|
|
W hat is daylighting? In its most general sense, it is the use of natural light to illuminate the interior of a building. The goal is to make the architecture and life within it visible. The challenge is to bring light through the building envelope in a balanced way. Provide enough illumination but not too much; supply what heat might be useful but not too much; and dampen the natural cycle of the daylight hours to make the luminous environment steady and suitable for the tasks at hand.
What are the sources of natural light? The sun, of course, is the primary one. From a vantage point inside a building, the sky without the sun or the overcast sky is a principal source as well. The ground and neighboring buildings can be secondary sources, reflecting light into a room.
When light comes in a beam, it is most often directly from the sun but can also be from a mirrored surface. It brings high illuminance, casts distinct shadows and creates high contrast. We enjoy these qualities while relaxing on a sunny day but may find they interfere with tasks at work. When light is diffused it can be coming from the sky, from a matte surface or through a translucent medium. In this form it usually brings low illuminance, lower contrast and in some situations, almost no shadows at all. We tend to see fully diffuse light as gloomy, as on an overcast day, but find it useful for office tasks.
The direct sun beam and diffuse sky light can both contribute. To some, daylighting is more specifically the use of diffuse natural light to provide the basic illumination in a building. The source may be the overcast sky, the diffuse clear sky, or diffusing glazing elements. Much of the research on natural lighting is based on this approach. In this article we will consider both direct and diffuse illumination. Direct illumination brings with it the liveliness of sunlight but also greater potential for glare and overheating. Diffuse illumination tends to be lower in quantity but steadier and easier to control.
 Given the rectilinear nature of most buildings, it is useful to think of natural lighting techniques in terms of bringing the light in either from the top or from the side (Figure 1). What follows is an examination of the techniques used in both approaches.
SKYLIGHT STRATEGIES
Some spaces can be better illuminated from above (i.e., top lighting), using skylights. A field of skylights can spread light evenly throughout a single-story building, or the top floor of a multi-story building, and effectively provide the basic illuminance needed to walk around. There is, however, a greater risk of overheating.
 There are many ways to glaze and shape the skylights to complement the building’s design and uses(Figure 2). A simple skylight with clear glazing that is essentially horizontal gives a view of the sky and highly variable natural lighting. View and variability can be positive attributes in spaces that do not have demanding visual tasks, such as lobbies and lounges. This type of skylight generates high contrast much as a simple window does, if it allows a view of bright sky surrounded by dark interior surfaces. Shaping the skylight opening to create a sort of well or coffer can reduce the contrast by illuminating surfaces adjacent to the glazing. Light distribution improves if these surfaces are sloped outward from the opening to the ceiling.
Clear glazing also creates the potential for direct glare from the sun or a bright sky, and for reflected glare caused by the sun or a patch of sky bouncing off shiny surfaces. Use of diffusing glazing or a matte reflecting surface under the opening can dampen the variations in light level through the day, distribute illumination widely and control contrast.
 A more complex approach to top lighting is to make the glazing vertical (Figure 3). The resulting architectural shapes are known variously as saw-tooth skylights, monitors and clerestories. A challenge of this approach is choosing the orientation. Early versions of saw-tooth skylights, common on industrial buildings, faced the glass north to get steady illumination with little glare throughout the day. Designers looking for passive solar heat gain have tried facing the glass south, using baffles and shading devices to control the direct sun. Designs orienting the openings east and west generally benefit from a reflecting surface to intercept low-angle sun and diffuse it for useful illumination. Other characteristics associated with orientation are generally the same as for windows.
WINDOWS AND SKINS
Side lighting may be done with windows or with an entirely transparent or translucent building skin. This strategy can provide views along with greater potential for glare.
When lighting from the side, orientation affects the quality and quantity of illumination. Facing south (in the northern hemisphere) gets the most hours of direct sun. Control with a fixed horizontal shading device is straightforward at this orientation. With a clear sky the light color is considered “warm.” By comparison, glazing oriented to the east or west gets fewer hours of direct sun. However, because of the lower sun altitude, glass facing east or west receives the maximum heat gain on a vertical surface in a short period of time. Direct sun at these orientations is the most difficult to control, also because of the low altitude. The color of natural light in a room facing only east or west is highly variable throughout the day, from the orange of sun at the horizon to the blue of the sky alone. Facing the glazing north gets little or no direct sun at most latitudes and is therefore the easiest orientation to control. The color is what we call “cool,” and the illuminance varies the least throughout the day.
