EnergyWise House: Building a Smarter, More Efficient Window

Courtesy of BobVila.com

Homeowners love windows – the light they bring, the views they frame, the feel they give to homes. Energy experts hate windows – the heat they bring, the heat they drain, the added energy consumption they cause. The race is on to develop the technology that will allow builders to incorporate windows and homeowners to place them wherever they want without fear of skyrocketing energy costs.

Low-e Coatings

Windows have continued to improve over the years, first with insulated glass units that provided a buffer zone of air between two panes of glass to reduce the heat loss incurred by single pane units during cold weather. Improvements continued with the advent of low-e or low emissivity coatings. These microthin metallic coatings reflect heat, sending it back where it came from. Low-e coatings significantly improve the thermal quality of insulated glass units, helping homes to stay warmer in the winter and cooler in the summer. Adding argon or krypton gas fill between these coated panes adds to the insulative properties of the window unit.

At the high end of available window technology is spectrally selective coatings. The coatings are applied to the panes of glass to reduce heat gain by blocking selected rays from entering the home. This enhancement to low-e coated glass helps further reduce heat loss from inside and heat gain from outside. It also serves to protect furniture and fabrics within from sun bleaching. With gas-filled, spectrally selective coated glass in a well-constructed insulated glass unit, the R-value of a window can approach that of a well-insulated wall. Still, short of drawing the shades or awnings, beating sun challenges all windows and causes solar heat gain that can compromise any energy budget.

New Window Technology

Companies are now researching technologies that will allow maximum visible light to enter the home while blocking the rays that cause heat buildup within the home. Enter switchable windows that respond with a flick of the switch to darken, lighten, or almost completely block the sun from entering living space. Technology already exists to bring switchable privacy glass (SPG) to homes, but these windows don't block heat gain or ultra violet (UV) light penetration. SPG changes from clear to milky white when switched, which allows light to enter the interior space while the view is blocked from outside. Marvin Windows brought this technology to the residential market, but was forced to abandon it due to failures. "The quality of the glass and reject rate became frustrating for both our supplier and us and were hurting sales," says Anthony Head, Regulatory Product Planner for Marvin. With Marvin's longstanding commitment to quality, durability, and a 20-year warranty, they had no choice but to discontinue the line. "There's still quite a bit of it operational in the field," Head says. "We continue to monitor this technology today in our advanced research area. The most promising technology seems to be glass that isn't a privacy glass, but is a variable tint type of glass."

The prize will be to discover a switchable tinted window that responds to light and heat gain by absorbing rays while allowing light to penetrate. The only problem is that any such technology must clear a number of hurdles before making it to the marketplace. There are many companies in competition to develop an electrochromic or chemical window solution that can darken and lighten windows at the flick of a switch or by automatic sensor. "Much of this development is heavily cloaked in secrecy as patents are sought and technology continues to develop," explains Head.

Building a Better Window

The key stumbling blocks are threefold: durability, thermal stability, and reliability with regard to switching. Gentex Corporation, a Zeeland, Michigan company, known for its electrochromic automobile mirrors that automatically darken and lighten to eliminate glare, is undergoing its own tests for electrochromic windows. "We've made increases in durability, fairly significant ones," says Tom Guar, Vice President of Chemical Research for Gentex. Still, the goal has to be to produce a window that can stand up for 20 years. "When you think about it, it's a pretty tough environment," Guar says of the stresses a window designed to absorb heat must endure. "If you think about your glass pane absorbing great amounts of light and heat, it will get very hot, maybe even crack the glass," he says. Add to that susceptibility to water penetration, which is a problem for all insulated glass units (IGUs), and oxygen penetration, and there's a lot to overcome. Marvin struggled with making sure the film adhered to the glass and the seal remained undisturbed. Any future product will need to address the same issues.

Faribault, Minnesota's Sage Electrochromics, makers of SageGlass, has developed an electrochromic window technology that has passed all four levels of testing by the Department of Energy. To get DOE approval, Sage must demonstrate that their product is durable as well as efficient. "There are three key reasons why this approval is so critical," says Mike Myser of Sage. "The first is durability. In order for a window company to sell our product they need to know it's durable, and will withstand the thermal stress and UV degradation of the sun." Sage is the first to receive DOE approval for durability. With this approval in hand, Sage can partner with glass and window manufacturers to create a product that will be available to consumers for home use.

Making Window Efficiency Affordable

"I think their technology is very good," admits Guar. Still, as he sees it, the key will be to produce a window that is affordable, reliable, and durable. The manufacturing implications are huge, he says. "Handling large sheets of glass is non-trivial," Guar says. Covering that glass with a chemical film is even harder. "It's not an easy thing to accomplish," Guar says. Still, the right partnership would make the glass available to window companies who feel they could provide a dependable seal for this technology. The fact that Sage is partnering with Honeywell to provide the switching capability gives a great boost to their product.

Granted, there are technologies available that use advanced chemical solutions to address energy efficiency. Heat Mirror is an excellent product that was developed to insulate glass from heat loss and heat gain. Heat Mirror windows have an insulative reflective barrier that is suspended between the panes of glass. It is spectrally selective and allows visible light to enter while blocking near infrared rays that cause heat buildup. It functions like a third layer of insulation and greatly increases the R-value of walls with windows. But, while the R-value with Heat Mirror windows is exceptional, the cost for insulating windows of this caliber is very high. And they're not switchable. The key to winning the high efficiency window race, most experts agree, is to produce an affordable technology that can be mass produced and last for the lifetime of a standard window warranty.

Changing the Way We Work With Windows

SageGlass plans to offer an affordable technology that is durable, reliable, and ripe for the market. "We're trying to change an industry," explains Myser. Consumers want more glass, but they demand efficiency and control. Bringing electrochromic device technology to the window industry will allow homeowners to control the amount of light and heat that enters the building. "People want skylights, they want windows in their homes, the light it brings, but what do they do about the heat? We've solved that. Switchable control means controlling both the light and the heat," Myser says. Since the SageGlass coating is applied to only one of the panes of glass, their hope is that they can provide affordable switchable technology to the window industry and its customers.

"People understand this technology and want to buy it," Myser says. For homeowners it could revolutionize window placement and home design. It would mean windows where you want them, not just where heat gain can be reasonably avoided. This technology may even allow designers to place windows on west-facing facades without sustaining the incredible burden a beating western sun places on the air conditioning system. It may also mean designing for the glorious morning sun that so many homes currently avoid. Better yet, since this technology is device-controlled, it can even be programmed to darken or lighten automatically so that indoor climates can be effectively controlled even when the homeowner is absent.

The EnergyWise House in an ongoing project that examines energy efficient building techniques, materials, and methods.