As long as there have been buildings, there have been windows. Humans crave natural light and have found ways to bring it inside for centuries. Glass helped achieve this feat while keeping unwanted elements out of the space.
“Throughout the history of man, there has been a history of desire for freedom and light,” says John Van Dine, president at SAGE Electrochromics Inc., Faribault, MN. “Since man came out of the cave, we’ve wanted to have it. We all feel better when we don’t feel trapped by the walls of a building. When we have a view and a connection to the outdoors, it’s a real boost to our psychological health.”
While the origin of glass goes back as far as 5,000 years, the use of windows dates back to the end of the third century, according to the McLean, VA-based National Glass Association.
The first window glass, purported to be developed by the Romans, started out as thick and translucent, and was made by hand. It served the purpose of letting light in, but it did not provide a view. However, in 1291, glassmakers on the Italian island of Murano perfected a clear, transparent glass that they called “cristallo.”
Craftsmen made window glass by hand well into the Middle Ages by blowing molten glass into a flat disc. The disc was spun, and centrifugal force caused the glass to thin out and flatten. These glass discs were then cut into small panes and pieced together to make windows.
Improvements to glass and its visibility continued. By the end of the 17th century, various techniques from England and France had improved views. By the 1900s, mass production became the norm. But, the drive to improve glass - not only for visibility, but also for energy efficiency and structural issues - continued.
The biggest evolution in windows over the years has centered on energy efficiency and comfort.
“Glass has had to progress substantially; it is a big element [in a] building [due to] its attractiveness,” notes Tom Schwartz, president and senior principal at Simpson Gumpertz & Heger Inc., Waltham, MA. “Glass was initially designed to bring light in and keep rain out, but as time has progressed, our demands on building enclosures have increased dramatically.”
Insulating glass (windows formed from multiple layers), Schwartz says, is the major trend of the century. “It’s not all that new, but the commercial success and attractiveness of it has grown in the last 40 years,” Schwartz says. “The 1970s energy crisis really boosted its profile.”
Early versions of insulating glass had at least two sheets of glass separated by a spacer to keep the window and the air inside hermetically sealed. These early windows were not reliable; seals failed frequently. Improvements since then have been dramatic, particularly in the last 30 years. Layers of glass have grown from two to three or four. Other products, such as suspended coated films (SCFs) in the air space, make for a lighter (but highly efficient) window that can greatly boost R-values.
The introduction of low-emissivity (low-E) coatings has also boosted window efficiencies. Applied to the glass surface, these spectrally selective coatings reflect certain energy wavelengths and have become widely accepted in commercial applications. “Energy and comfort is a big driver for what’s going on these days,” Schwartz says.
Experts believe that windows will become even smarter down the road. The introduction of insulating glass, spectrally selective coatings, and SCFs has jumpstarted more developments aimed at boosting energy and building efficiency.
Take electrochromic glass, for instance: These windows alter visual transmittance by an electrical current that changes transparency and optical quality. While this product is still relatively new and expensive, the technology offers promise.
“With the advent of electronically tinted glazings, you can look at glazings more as an energy/comfort/convenience appliance than just an exterior building material,” Van Dine points out. “Once you bring an electrical capability to a window, it can become a security device. You can tell if a window has been breached. You can also implement display mechanisms on the glazings so you can tell what the temperature or humidity is outside. You can also make them into signage. A number of things stem from the implementation of electronically tintable glass.”
Robin Suttell, based in Cleveland, is contributing editor at Buildings magazine.