Imagine the simplest window. It gives a view out. It allows natural light into the room. It does little to modify the light from the sun or the sky or the ground. It creates an illuminated area near it, but does not project light far into the room. When viewed from some distance to the interior, an unshaded window can give high contrast, depending on the scene outside. Viewed against a bright scene beyond, an object or person might be seen in silhouette.
 The surfaces surrounding the window and forming the opening reflect light. If these surfaces are extended, say by thickening the wall, they diffuse a small amount of illumination into the room (Figure 4). This reflected light reduces contrast between the exterior and the interior. Angling or “chamfering” the surfaces enhances this effect. Window niches in thick masonry walls diffuse light with broad angled surfaces. Movable shutters at each window add control to accommodate a variety of sun and sky conditions.
Fixed elements outside the window will, of course, affect how much light comes in and also how much contrast there is between inside and outside. Cornices, porches, awnings, shelves and similar devices all can reduce the amount of sun and sky light that comes through the window directly. Some of these features will also direct ground-reflected light through the window. On the south-facing side, generally horizontal surfaces above the window are best at limiting summer sun and in reducing the sky contribution. On the east and west, vertical slats or dense planting in front of the window are effective at blocking low-angle sun.
Shading devices that move are usually part of any arrangement for controlling glare, contrast and illuminance. Choosing among the infinite number of possibilities is again an issue of balance: how to get illumination without glare; how to get both view and privacy. Translucent shades and blinds or shutters with operable slats are common solutions.
 An approach that complements the use of movable shading is to divide the window opening with a beam so that there are, in a sense, two windows—one above the other (Figure 5). If the bottom window is covered to prevent glare, reduce the light level or maintain privacy, the upper window can still let the sun shine in. Dividing the window also begins to deal with the dilemma of having too much light at the perimeter by the exterior wall but not enough at the points of the interior farthest away.
Adding a sort of shelf at the division between an upper and lower window refines this approach. Usually the shelf is outside, shading the lower window. The top of the shelf reflects light in through the upper window. The top surface may be a specular, semi-specular or diffuse reflector depending on how the light needs to be distributed and what effect is desired. While the shelf can be effective in redistributing natural light and preventing glare, it typically does not increase the average illumination in a room. Generally a light shelf, like all fixed horizontal shading, is most effective on a façade facing south. Working such a device into an architectural design is a project in itself. A light shelf tends to be a large object, needing structural support and detailing to function properly and complement the appearance of a façade.
MORE QUESTIONS
As the shaping of an opening is important, so is the placement of a window or skylight relative to the walls, floor and ceiling of the room it is illuminating. For example, if a window is located with one edge where an interior partition meets the exterior wall, the partition becomes brightly lit. The lighted area minimizes the contrast between that interior surface and the exterior scene. Depending on the orientation, the partition can also reduce direct glare by intercepting and diffusing some direct sun. The partition serves as a diffuse reflector as well, distributing illumination around the room.
The reflectances of the surfaces also play a role. It is intuitive that a room with dark finishes will have less illumination on average than one with light finishes. What may not be obvious is that illuminance in the room with darker finishes will be less uniform than in the one with lighter finishes.
Facing infinite design possibilities can be daunting. Rules of thumb can be helpful in getting started. One place to find them is Lawrence Berkeley National Laboratory, http://windows.lbl.gov/, where extensive research continues on windows in combination with other building systems. The website states that conventional windows provide daylight in the outer 10 ft to 12 ft of a perimeter space, or about 1.5 times the window head height. For effective daylighting the lab suggests that window area be about 30 to 40 percent of wall area. There is a detailed guide available for window design for commercial buildings.
Research by the Heschong Mahone Group suggests several guidelines for skylight design, that is, with the glazing essentially horizontal, including bubble skylights and flat glass units that are sloped to drain. The researchers recommend that the total open area not exceed 5 percent of the daylighted floor area, and that the glazing material be diffusing. They note that the angle of slope or splay of the sides of the skylight well works best in the range of 45 to 60 deg from the horizontal. The firm makes available spacing guidelines and other studies online at www.h-m-g.com/downloads.htm.
Finally, studies of window placement and the effect of room finishes can be found at the Daylight Design web pages http://sts.bwk.tue.nl/daylight/ of the Technical University Eindhoven Faculty of Architecture, Building and Planning.
Addressing all the variables and constraints means massaging the building envelope form into something that works with the structure and program. Making a creative response to requirements is at the heart of architectural design. Ultimately, daylighting is architecture. off of our faces.
|
