Methodology for ENERGY STAR Savings Estimates for Windows

Savings estimates are based on energy calculations performed by D&R International using RESFEN 6 for typical homes 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. Eight home profiles were modeled for each city, with variables for construction vintage, home size, and heating and cooling equipment. The "typical home" is assumed to be the default new construction and existing construction assemblies in each location. The one-story home is 1,700 square feet and the two-story home is 2,600 square feet. It is assumed that the window area of the homes is equal to 15 percent of the floor area and the windows are equally distributed in the ordinal directions. Heating and cooling systems for each model include either natural gas heat and electric air conditioning or an electric heat pump.

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. For single-pane, clear windows, an aluminum frame model was selected for cities in the South-Central and Southern Climate Zones, while a wood/vinyl frame model was selected for cities in the Northern and North-Central Climate Zones. For double-pane windows, clear-glass windows were selected from the library with wood/vinyl framing. Savings calculations are for whole-home replacements with ENERGY STAR Version 6.0 windows in all climate zones. The energy savings for each city were calculated by averaging the eight home profiles. Climate zone savings estimates represent the average savings among the cities in each climate zone. All cost savings calculations used 2013 and 2014 average natural gas prices and 2012 and 2013 average electricity prices for each state from the Energy Information Administration (EIA).

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