How Windows Affect Heating and Cooling Costs
Is it really a big deal to get the right type of low-e coating? The charts on this page suggest that it is well worth the effort to select the right product. In general, houses in northern, heating-dominated climates benefit from high SCs that promote solar heat gain, even without special "solar" design features. As the charts show, using low SC products in Madison and Seattle increases the total energy load. In Madison, high SC windows provide a net heat gain, while low SC windows cause a significant loss.
Houses in cooling-dominated climates benefit greatly from the low SC products. Los Angeles is a bit surprising. The difference in total energy load is small between the high SC and low SC options. The house with a high SC had a slightly lower overall energy use, but showed a wide swing between heating and cooling. With low SC windows, the energy use was more moderate for both heating and cooling.
Some manufacturers have selected low SC coatings as their standard product offering. That could be a problem for northern buildings, because they would lose "free" solar heat. The problem is serious for buildings oriented and designed specifically to capture solar heat. Using a the wrong low-e could be a disaster in a solar home.
Some designers like to mix different low-e coatings on different sides of a home. That makes logical sense, but as a general rule it's probably not necessary. There's only one situation that would justify mixing your window order: The building is in a northern climate where you would normally choose a high SC and it has many large, west-facing or east-facing windows that will be unshaded and prone to overheating. Those sides might need windows with a lower SC. To know for sure, you should conduct a computerized energy analysis of that specific building. (Commercial buildings will likely benefit by selecting a specific glazing for each facade.)
These charts are based on computer analysis using the RESFEN computer program. It calculates the heating and cooling performance and cost of residential window systems. To produce the numbers for these charts, a standard house was modeled. It was 1540 sq. ft. The window area was 16 percent of the floor area with windows distributed evenly on all four sides. The assumed window U-value was 0.30. The heating system was a heat pump (COP 2.2). Regional average electric rates were used for each city. RESFEN was developed by the Building Technologies Program at Lawrence Berkeley Laboratories.
Special thanks to these people for their assistance with this article:
- Ross McCluney, Florida Solar Energy Center
- Robert Sullivan and Steve Selkowitz, Lawrence Berkeley Laboratories
- Gary Curtis, Oregon Department of Energy
This article appeared in Energy Source Builder #35 October 1994