Design considerations for window performance

Orientation in the Southern Zone (Cooling Dominated)

In predominantly cooling climates, the goal is to face most windows north, where there is little direct exposure, or to the south, where they can be designed with overhangs that will keep out most of the hot summer sun. Overhangs are much less effective against the lower angles of the east and west sun. Therefore, simply reducing the size and number of east and west windows can be the best strategy.

The figure illustrates the impact of different window orientations on annual energy costs for a typical house in Phoenix, Arizona. Due to intense solar heat, orientation has a significant impact when windows with a high SHGC are used (Windows A and B). When higher-performance windows with low-solar-gain low-E coatings are used, window orientation has a greatly diminished impact on energy use (Window D).

All of the cases shown have average window area and shading conditions. If there were no shading or greater glazing area, the less efficient glazing would perform worse in comparison to the low-solar-gain low-E windows.

Window Glazing Frame U-factor SHGC A Single, Clear Metal ≥1.00 ≥0.61 B Double, Clear Metal 0.71-0.99 ≥0.61 C Double, Mod-Solar-Gain Low-E Metal 0.56-0.70 0.26-0.40 D Double, Low-Solar-Gain Low-E Metal, Thermal Break 0.41-0.55 ≤0.25

Equal	North	East	South	West

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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