Selecting Residential Window Glazing for Optimum Energy Performance

This Update discusses the results of a research project that examined the impacts of two types of high-performance window glazing (low solar heat gain and high solar heat gain) on energy consumption in residential applications.

Windows affect energy loads on houses in two ways: they allow solar energy to enter the house and also allow a significant amount of heat to escape in winter and enter in summer, as their insulating value is much lower than that of the surrounding walls. In winter this solar energy can help heat the house, whereas in summer, it contributes to excess heat, which then has to be removed or dealt with in some way.

Figure 1. High solar heat gain (HSG) and low solar heat gain (LSG) glazing

Various research studies have shown that in a typical Canadian house with conventional double-pane clear-glass windows, solar gain typically provides from 10% to 27% of the total heating energy for the house. However, the heat loss through these same windows during the heating season may account for more than 27% of the total heat loss for the house.¹

Factors that influence savings The house thermostat setting, the choice to cool or not to cool in summer, the orientation of the house, the size of windows, and the choice of shades are just a few of the factors that influence overall savings. Additionally, regional climate and the cost of the available heating fuels (electricity, oil, natural gas or propane) play an important role in determining the cost savings to be gained from using these high-performance glazing systems.

One way of achieving better performance is by using high-performance low-emissivity (low-e) coated glazing, which provides better thermal performance than clear glass. However, the products available on the market vary greatly when it comes to taking advantage of solar gain. They range from those with low solar heat gain (LSG), which greatly reduce the entry of solar radiation into the living space, to those with high solar heat gain (HSG), which allow greater solar gain. The LSG windows provide an advantage during cooling season while the HSG windows are beneficial during winter.

In general, Canada is a heating dominated climate — so at first glance, installing HSG glazing would be an obvious choice. But with the rising cost of electricity for summer cooling and the concerns over peak electrical loads during hot summer afternoons, the advantages of LSG glazing during cooling season are becoming more significant. Many other factors affect annual savings from low-e coated glazing, making it challenging to answer the question: Which low-e coated glazing — HSG or LSG — will provide the best energy and cost savings performance for Canadians?

Window features for improved energy performance Modern windows can be equipped with a variety of features to improve energy performance. These features include double or triple glazing, warm-edge spacers, gas filling between the panes (argon, krypton or a mixture of both), and a variety of coatings on the glass. All of these features are aimed at improving the U-factor of the window, hence reducing the heat loss through the window. Low-e coatings. A low-e coating is a thin metallic layer applied to the surface of the glazing to improve energy performance. It does so by reflecting the long-wave infrared radiation (heat) portion of the spectrum, while still allowing part of the solar spectrum to pass through it. Thus, heat is reflected back into the house in winter (or kept out in summer), and the glass remains "see-through." Only a small portion of the solar spectrum is visible light, and how the low-e coatings deal with the remaining "invisible" portion of the solar spectrum can vary. An LSG coating reflects most of the invisible solar spectrum, helping to keep solar gain to a minimum, while an HSG coating transmits most of the solar spectrum and its accompanying heat gain. The location of the coating also makes a difference: an LSG coating is typically located on the exterior pane of the window, to reflect heat out of the building; an HSG coating is typically located on the interior pane of the window, to reflect towards the inside (see Figure 1).

In order to answer this question for a specific case, researchers at the Canadian Centre for Housing Technology (CCHT) in Ottawa conducted a side-by-side, whole-house comparison of two different types of window glazing. They then used the results to calibrate a model for examining glazing performance in a number of other locations across Canada, for different circumstances.

Source : http://irc.nrc-cnrc.gc.ca