Design considerations for window performance

Shading in the Central Zones (Heating and Cooling)

In climates where there is both a significant heating and cooling season, there is a desire for solar gain in winter while preventing gain in summer. This can be accomplished with overhangs on the south side and other shading devices elsewhere. As shown in the figure for Sacramento, the cooling season benefits of shading are notable.

Reliance on any form of shading is not as important when windows with a low solar heat gain coefficient (SHGC) are used. Using low-solar-gain low-E glazing (Windows C or 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. However, even with high-performance glazing there are summer comfort and glare control benefits from shading.

With more west-facing window orientation or greater glazing area, the energy penalty of not using strategic shading or high-performance windows would be even greater.

Window Glazing Frame U-factor SHGC A Double, Clear Non-Metal 0.41-0.55 0.41-0.60 B Double, High-Solar-Gain Low-E Non-Metal 0.31-0.40 0.41-0.60 C Double, Low-Solar-Gain Low-E Non-Metal 0.31-0.40 ≤0.25 D Triple, Low-Solar-Gain Low-E Non-Metal, Thermally Improved ≤0.20 ≤0.25

None	Interior	Overhangs	Typical	Maximum

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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