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Design Considerations for Window Performance

Advanced window technologies can have a major effect on comfort and on the annual energy performance of a house. However, there is a broader and possibly more significant impact of the recent revolution in window performance. Because the new glazing technologies provide highly effective insulating value and solar protection, there are important implications for how a house is designed.

There is a long-established set of window design guidelines and assumptions intended to reduce heating and cooling energy use. These are based, in part, on the historical assumption that windows were the weak link in the building envelope. These assumptions frequently created limitations on design freedom or generated conflicts with other performance requirements, such as view. Traditional considerations include orientation, amount of glazing, and shading requirements for windows. The new technologies challenge the validity of many of these restrictive assumptions.

Windows are one of the most multifaceted components of a home. They affect the aesthetics of the home, provide for views, ventilation, and daylight, and have a major impact on the comfort, energy consumption, maintenance requirements, and cost of the home. The designer must determine the amount and placement of glazing to provide natural ventilation in summer, solar heat gain in winter, or simply to allow egress from bedrooms as required. Decisions about window size and placement are also integral to the exterior appearance of the home and the interior aesthetic qualities of the spaces. Possible design objectives include creating a sense of spaciousness and providing natural light or particular views. These architectural decisions are interwoven with site design.

To develop a useful set of guidelines for window sizing, placement, and other building design concerns, it is necessary to consider the multiple design objectives related to windows. These are:

The first three of these—providing views, light, and fresh air—represent traditional window functions that do not change fundamentally as new technologies are introduced. However, the second three objectives—decreasing summer heat gain, decreasing winter heat loss, and providing winter heat gain—are influenced considerably by the ability to select high-performance windows. Guidance to understand the impacts of these objectives can be found in the climate guides for new windows and replacement windows.