Issues in Window Selection: Energy-Related
Heat Transfer Mechanisms | Measuring Properties | Overview of Energy Use | Codes and Standards
Energy Codes and Standards
Energy codes and standards are one of the main tools used to improve energy efficiency in buildings. In most places in the United States, the commonly referenced energy code is ASHRAE Standard 90.1-1999. This code has two paths--the Building Envelope Option with prescriptive limitations on allowable window area and window properties (U-factor and SHGC). Alternatively, a performance standard called the Building Envelope Trade-off Option can be used to meet a total envelope performance number. This allows for design flexibility in window area and properties, and provides credit for higher VT windows that enhance daylighting.
While the performance approach has a number of benefits, the prescriptive requirements give a good general sense of the limitations that codes place on window design and selection. The prescriptive part of ASHRAE 90.1-1999 indicates the following:
- The total vertical fenestration area, including both fixed and operable windows, should be less than 50 percent of the gross wall area of the building.
- The allowable window area can be increased to 75 percent at street level if the first story is not higher than 20 feet and has a continuous overhang with a projection factor of 0.5 (the depth of the overhang = 0.5 x the window height).
- The total skylight area must be less than 5 percent of the total gross roof area.
- The U-factor and SHGC of all types of fenestration cannot exceed limits prescribed for the appropriate climate zone. Distinct requirements are given for fixed and operable vertical glazing as well as different types of skylights. A higher SHGC level is allowed for north-facing windows. Windows with higher SHGC levels can also be used when there are permanent projections (overhangs) over the window.
Figure 2-28 shows a simplified set of U-factor and SHGC requirements for fixed vertical windows in 26 climates. Figures 2-29 and 2-30 shows which of nine window types used throughout the book meet the code in Chicago and Houston. If a window does not meet the code but can meet it with a permanent projection (overhang), the minimum size of the projection is given in the table.
