Article #1423 - AEDG Implementation Recommendations: Daylighting Window 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
.
Visual Light Transmission (VLT) (Climate Zones: 1 2 3 4 5 6)
The amount of light transmitted in the visible range affects the view through the window, glare, and daylight harvesting. These functions are not always best served by the same glazing product. For the effective utilization of daylight, the highest VLTs (0.50 - 0.70) should be used in the glazing located between 6 ft above the floor up to the ceiling. The view windows below 6 ft do not require such high VLTs, so values between 0.35 and 0.50 are acceptable to achieve recommended SHGC values. Higher VLTs are preferred in predominantly overcast climates.
VLTs below 0.35 may appear noticeably tinted and dim to occupants and may degrade luminous quality. However, lower VLTs may be required to prevent glare, especially on the east or west façades or for higher window-wall ratios. Lower VLTs may also be appropriate for other conditions of low sun angles or light-colored ground cover (such as snow or sand), but adjustable blinds should be used to handle intermittent glare conditions that are variable.
Color-Neutral Glazing
The desirable color qualities of daylighting are best transmitted by neutrally colored tints that alter the color spectrum to the smallest extent. In particular, avoid green and bronze colored glazing.
Reflective Glass
To the greatest extent possible, avoid the use of reflective glass or low-e coatings with a highly reflective component. These reduce the quality of the view and the mirrored effect is unpleasant to occupants after dark.
Light to Solar Ratio
The use of high-performance and selective low-e glazing reduces the visual light transmission (VLT) proportionately less than do reflective coatings or tints. Dividing the VLT by the solar heat gain coefficient (SHGC) is a good rating of the performance of the glass. If the result is less than 1.0, then the glass is a poor choice for visual quality and daylighting. If the result is higher than 1.55, it is a high-performance option.
Effective Aperture (Daylight)
The Window Wall Ratio (WWR) times the Visual Light Transmission (VLT) results in the "effective Aperture" predicting the day lighting potential of the glazing. Depending on the latitude and predominant sky conditions (clear or overcast), Effective Apertures of day lighting are generally between 1.5 and 3.0. The smallest Effective Aperture that will meet day lighting needs should be pursued. It is unlikely that sufficient day lighting savings or user acceptance will be realized with Effective Apertures much less than 1.5.
Preferred Window-Wall Ratios (WWR)
For view and a positive connection to the out-of-doors, people prefer a minimum 20 percent to 30percent ratio of window area to wall area. Glazing the wall areas below desk height (0-30 in. above the floor) offers little or no benefits for daylighting an office or view for the occupants.
High Windows
High, continuous windows are more effective than individual or vertical windows, to distribute light deeper into the space and provide greater comfort for the occupants. Try to locate the top of windows close to ceiling line (for day lighting) but locate the bottom of windows no higher than 48 inches (for view). Consider separating windows into two horizontal strips, one at eye level for view, and one above to maximize daylight penetration.
High Ceilings
More daylight savings will be realized if ceiling heights are 10 feet or higher. Greater daylight savings can be achieved by increasing ceiling heights to 11 feet or higher and specifying higher VLTs (.60-.70) for the upper glazing than for the view windows. North facing clerestories are more effective than skylights to bring daylight into the building interior.
Light Shelves
Consider using interior or exterior light shelves between the daylight window and the view window. These are effective for achieving greater uniformity of day lighting, and for extending ambient levels of light onto the ceiling and deeper into the space. Some expertise and analysis will be required to design an effective light shelf.
Window and Office Placement
Daylighting is more cost effective if open plan workstations are located on the north and south side of the building since open plan areas are more continuously occupied and achieve lower savings from occupancy sensors. The open configuration also absorbs less light, and inter-reflections provide a more uniform distribution of light deep into the space. The control of heat and glare on the east and west facades is difficult, because daylight and views are blocked in an effort to properly control the low sun angles. By placing private offices on the east and west, occupants can individually control their blinds, and thereby control thermal discomfort and glare.
Interior Sun Control
Similar to exterior sun control, horizontal blinds on the south windows and vertical blinds on the east and west are most effective. In northern latitudes, low angles of sun can enter the north windows on summer mornings and afternoons. Vertical blinds that retract fully for the middle of the day are recommended for these conditions. Perforated blinds and translucent shades may cause glare when hit by direct sunlight.
, Atlanta, GA.
Advanced Energy Design Guide for Small Office Buildings
. Reproduced with permission. All rights reserved.