High Performance Windows

Windows in any building that is heated or cooled.

windows, heating, cooling, energy use, energy loss, thermal performance, U-factor, emissivity, low-e coatings, insulation, solar heating, solar heat gain, SHGC, electrochromic film

. . . Passive Solar Heating and Cooling is a simple, inexpensive energy source, but that energy won't help much if it is lost through poorly insulated, single-pane windows.

According to the US Department of Energy's Lawrence Berkeley National Laboratory, windows account for 2/3 of energy used in the US to heat and cool buildings, totaling 4 quadrillion BTUs (or "quads") of energy, over 4% of the total US energy use! This energy use is due to both heat loss from the inside in the winter, which has to be replaced by heating systems, and heat gain from the outside in the summer, which has to be removed by cooling systems. Fortunately, new window design and technology can practically eliminate that use with better thermal performance.

High performance windows are energy efficient because their thermal loss, or U-factor, is low. Some high performance windows are called "low emissivity windows" or "low-e windows", referring to the low emission of infrared radiation (heat) due to low-e coatings on the glass. Advanced high performance windows can reduce energy losses to nearly zero, achieving energy savings that rival those of the best insulated walls.

Emissivity

Glass has a fairly unique property in that it is quite transparent to visible light but relatively opaque to infrared radiation, i.e., heat. Solar energy passing through glass windows is absorbed by other materials inside which reradiate it as heat energy. Since windows don't transmit this heat out as well as they transmit light in, the heat is trapped. This is precisely why passive solar buildings, solar collectors and greenhouses work so well to heat spaces.

Emissivity is a measure of the ability of a material to absorb and radiate or emit energy. Dark dull materials have high emissivity. Reflective materials like glass and polished metals have low emissivity, i.e., they are poor absorbers and emitters of thermal energy. (Specially formulated glasses, like borosilicate or "pyrex", have lower emissivity than normal glass.) This is why they feel cool to the touch compared to other materials in the same environment. However, even though glass has low emissivity, a single-pane glass window will not insulate as well as the same area of a well-insulated wall (that is, it's R-value is lower). As a result, there is considerable heat loss through the typical window.

High Performance Windows

One way to improve a window's energy efficiency is to increase its insulation ability - to lower its U-factor - by making it from two or more panes of glass, separated by either a vacuum (as in a thermos bottle) or an inert gas that conducts less heat than air does, like argon or krypton.

Another way to improve a window's energy efficiency is to surface the glass with a low-emissivity (low-e) coating, an ultra-thin, practically invisible layer of metal or metallic oxide. These coatings help reflect radiant infrared energy, which helps keep it inside during the winter, and outside during the summer, adding to the low-emissivity of the glass itself. The solar heat gain coefficient (SHGC) of low-e coated windows varies. Windows with a lower SHGC reflect and keep more of the sun's heat out, which is more suitable for areas where cooling is the greater concern; windows with a higher SHGC allow more of the sun's heat in, which is more suitable for areas where heating is the greater concern.

High performance windows utilize one or both of these methods to reduce energy loss. Low-e coatings of tungsten oxide, often called electrochromic films, can even be switched from clear to tinted electronically, thereby varying the SHGC. New window prototypes using these coatings can enable windows to have dynamic solar control, providing an all-climate, all-year, solution to energy loss.

Effects on Passive Solar Heating

Keep in mind that passive solar heating is not so much the direct passing of infrared energy from the sun through the glass into the building. Rather it is a function of passing light through the glass which is absorbed by high-emissivity materials and reradiated as heat inside. While low-e coatings further reduce the direct transmission of infrared through the window, as long as sunshine pours through the glass it will still act as a source of solar heat. Hence, low-e windows can still be used for passive solar heat while dramatically increasing their ability to hold heat inside, and in the summer they will keep more heat out than normal glass windows do under the same conditions of being shaded or not.

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Gary Swift, 11 November 2008.
Last updated: Friday, 21-Sep-2012 19:34:11 MDT