For architects and builders concerned with a building’s energy efficiency, windows have always been a double-edged sword. On the one hand they offer views, natural light, and heat in the winter when the sun is shining. On the other hand, they’re holes in the building’s envelope allowing for heat transfer in the wrong direction and glare when the sun’s rays are low. In south-facing rooms in particular it’s always a challenge to offer a comfortable environment, and natural daylight during midday without drawing blinds or having the air conditioner work overtime. One technology that offers a solution but is still relatively new is electrothermal glass, also known as tintable or dynamic glass.

SageGlass was founded in 1989 but took 14 years of research and development before launching its electrothermal glass product. This glass is able to change its tinting to block or allow the sun to enter a building. If you’ve ever been in a building which is enveloped in glass, you might have had the uncomfortable experience of having the sun beat down on you. It’s worse than being outside because there’s no breeze to combat the heat, and the only way to avoid glare is to use sunglasses, which, if you’re inside, isn’t really a great solution.

Advantages:  Tintable glass cuts glare and heat during the hottest times of the day and year but also allows the sun to enter during colder times of the year. The advantages are increased energy efficiency for both the heating and cooling system, as well as permitting  natural daylight without the glare which helps plug load. It also means that blinds aren’t needed, except in areas where privacy is a concern.  A study of increased energy consumption saw that windows with SageGlass increased a building’s energy efficiency  21% over ASHRAE baseline building requirements. Heating and cooling systems will work better, particularly on extremely hot days.

Applications: This glass has been installed in several commercial and institutional building projects but also has applications in high-end residential setting. The advantage of tintable glass is that it permits views while preventing excess heat and glare.  The glass can be specified by any window company, currently for residential applications, Marvin Windows and Doors uses this glass.

Regarding overall energy efficiency of the glass, not only does it allow or prevent radiant heat (heat from the sun) passing through, but it is also treated with a low-emissivity coating to increase the glass’s thermal rating. SageGlass works very closely with all window manufacturers using its glass to make sure it is installed into the window frames correctly. In residential applications, it has been combined with both double and triple glazed windows.

Solar Heat Gain Coefficient: One of the measurements of energy efficient windows is the solar heat gain coefficient. It is the amount of heat that passes through a window from the sun’s rays and is measured on a scaled from 0-1. The average low-e double glazed window has a solar heat gain coefficient of .47, meaning that it lets in 47% of the sun’s heat that is hitting the window. A window coated with SageGlass has a variable solar heat gain coefficient that ranges between 0.09 and 0.49. When the window is fully tinted, only 9% of the sun’s heat gets through. The solar heat gain coefficient of regular glass is static.

Technology: The technology SageGlass developed involves five layers of nano-ceramic coatings. When a small current of electricity is applied to the glass lithium ions and electrons transfer from one layer to another causing the glass to darken. It can be controlled manually or built into a building automation system. In case you’re wondering about whether the electricity applied offsets the gains in energy efficiency, the voltage applied is very small and the benefits of the tinted glass far outweigh the costs.

When used in residential applications, regarding the electrical requirements, the same person installing your internet cables is the one specified for installing this glass.

Cost: While the company couldn’t give a cost for this glass because it depends on so many factors, it is definitely a product intended for high-end residential applications. However, I can see plenty of residential applications for this glass — especially in Toronto where there is a significant number of high-end condos being constructed. Most of these condos are made out of glass, and the upper floors are all about the views. This glass would be a perfect application.

For more information on SageGlass and electrothermal glass, visit

Photo courtesy of Sage Glass

BEC Green

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