and outdoor temperatures. The lower the U, the less  heat is transmitted. The higher the R value, the greater its resistance to heat  transfer.  

 Working with higher-rated glass enables builders to improve their projects'  energy efficiency, and knowing the R and U values of the glass in their  building can help co-op and condo administrators make better choices to reduce  energy costs.  

 “New glazing units are being installed that have extremely good insulating  properties,” says Lister. “They contain additional layers of glass or clear plastic. This glazing may have eight times—or more—of the insulating properties of regular single glazing.”  

 “Specifiers who draw up plans and building owners now recognize that glass can be  an attractive and green option, further encouraging its use in buildings, both  on the exterior and the interior,” adds Jayson.  

 Speaking of costs, Lister says, “Glass curtain walls are usually more expensive than masonry materials—but they make more sense from a marketing perspectiveif the building has a great view.On some building sites, it also makes sense to have prefabricated glass curtain  wall sections delivered to the site rather than build a masonry wall on-site.Glass curtain walls are usually faster to build than masonry, and speed saves  the developer money.”  

 How it's Made

 Of course, one of the most important concerns about an all-glass building is  privacy, says Jayson—but there are engineering requirements as well. “The see-through property of glass can complicate the insulating aspects of the  building envelope,” he says.  

 “Otherwise, glass facades have pretty much the same areas of concern as masonry  ones. They have to be designed to withstand specific weights, wind loads,  stress, deflection, etc. within the assigned span limit. While glass is more  susceptible to breakage than other materials like granite, even granite facades  have to be designed with the same set of considerations in mind.”  

 It’s important to understand that there is a difference between architectural glass  and window glass that you might buy in a hardware store. “The most important difference is that it is almost always double-glazed—two pieces of glass are glued together with sealant but kept about 3/4" apart to  create an airtight space,” says Lister. “This airspace approximately doubles the insulation value of a glazing unit  compared to a conventional piece of single glass.This airspace can befilled with special gases (argon or xenon) which can almost double the  insulation value again (4x the insulation of a single glass).”  

 Layering also makes glass far more resistant to damage from attempted break-ins,  vandalism, and natural disasters like storms and earthquakes, says Lister. “That's achieved by tempering, which is a heat treatment process, and laminating  multiple layers of glass with a tough, clear polymer plastic between each  layer. Glass terrace doors are usually made from laminated glass, for example.”  

 Another difference is in the coating. “The glass in a glazing unit frequently hasan almost invisible 'Low-E', or low emissivity coating that partially reflects  thermal radiation, but mostly permits sunlight's visible radiation through,” Lister continues. “This reduces heat gain from the window in warm weather.Low-E coatings are most desirable in east and west facing windows that get the  full brunt of direct sunlight in the morning or evening.”  

 Architectural glazing units also differ from more conventional glass in the way  they are installed in a window frame. “Single glass is usually waterproofed with glazing putty, which is available at  any hardware store,” says Lister. “Architectural glazing units are very carefully installed using dry gasket  systems.”  

 Making it Last

 Once installed, architectural glass has a long life span, but exactly how long  depends on the quality of the installation. “This can be maximized by selecting a glass manufacturer and supplier together  with a glass installer that have strong reputations for quality and expertise  in the industry,” says Jayson.  

 The first glass curtain wall building—The Hallidie Building in San Francisco— was built about 90 years ago, so Lister explains that there isn't as much of a  tradesman's tradition of maintaining and preserving curtain wall buildings as there is for other materials like brick or limestone.  

 Older buildings that had regular steel connections between the curtain wall and  the steel frame have since required complete replacement of the curtain wall,” he says. “Lever House, for example, on Park Avenue in Manhattan was built in 1952 and  reskinned in 2000.Newer buildings use stainless steel and aluminum, which are much less prone to  deterioration than regular steel.”  

 Most failures, he explains, occur in the gasket systems between the glass and  the frame.“Double-glazed systems will eventually fail and allow air and moisture to enter  the space between the two layers of glass,” Lister continues. “The moisture and dust clouds the glazing unit.Gasket and glazing unit failures occur between 25 to 50 years after a building  is built.Most failures can be addressed by replacing the glass, but leaving the frame in  place.This work obviously has a major impact on residents.”  

 On the other hand, Lister says that stone, terra cotta and brick usually last  hundreds of years.  

 Taking care of the glass will help it to last longer, “The maintenance of glass facades can depend on the specific glass used and the  site,” says Jayson. “Important considerations include glass coatings—the permanence of any applied decorative or performance finishes—and access to the glass itself. Every installation is different, but most glass  can be cleaned with a simple squeegee and warm water or conventional,  non-abrasive glass cleaners.”  

 As time goes on and fashion and economics shift, glass has arisen as a key  component of modern architecture. Its endless possibilities and surprising  strength have taken it out of its usual place in the window and fully  assimilated it into nearly every other part of the building.    

 Lisa Iannucci is a freelance writer and author, who lives in Poughkeepsie, New  York.