Design considerations for window performance
Window Area in the Southern Zone (Cooling Dominated)
The traditional approach to reduce heat gain is to reduce the total glazing area. However, low-solar-gain low-E windows in combination with optimum orientation and shading can minimize cooling load impacts.
The figure illustrates the impact of window area on annual energy costs for a house in Phoenix, Arizona. If windows with a high SHGC are used (Windows A and B), increasing the glazing area has a very significant impact on the cooling load. The annual energy use for a house with low-solar-gain low-E glazing (Windows C and D) still exhibits the same basic pattern, but the differences are not nearly as great in relative or absolute terms.
While increasing glazing area does increase energy use in this climate, it will not have nearly as profound an impact when high-performance windows are used. In all cases, cooling season energy use can be further reduced by shifting the window area to preferred orientations and employing shading strategies.
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Note: The energy performance figures for the 0.15 window to floor area (WFA) 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 equal window distribution 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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