The Basics Sidelighting is about controlling the interrelationship of a daylight source, window(s), and a room. Here is a detailed look at each of these elements.
The Source Sun, sky, and reflected light are all components of daylight. But there is another element that is part of all three—heat. For a standard window, skylight is typically the preferred source due to its high light-to-heat content, all day availability, and absence of direct sun—the ultimate source of both heat and glare. Another ideal source that can minimize the amount of heat entering the building envelope is sunlight reflected from an exterior surface, for example, from an adjacent building. However, since the sun is moving, its availability may be of shorter duration and needs to be carefully planned.
Daylighting strategies vary with the light source. Each site has a predominant sky type and set of site conditions that a designer needs to define as they conceptualize and facilitate their design. Two important sky characteristics to consider are brightness distribution and variability.
All skies are not created equal. There are an infinite variety of skies that must fit into three standardized conditions: clear, overcast, and partly cloudy. A good illustration of this is the inverse brightness relationship of a clear versus an overcast sky. See Figures 2 and 4. For a clear sky, the area near the horizon is about three times brighter than the sky overhead. For an overcast sky the inverse is true. For Sidelighting, the resultant effect is that clear skies tend to provide more illumination per window area than overcast skies since the window’s vertical aperture faces a region of sky of higher luminance. Each sky type also has some unique and subtle brightness variations. The brightness gradient of an overcast sky is radially symmetric, while the brightness distribution of the other two sky types distribution are asymmetric. For a clear sky the darkest part of the sky dome is the area 90 degrees opposite the sun. For a partly cloudy sky, the same area is usually the brightest part, not including the sun, due to sunlight reflected off clouds. Since the sun is moving, these areas of light and dark may move with it and may be right in front of your window.
Obstructions outside the window are another modifying factor. If they rise more than 25 degrees above the center of the window it is highly probable that daylighting with skylight alone will not be feasible since a significant portion of available daylight is lost. This obstruction limit is based on a general rule: The minimal amount of overcast sky that a window needs to be exposed to, to admit sufficient daylight is 65 degrees. See Figure 14. However, if there is a clear or partly cloudy sky and the building has a high reflective exterior surface, the building may become brighter than the sky and provide an ample supply of reflected light.
Variability is another major concern. Overcast skies, as well as regions of clear skies that do not contain sun, are fairly constant, but partly cloudy skies are highly variable, they cause constant patterns of change in interior brightness many times over the day, hour, and minute as clouds of varying density pass in front of the sun. These changes may seem more pronounced near the boundary of the daylight zone, since illumination is far from its source and it can be compared with the constant output of electric light. Here a continuous dimming system is an ideal solution, where photocells, dimming ballasts, and control systems work together to modulate electric light to maintain a prescribed light level in response to available daylight.
The Window
Until the day designers are able to control sky and sun, the window will remain a first line of control. Every element of its design, from its size and location, the the type of glazing selected and the detailing of the aperture itself has some effect on the distribution, quantity, and color of daylight admitted.
Let’s start with distribution the most oft-repeated Sidelighting rule of thumb: Daylight illuminates an interior to a depth approximately twice the height of the window. See Figures 5 and 6. Looking at the window all in section, the rule makes perfect visual sense. Light from the sky enters the window at a downward angle, so the higher the window is above the floor, the further daylight extends into the interior. By locating windows on opposite sides of the room you effectively double the daylighted room depth. Following this principle, if a vertical aperture is designated to provide daylight only, locate it as high on the wall as practical in order to maximize daylight penetration.
Right-sizing is an essential to all fenestration design. It is the principle of adjusting window area and the glazing transmittance to control the amount of light flowing into a space. For example, a large area window with low transmittance glazing may provide an equal quantity of light as a small window with high transmittance glazing. A general rule for a well-positioned window with clear glazing is: The amount of glass area needed to daylight a room is approximately 25 percent of the room’s floor area. If low transmittance glazing such as grey-tinted glass is used, the fenestration area can increase proportionally, however it is wise, and often code required, to limit the window-to-wall area to around 35 percent to prevent excessive heat transfer through the building envelope. See Figures 15 and 16.
Aperture orientation and exterior surface characteristics are also important elements to consider in relation to the “color” of daylight. Each cardinal sun direction has a particular overall color due to the amount of atmosphere that sunlight passing through absorbs, or more accurately, scatters, blue light. North exposures have cool-toned light lasting all solar day (duration of daylight from sunrise to sunset). South exposures have warm-toned light lasting most of the solar day. East and west exposures have even warmer-toned light, but it only lasts half the solar day. Since a window aperture sees both direct and reflected light, the colors and surfaces that make up the exterior, from green grass to freshly fallen snow, will influence how daylight renders interior surfaces and objects, even including the complexion of skin.
The Room The room element is all encompassing since it includes everything else besides the window: room geometry, interior surfaces and finishes, partitions, furniture and even the occupants.
The ideal sidelighted room is proportioned so that its depth is not much greater than its width. Even though daylight can penetrate a substantial room depth, a designer does not want to create a tunnel-like effect. Room surface finishes are next for consideration. By selecting a higher than 50 percent reflectance color for the wall opposite the window, we can help guarantee that an occupant will perceive the room as bright, rather than gloomy. See Figures 7 and 8. A ceiling with a high reflectance value, greater than 80 percent, also improves the apparent brightness of the interior, but it really helps with evening the distribution of illumination through the depth of the room. As a general rule, use high reflectance finishes throughout the interior since they increase the number of times light rays interflect before being absorbed by the room itself.
Size and placement of furniture in the room also need to be well considered so as not to inhibit the lateral flow of light. For example, if the room is a library with book stacks, orient them perpendicular to the window wall so light passes through them. The same is true for rows of shelving in retail and warehouse scenarios. If the room is an open plan office, try to use low partitions so light can flow over them, and deeper into the space. If it is a band of perimeter offices with full height walls running parallel to the window wall, designate the upper part of the partition a clerestory to allow adjacent spaces, such as corridors, to borrow light.
Developing a select set of daylight-friendly details is a good way to bring room elements together. For example, canting the ceiling to meet the top of a window eliminates the dark corner between window wall and ceiling. It also maximizes daylight penetration since it effectively raises the height of the window. Splaying the opening around the window reduces contrast between window glazing and the interior surface of the window wall, making the view out more comfortable.
A Parting Strategy The Dual Window concept is key to all Sidelighting strategies. It enables a window to provide two primary functions: daylight and view. Since each requires a different set of glazing characteristics, the aperture is divided into two so that each part may be optimized to serve the intended function. The upper half of the window, located high on the window wall, is designated the daylight window and outfitted with high transmittance clear or translucent glazing. The lower half of the window, located somewhere around eye level, is designated the vision window. See Figure 9. It’s glazing is given a lower visible transmittance that balances the brightness of the exterior with that of the interior wall surfaces surrounding the aperture to enable the view. Since the area of both apertures combined are limited by the room’s wall area as we mentioned earlier, the most judicious approach is to dedicate more fenestration area to daylight function than view function. Separating daylight and view windows into two distinct sets of fenestration is a creative variation. For example, daylight fenestration may be a band of 18-inch-tall clerestory windows that run along the window wall right below the ceiling with an exterior overhang to provide solar control. For view function, individual, smaller area windows may be strategically placed in the lower part of the wall exactly where they are needed.