A key determinant of a window's energy efficiency is its glazing and with windows accounting for more than a quarter of the annual space heating needs in a typical new home, using the most energy efficient glazing is a key to conserving energy and realizing cost savings.
Glazings—not all created equal
As there are more than 300,000 types of window products on the market, and the areas and types can vary widely with the vintage of homes, making the right choice of glazing can be a challenge. Low-emissivity (low-e) coated windows provide improved thermal performance over clear glass, but the many types of low-e coatings vary greatly when it comes to solar performance.
There are many factors to consider when choosing a glazing, including rising cooling loads, peak electric demands, and heat loss coefficients (U-value or u-factor). But one of the most important determinants of a window's energy efficiency is its Solar Heat Gain Coefficient (SHGC): the fraction of incident solar radiation that the window admits. The SHGC is a number between 0 and 1, with low solar gain products having an SHGC of less than 0.30 and high solar gain products an SHGC greater than 0.30
CCHT window glazing project
To determine the impact of using high versus low solar gain glazing systems , the National Research Council of Canada (NRC), Natural Resources Canada (NRCan), and Canada Mortgage and Housing Corporation (CMHC), with contributions from 37 companies, used the Canadian Center for Housing Technology (CCHT) twin-house facilities to compare high solar heat gain (HSG) windows and low solar heat gain (LSG) windows.
Thirty-one glazing units were tested in four types of windows—casement, fixed casement, fixed, and patio door—on north, south, east, and west-facing walls. Both the high HSG and low HSG windows were double-glazed and low e-coated. Their U-Factors were:
- HSG U-Factor: 1.76 W/m2o K(0.31 Btu/hr.ft2.o F)
- LSG U-Factor: 1.62 W/m2o K(0.285 Btu/hr.ft2.o F)
Results for HSG and LSG windows were compiled for locations throughout North America and the results for 10 Canadian locations indicated that:
- HSG offered 13% to 17% energy cost savings compared to conventional windows
- HSG offered annual energy cost savings of $117-354 CDN
- LSG offered 8% to 10% energy cost savings compared to conventional windows
- LSG offered annual energy cost savings from $71-$203 CDN
Lessons from the field include the observation that energy efficiency benefits are dependent on the mode of operation, orientation and climate, and that combinations of glazing (e.g., LSG on the north, HSG on the south) can generate the best results.
Energy Star criteria
Using ENERGY STAR certified products is an easy way to select energy efficient windows. To take advantage of passive solar gains, new specifications for the ENERGY STAR in the Fall of 2010 will ensure a minimum SHGC of about 0.22 and U-value of 2.0.
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