In “Mastering Sidelight,” (April/May 2007, p. 93–97) a methodology for sidelighting was introduced along with a set of guidelines for relating sky view, aperture, and room. The present article follows with an in-depth discussion of how to design for an equally valuable interrelated resource—view. It describes how to separate the window into view and daylight function, then optimize for view by considering three fundamental physical properties of aperture: size, transmittance, and location.
Mining the Viewshed
Views are a distinct site amenity. Similar to a site's available daylight, a sustainable design effort needs to recognize and make use of views. Incorporation of local materials, such as stone or wood, not only reduces energy, but also imbues a project with an identity that is part of the local vernacular. The same concept can be applied to view. Identifying and hierarchizing a site's view corridors—natural and man-made, sky and earth, near and far—establishes a site's “viewshed.” Implemented as a concurrent process with other energy flows from sun, wind, and light, it allows for planning of form and fenestration. See Figures 1 and 2.
View vs. Daylight
Windows designed primarily for view do not provide adequate task illumination. Conversely, windows designed for daylight do not necessarily provide view; in fact, they often contribute some type of glare. The split window strategy presented in our earlier article addressed these conflicting functional requirements. See Figure 3. The approach has three basic steps: 1) separate the window into two parts; 2) optimize size and transmission for each function; and 3) locate the view window within the field of view, and the daylight window above the field of view near to the ceiling.
To develop a deeper understanding of view function we will explore the interrelationships between three characteristic window aperture properties: size, transmission, and location.
Window size is measured in relation to wall area and described by the window-to-wall ratio (WWR). There is a minimum size for a window to provide adequate view, just as there is a minimum size for a window to provide adequate light. The main difference is that the threshold size for view is based on the viewer rather than the wall. To maintain a certain angular area, the aperture needs to increase in size as the viewer moves further from the window wall. According to the British Standard Daylight Code, for a distance less than approximately 25 feet, a minimum .20 WWR is required, and for a distance greater than approximately 50 feet, a minimum .35 WWR is required. See Figure 4. Several studies over the past few decades have shown that the majority of people find windows sized to a WWR above about .30 provide sufficient view. It is important to note that exceeding these thresholds to enhance view can become problematic unless specific attention is paid to glazing thermal properties. Large glazed areas may admit excessive solar heat gain during the cooling period, and lose heat during the heating period. The radiant and thermal effect of large glass areas also can cause discomfort for occupants seated near windows in the form of drafts, chills, and overheating, and thwart the occupant's ability to enjoy available view.
Visible transmittance (VT) is a percentage of visible energy admitted and measures glazing transmission. The minimum acceptable VT to provide adequate view can be keyed to the window wall's cardinal orientation and the predominant sky condition: overcast, clear, or partly cloudy. The three sky conditions have different characteristic ranges of illuminances and sky ground brightness relationships. Under an overcast sky, the ground typically is darker than the sky and the exterior illumination level is low. Under a clear or partly cloudy sky condition, the ground typically is brighter than the sky and the illumination levels are two to 10 times higher than an overcast sky. For these reasons, as a general rule, lower VT glazing is more acceptable in clear or partly cloudy sky conditions, and higher VT glazing in overcast conditions. See Figure 5. In RP-5-99 IESNA Recommended Practice of Daylighting, a related study is cited where, for spectrally neutral glass, the majority of people found a 38 percent VT acceptable for predominantly overcast skies and a 32 percent VT acceptable for clear skies. Keeping in mind the basic concept of light sunglasses for overcast skies and dark sunglasses for clear skies, this range can be extended slightly in each direction. During partly cloudy sky conditions, the area of the sky opposite the sun can have excessive luminance levels requiring low-end of acceptable VT. Views toward the North (in the Northern Hemisphere) of high-reflectance buildings under similar sky conditions also may call for a similar solution. It is critical to understand that adjusting glazing VT to allow view of direct sun is not possible since it would prohibit all views. Another factor to consider is the location of high-VT and low-VT apertures within the same field of view as the juxtaposition allows comparison and may affect the acceptability of the lower VT.
A window's view frame can be deconstructed into three layers: sky, horizon, and earth. The sky, located toward the upper part of the wall above eye-level, shows distant background. The horizon, located at eye-level, shows mid-ground. The earth, located toward the bottom of the wall below eye-level, shows foreground. Not all view frames incorporate all three elements; some may include just one of these components. The interpretation of view is a subjective phenomenon, but a compelling visual conclusion, illustrated in the example shown, is that views with all three layers, or at least two, may be the most satisfying for the occupant. See Figure 6A, 6B, and 6C.
Consider a minimum room height of 9 feet, a minimum daylight window height of 18 inches optimally placed at the top of the window wall, and a daylight penetration depth twice the daylight window head height; the datum separating daylight and view functions falls at approximately 7 feet 6 inches above finished floor (AFF). This offers some logic behind the manual calculation method option for the Leadership in Energy and Environmental Design's (LEED) Indoor Environmental Quality Daylighting credit 8.1 that considers any window area below 7 feet 6 inches AFF (down to 2 feet 6 inches AFF) to be noneffective for daylighting and designated for view purposes only. See Figure 7.
The Window in Balance
Daylighting also depends on aperture size and transmission, but unlike the fixed nature of these properties for view function, the relationship is inverse.
Total daylight admission depends on effective aperture (EA), the product of WWR and VT. The three windows in Figure 8A, 8B, and 8C all have the same EA, so they admit equal quantities of daylight into the room. The top room's window has no glazing, so it has a VT of 100 percent. The bottom room's window fills the entire wall, so its WWR is 1.0. To keep the EA constant in each room, aperture VT is increased or decreased in inverse relation to the WWR. The middle room's window (.40 WWR x .70 VT) represents a typical window that tries to balance daylight and view. The WWR is .40, just below the maximum value allowed in U.S. energy codes using the prescriptive method—meaning criteria are described, as opposed to the performance method where compliance is based on gathered data. The VT is 70 percent, representing a neutral color high-performance glazing on a clear low-iron (less green tint) substrate.
Our example should help with understanding the intent of the manual glazing factor calculation method LEED offers to achieve daylighting credit 8.1. As part of this calculation, any window with a VT below 70 percent is to be considered for view purposes only, and it requires the area of the window to be considered for daylighting to be above a 7 foot 6 inch datum and have a minimum VT of 70 percent. In other words, using glazing with a fixed VT, it is difficult to get one window to do it all.