Design considerations for window performance
Shading in the Southern Zone (Cooling Dominated)
The best place to shade a window is on the outside, before the sun strikes the window. This can be accomplished with overhangs on the south and other shading strategies elsewhere. To most effectively reduce solar heat gain on the interior, the shade or drapery used to block the sunlight should have high reflectance (light color). The best strategy in hot climates is good shade management combined with low solar heat gain coefficient (low SHGC) windows.
In addition to shading the building from direct sun, trees have been found to reduce the temperature of air immediately around them by as much as 10°F (5°C) below the temperature of the surrounding air due to evaporation of moisture. Trees and bushes can provide strategic shade from low east or west sun angles that are extremely difficult to shade architecturally.
Reliance on any form of shading is not nearly as important when windows with a low solar heat gain coefficient are used. Using a low-solar-gain low-E coating (Window D), results in great energy cost reductions for all conditions even with no shading. This is because the glazing itself provides control of solar radiation, so these additional measures become less important in terms of energy use. However, even with high-performance glazing there are energy, summer comfort and glare control benefits from shading.
With more west-facing window orientation or greater glazing area, the energy penalty of not using shading or high-performance windows would be even greater.
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| None | Interior | Overhangs | Typical | Maximum |
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Note: The energy performance figures for the typical shading case shown here were generated with regression expressions provided by Lawrence Berkeley National Laboratory (windows.lbl.gov/EStar2008). The difference between the typical case and shading variations were generated using RESFEN 5.0. Results assume a typical new construction 2250 sq ft house with 337.5 sq ft of window area distributed equally on all four sides. 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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