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Methodology for ENERGY STAR Savings Estimates for Windows

Savings estimates are based on energy calculations performed by Lawrence Berkeley National Laboratory (LBNL) using RESFEN 3.1 for a typical home in 93 U.S. cities. RESFEN is the standard software program used for calculating the impact of windows on heating and cooling costs for residential new construction or existing housing stock. The “typical home” is defined as the RESFEN default home — a 2,000 square foot, single story, detached house with 300 square feet of window area (15% of floor), gas heat and electric air conditioning with RESFEN default operational assumptions.

Representative windows were selected from the RESFEN window library. For ENERGY STAR qualified windows, types were selected for each city based on the city's ENERGY STAR climate zone (Northern, North/Central, South/Central, Southern). For single pane windows, a wood/vinyl frame model was selected for cities in the Northeast and Midwest census regions and an aluminum frame model was selected for cities in the West and South census regions, reflecting the dominant frame types reported in the Energy Information Administration (EIA) 1997 Residential Energy Consumption Survey (RECS 97). For typical alternative windows, double-paned, clear-glass windows were selected from the library with wood/vinyl framing for cities in the Northern, North/Central, and South/Central climate zones and aluminum framing for cities the Southern climate zone. Where more than one library window qualified, the window that generated the lowest (most conservative) savings estimates was selected.

Regional population-weighted savings estimates were created by first setting performance for each U.S. county equal to the performance of the nearest RESFEN city with a similar climate as defined by the climate zones in the 2004 Supplement to the 2003 International Energy Conservation Code. ENERGY STAR climate zones are based on RICC climate zones and the RICC code change proposal was adopted into the 2004 Supplement to the 2003 International Energy Conservation Code. U.S. Census (2000) county level population data were then used in combination with this data set to calculate regional population-weighted averages. All calculations used EIA 2005 average gas prices and year-to-date through June 2006 electricity prices.

Hawaii was excluded from this analysis as RESFEN operational assumptions applied in the base data set diverge significantly from the norm in Hawaii and generated unrealistic savings estimates.

Cost savings were rounded to the nearest multiple of five to reflect their characterization as estimates.

For more information on RESFEN, visit Lawrence Berkeley National Laboratory (LBNL) Exit ENERGY STAR.