Today, a working knowledge of daylighting needs to be part of every lighting designer’s skill set. But in fact, daylighting is a broad term that encompasses many methods and techniques. This article explains some of the fundamental principles of daylighting with windows and clerestories, aptly termed Sidelighting, along with a set of important strategies. The concepts presented are applicable to a wide range of projects types where daylight enters the building form from the perimeter from the open plan office in a multi-story office tower, to the single room classroom.
The first step toward mastering Sidelighting is to realize that it is literally “light from the side.” It is basically a two-part process. The first task is to admit the proper amount of light into the interior by adjusting the size and transmission of the window aperture relative to the room area. The second task is to apply an optimal combination of room-related design decisions to guide and maximize the primarily lateral flow of light away from the window wall, deep into the interior.
Although it sounds simple and straightforward, creating a visually comfortable luminous environment by directing light from one side of a room to the other is an artful challenge. For example, imagine if a lighting designer tried to construct the electric lighting equivalent of a typical window using some type of lensed fluorescent troffer recessed into a wall at eye level. Now consider the apparently conflicting requirements: Occupants viewing computer displays seated anywhere from 2 to 20 feet away from the lighting set-up, who in turn must receive glare-free, adequate illumination, yet also maintain an eye level view of the light fixture. Do not fear, this article will provide at least one or two creative ideas on how to make this lighting condition a more favorable situation.
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.
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 is 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 reflectance 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.
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, to 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 wall 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 important elements to consider in relation to 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 last 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 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. Its 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 just where they are needed.
All windows incorporate some type of solar shading into their design. Shading may be accomplished by the position of the window itself, for example by locating it high up, or in a northern corner of a room. However in most windows, shading is addressed separately from the aperture itself using a shading device. Horizontal elements such as overhangs and light shelves work well for orientations within 45 degrees of south since the sun is high in the sky. Vertical elements work well for elevations within 45 degrees of east or west since the sun is low in the sky. Shading devices serve a dual purpose: To reduce heat and to prevent glare. The most efficient way to manage the solar heat gain absorbed by the shading device is keep it outside of the building envelope. To control glare, consider the brightness of the room side view of the shading device in comparison to the surrounding room interior, and use surface finish and geometry to keep within an acceptable brightness ratio.
Light shelves are functional for facades oriented within 90 degrees of south. The exterior portion of the shelf shades the lower window from direct sun to reduce the transfer of heat into the building envelope when it is not needed. Extend the shelf into the interior to prevent glare in near-window task areas by redirecting sunlight up on to the ceiling plane. See Figure 13. It is important to note that a standard light shelf does not increase overall daylight in the room; rather it decreases illumination near the window wall where it would otherwise be the highest. By doing so, it fulfills its primary function to even out the illumination gradient from the window to the rear of the room.
Design for Climate Zone
The action of the shading device, the ability of the glazing to reject solar heat gain and resist the transfer of heat through the building envelope, are all performance characteristics that can be optimized to suit a particular climate zone. In an area where cooling is a major building expense, shading window apertures throughout the cooling period and using sun-protecting glazing will make a building less dependent on its mechanical systems and save energy. In regions where heating is a major cost, allowing sunlight into the interior building envelope to heat a large mass that in turn releases heat long after the sun has set, is a well established passive solar heating strategy. Using a window system with a high resistance to the transfer of heat, measured by U-value, will work to keep this passive collection of energy inside the building envelope.
Integrate Electric Light
In order to maximize your daylighted floor area, daylight first, then supplement with electric light. After this, explore using light for general illumination during the day, which at night can be lowered or completely removed, as a person’s eyes adapt to lower light levels in preparation for sleep periods. Whatever your strategy, make sure to implement a daylighting control system, whether it is a combined daylight and occupancy responsive dimming system, or a person whose sole responsibility is to switch light fixtures and operate shades.
Remember, if the building’s electrical demand for light and heat is not being reduced, it is not daylighting. Keep these basics in mind and you too will master the age-old art of this renewable energy source.