Design considerations for window performance
Orientation in the Central Zones (Heating and Cooling)
In climates where there is both a significant heating and cooling season, orienting windows to the south will result in greater solar gain in winter while overhangs can be designed to reduce summer solar gain. East and west windows are difficult to shade and increase cooling loads. The results shown in the figure indicate that, as expected, south-oriented windows perform best. These results are the annual heating and cooling costs of a typical home with half of the windows facing one direction and the other half distributed evenly on the other three sides.
The difference between orientations is most notable when clear-glazed (Window A) or high-solar-gain low-E windows (Window B) are used. With these windows, western orientation can significantly increase cooling energy use. The impact of orientation is diminished when windows with lower SHGCs (Windows C and D) are used.
Window orientation in a house is often dictated by views and factors other than optimal solar gain. By using high-performance windows, any orientation can result in an energy-efficient house. For example, when the house has low-solar-gain low-E windows (Windows C and D), any window orientation uses less annual energy than a south-facing orientation with clear double-glazed windows (Window A). All of the cases shown have average window area and shading conditions. If there was no shading or greater window area, the difference in energy costs between less efficient and more efficient windows would be greater.
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| Equal | North | East | South | West |
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Note: The energy performance figures for the equal orientation case shown here were generated with regression expressions provided by Lawrence Berkeley National Laboratory (windows.lbl.gov/EStar2008). The differences between equal distribution and different orientations were generated using RESFEN 5.0. Results assume a typical new construction 2250 sq ft house with 337.5 square feet of window area (15% of floor area) and typical shading strategies. U-factor and SHGC are for the total window including frame. The costs shown here are annual costs for space heating and space cooling only and thus will be less than total utility bills. Costs for lights, appliances, hot water, cooking, and other uses are not included in these figures. The mechanical system uses a gas furnace for heating and air conditioning for cooling. Natural gas prices used are projections of the average natural gas price for the heating seasons of 2010-2020 in real 2009 dollars. Projections are based on state-specific natural gas retail price data by the Energy Information Administration (EIA) for the heating seasons of 2006-08 and are adjusted based on EIA projections of national natural gas price trends for 2010-2020. Electricity prices used are projections of the average electricity price for the cooling seasons of 2010-2020 in real 2009 dollars. Projections are based on state-specific electricity retail price data by the Energy Information Administration (EIA) for the cooling seasons of 2006-08 and are adjusted based on EIA projections of national electricity price trends for 2010-2020 (www.eia.doe.gov). RESFEN is a computer program for calculating the annual cooling and heating energy use and costs due to window selection and is available from Lawrence Berkeley National Laboratory (windows.lbl.gov/software/resfen).
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