Australia’s Sustainable Window Alliance (SWA) is using science to measure the properties of energy efficient windows for residential applications. The groups says that while windows represent only 8% of a home’s exterior surface, they can account for up to 60% of unwanted heat loss in winter and 91% of unwanted heat gain in summer … and that understanding how to best use different kinds of windows could reduce global carbon emissions by billions of tons each year.
Traditionally, a new windows’ U-value (a measurement how well a product prevents heat from escaping a home or building) has been the only factor most builders consider. U-value ratings generally fall between 0.15 and 1.20 and the lower the U-value, the better a window keeps heat inside a building. The SWA, however, says there is another important measure of a window’s energy efficiency called “Solar Heat Gain Coefficient” (SHGC).
This SHGC rating, illustrated as a number between 0 and 1, indicates how much heat from the sun is allowed into the interior of a building. The lower the SHGC, the more your new windows will block solar heat gain, which is particularly important during the summer cooling season in the warm climates. By contrast, people in colder areas may want solar heat gain during the cold winter months to lessen the cost of heating the home. These homeowners may even want to purchase new windows with different SHGC ratings for different walls of their house, depending on sun exposure. For example, walls that face away from the sun could benefit from whatever solar gain is available while those facing the sun could be optimized to limit solar gain.
Across the world, the University of Ulster in Ireland is also working to become a leader the search for better glass for new windows. The university is evolving technologies like dynamic glazing, which switches between clear and tinted glass on demand, triple glazing, and hermetically sealed vacuum glazing. Of the three, triple glazing combined with judicious use of low-e coatings offers the most energy efficiency gains for the money. But the added weight of an extra pane of glass means that sash has to be made from a material that is strong but also doesn’t conduct heat like aluminum or steel. Fiberglass is one such material.
Michael Barclay, an engineer in Australia suggests that a lack of awareness of these new windows is still the biggest issue. “Most architects and engineers would rather stick with what they know,” he says. “It’s a matter of educating professionals from architects to fenestration designers. Unless we stay on top of energy performance, the size of the window will shrink and opaque building materials, like brick or wood, will take over the façade. What you’ll have is a trend towards a box.”
Cutaway of Triple Glaze Windows