ARCHIVED - NRC opens a window on energy efficiency

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February 08, 2008— Ottawa, Ontario

All 31 windows in the Canadian Centre for Housing Technology test house were swapped in a single day.
All 31 windows in the Canadian Centre for Housing Technology test house were swapped in a single day.

Windows say a lot about the style of a house, and even more about its energy efficiency. To help Canada's construction industry provide the most energy-efficient windows, a team at the NRC Institute for Research on Construction (NRC-IRC) has completed an extensive research project on different window glazing systems to discover how much energy can be saved by changing certain characteristics of the glazing system. This project was conducted in collaboration with Natural Resources Canada and Pilkington North America ― a major international glass manufacturer.

The Canadian Centre for Housing Technology is a partnership between NRC, Natural Resources Canada, and Canada Mortgage and Housing Corporation.

During the winter, a window (a single pane of uncoated glass) loses up to 10 times more heat than an equivalent area of an outside wall. Windows specifically designed to conserve heat in winter and reflect sun in summer can significantly reduce home energy consumption.

Modern windows are designed to improve energy performance. The number of panes (double or triple glazed), the type of spacer between the panes, the gas fill (argon, krypton or a mixture of both), and a variety of coatings on the glass all help to reduce heat loss through the window. With rising cooling loads and associated peak electric demands in large urban centres, the selection of suitable windows for houses is increasingly important.

A pyranometer mounted in a south-facing window measured the transmitted solar radiation.
A pyranometer mounted in a south-facing window measured the transmitted solar radiation.

For the past 25 years, Dr. Hakim Elmahdy, a research officer with NRC-IRC, has been assessing the energy efficiency of various window glazings. "Over the last decade, window glazing technologies have matured to cut energy costs through greater solar gains and higher insulation values," says Dr. Elmahdy. "But certain low-emissivity glass coatings do far better than others in climates like ours."

He explains that the actual solar heat gain through the glazing system is greatly affected by the optical characteristics of the coating on the glass. "Low solar heat gain low-e glazing (LSG) greatly reduces the entry of solar radiation into the living space, an advantage during the cooling season," says Dr. Elmahdy. "High solar heat gain low-e glazing (HSG), on the other hand, allows more solar heat gain into the room, which is very beneficial during winter."

What is solar gain?

Solar gain (also known as solar heat gain or passive solar gain) refers to the increase in heat in a space, object or structure that results from solar radiation. The amount of solar gain varies with the strength of the sun, and with the ability of any intervening material to transmit or resist the radiation.

Certain types of window glazings can maximize solar gain inside the building during the cold months to reduce the cost of heating.

What is a low-e coating?

A low-e coating is a thin, almost invisible, metallic layer applied to the surface of the window glazing. It improves energy performance by reflecting the long-wave infrared radiation (heat) while still allowing a large portion of the solar spectrum (with a shorter wavelength) to pass. Thus, heat is reflected back into the house in winter (or out in summer), and the glass remains see-through.

Only a small portion of the solar spectrum is visible light. How the low-e coatings treat the remaining "invisible" portion of the solar spectrum can vary. A low solar gain (LSG) coating reflects most of the invisible solar spectrum, helping to keep solar gains to a minimum. A high solar gain (HSG) coating transmits most of the solar spectrum and its accompanying heat gains.

Under Dr. Elmahdy's direction, NRC and the Canadian Centre for Housing Technology ran a series of tests at its twin R‑2000 house facility on the NRC campus in Ottawa. The research team monitored the summer and winter energy usage of two identical houses, with simulated occupancy, using two commonly used and different high-performance coated glass types. The windows in the "reference house" had a glazing with a low heat transmittance value and a high solar heat gain value. The windows in the "test house" had a slightly lower heat transmittance value and a much lower solar heat gain value. The results were used to calibrate a model that was then used to examine their performance in other geographic locations.

Manufacturers simply don't have the means to conduct extensive field experiments to verify or validate the effect of various glazing characteristics on the heating and cooling of a house. In most cases, the thermal performance characteristics of window glazings and their effect on the heating and cooling loads in buildings are based on computer simulations. Thanks to NRC, manufacturers now have far more realistic data about the energy efficiency of various window glazings in different cities across Canada.

Of the two glazings tested, the one with a high solar gain coating delivered greater annual energy and cost savings in Canada.
Of the two glazings tested, the one with a high solar gain coating delivered greater annual energy and cost savings in Canada.

The R-2000 standard, developed by Natural Resources Canada, defines the leading edge of cost-effective housing technology.

The project yielded relevant results for Canadian homeowners. "The total annual heating energy savings in the reference house considerably outweighed the increase in the air conditioning load when compared to the test house," notes Dr. Elmahdy. "The bottom line is that the windows with the high solar heat gain value delivered significant energy savings when both the heating and cooling seasons were studied."

To learn more about this study, contact Dr. Elmahdy at (613) 993-9752 or AbdelHakim.Elmahdy@nrc-cnrc.gc.ca. Or read the full study at http://nparc.cisti-icist.nrc-cnrc.gc.ca/npsi/ctrl?action=shwart&index=an&req=5755564&lang=en.

Enquiries: Media relations
National Research Council of Canada
613-991-1431
media@nrc-cnrc.gc.ca

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