Article #1393 - AEDG Implementation Recommendations: Glazing Design

The Advanced Energy Design Guide (AEDG) seeks to achieve 30 percent savings over Standard 90.1-1999. This guide focuses on improvements to small office buildings, less than 20,000 square feet. The recommendations below are adapted from the implementation section of the guide, and should be used in cooperation with the whole document.* The full design guide is available from the ASHRAE website, Advanced Energy Design Guide for Small Office Buildings .

To be useful and consistent, the U-factors for windows should be measured over the entire window assembly, not just the center of glass. Look for a label that denotes the window rating is certified by the National Fenestration Rating Council (NFRC). The selection of high-performance window products should be considered separately for each orientation of the building and for daylighting and viewing functions.

Window Wall Ratio

The window-wall ratio is the percentage that results from dividing the total glazed area of the building by the total wall area.

For any given WWR selected between 20% and 40%, the recommended values for U-factor and SHGC contribute toward the energy savings target of the entire building. A reduction in the overall WWR ratio will also save energy, especially if glazing is significantly reduced on the east and west façades. Reducing glazing on east and west façades for energy reduction should be done while maintaining consistency with the needs for view, daylighting, and passive solar strategies.

Windows with overhang.

Solar Heat Gain

Significantly greater energy savings are realized when sun penetration is blocked before entering the windows. Horizontal overhangs located at the top of the windows are most effective for south facing facades, and must extend beyond the width of the windows. The extension of the overhang depends on the latitude and the climate.

Vertical fins oriented slightly north are most effective for east- and west-facing façades. Consider louvered or perforated sun control devices, especially in primarily overcast and colder climates, to prevent a totally dark appearance in those environments.

Operable versus Fixed Windows

Operable windows offer the advantage of personal comfort control and beneficial connections to the environment. However, individual operation of the windows not in coordination with the HVAC system settings and requirements can have extreme impacts on the energy use of a building's system. Advanced energy buildings with operable windows should strive for a high level of integration between envelope and HVAC system design. First, the envelope should be designed to take advantage of natural ventilation with well-placed operable openings. Second, the mechanical system should employ interlocks on operable windows to ensure that the HVAC system responds by shutting down in the affected zone if the window is opened. It is important to design the window interlock zones to correspond as closely as possible to the HVAC zone affected by the open window.

Warm Climates (Climate Zones: 1 2 3 4 5 6)

Window Orientation

In warm climates, north and south glass can be more easily shielded and can result in less solar heat gain and less glare than do east and west facing glass. A good design strategy avoids areas of glass that do not contribute to the view from the building or to the day lighting of the space. If possible, configure the building to maximize north facing walls and glass by elongating the floor plan. During site selection, preference should be given to sites that permit elongating the building in the east-west direction and that permit orienting more windows to the north and south.

A good design strategy avoids areas of glass that do not contribute to the view from the building or to the daylighting of the space. If possible, configure the building to maximize north-facing walls and glass by elongating the floor plan.

Since sun control devices are less effective on the east and west façades, the solar penetration through the east- and west-facing glazing should be considerably less than that through the north- and south-facing glazing. This can be done by reducing the area of glazing, reducing the SHGC, or preferably both. Thus, the area of glazing on the east and west façades, times their respective SHGCs, should be less than the area of glazing on the north and south façades times their respective SHGCs. If each façade has a different area or SHGC, the formula becomes: ((W window area x W SHGC) + (E window area x E SHGC)) < (less than) ((N window area x N SHGC) + (S window area x S SHGC)).

For buildings where a predominantly east-west exposure is unavoidable, or if the application of this equation would result in SHGCs of less than 0.25, then more aggressive energy conservation measures may be required in other building components to achieve an overall 30% energy savings.

Glazing

Exterior sun control.

For north and south facing windows, select windows for both high visible light transmission and low solar heat gain coefficient. Certain window coatings, called solar heat gain coefficient. Certain window coatings, called selective low-e selectively transmit the visible portions of the solar spectrum while rejecting the non-visible infra-red sections. These glass and coating selections provide superior view and day lighting, while minimizing solar heat gain and window manufacturers market special "solar low-e" windows for warm climates.

For buildings in warm climates that do not utilize daylight responsive lighting controls, north and south windows should be selected with a solar heat gain coefficient (SHGC) of no more than 0.35 and visible light transmittance (VLT) of not less than 0.35, with higher VLT''s preferred. Assembly U-factors for fabricated windows and store-fronts in cold climates should be no more than 0.40. For west and east facing walls, minimize window area or utilize a reflective or tinted glass to reduce solar heat gain. East and west facing windows in warm climates should be selected for an SHGC of no more than 0.25 with VLT not less than the SHGC, and a VLT greater than 0.30 is preferred. All SHGC, and a VLT greater than 0.30 is preferred. All SHGC and VLT values are for the entire fenestration assembly, in compliance with NFRC procedures, and are not simply center-of-glass values.

For warm climates, a low SHGC is much more important for low building energy consumption than the window assembly U-factor. Windows with low SHGC values will tend to have a low center-of-glass U-factor, however, because they are designed to reduce the conduction of the solar heat gain absorbed on the outer light of glass through to the inside of the window.

Obstructions and Planting

Adjacent taller buildings, and trees, shrubs, or other plantings are effective to shade glass on south, east and west facades. For south facing windows, remember that the sun is higher in the sky during the summer, so that shading plants should be located high above the windows to effectively shade the glass. Fully shaded windows can be selected with higher SHGC ratings without increasing energy consumption. The solar reflections from adjacent building with reflective surfaces (metal, windows or especially reflective curtain walls) should be considered in the design, which may modify shading strategies, especially on the north façade.

Cold Climates (Climate Zones: 7 8)

Elongate building in East/West orientation to maximize windows on North/South.

Window Orientation

For more northerly locations, only the south glass receives much sunlight during the cold winter months. If possible, maximize south facing windows by elongating the floor plan in the east-west direction and relocate windows to the south face. Be careful to install window management devices for the south-facing glass that allow for passive effects when desired but that prevent unwanted glare and solar overheating. Glass facing east and west should be significantly limited. Areas of glazing facing north should be cautiously sized for day lighting and view. During site selection, preference should be given to sites that permit elongating the building in the east-west direction and that permit orienting more windows to the south.

Passive Solar

Passive solar energy savings strategies should be limited to non-office spaces, such as lobbies and circulation, unless it is designed so that workers do not directly view interior sun patches or have them reflected in computer screens. Consider heat absorbing blinds. In spaces where glare is not an issue, the usefulness of the solar heat gain collected by these windows can be increased by using hard massive floor surfaces, such as tile or concrete in the locations where the transmitted sunlight will fall. These floor surfaces absorb the transmitted solar heat gain and release it slowly over time, to provide a more gradual heating of the structure.