Building a Smarter, More Efficient Window

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 homeowners to place windows 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 ultraviolet (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 quality problems.

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. Many companies are 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.