Daylight can be defined as diffuse light through clouds or partially cloudy skies. In a cloudy climate, the diffuse sky is often the main source of useful daylight.
For climate conditions in which cloud cover exists (no visible sun for a substantial percentage of the year), we should design to optimize daylight. In this case, the light source is the sky, not the sun or sunlit surfaces. Still, some of the sunlight strategies also apply to daylight, such as using light efficiently, controlling the amount of light, and integrating with architecture. Because the overcast sky not a point source it is an area source.
Here are the basic approach to the daylighting strategies;
1. Maximize solid angle of the sky seen from the task or light-reflecting surfaces. In practice, this means that tasks cannot be too far from the aperture. (windows and skylights). However apertures can be larger for daylighting than for sunlighting.
2. Shade to prevent glare. avoid direct views of overcast sky because it is a bright source of potential glare. Shading is not needed on the building exterior since heat gain is not a problem from overcast sky conditions.
3. Do not block light. Do not use solid light shelves or overhangs. They are not effective for redistributing light in overcast sky conditions and may reduce the amount of daylight reaching the task.
4. Locate openings high. Openings should see the brightest part of the sky. The overcast sky at the zenith is about 3 times brighter than at the horizon. High window locations and horizontal skylights will provide the best access to light from overcast sky.
5. Shape space to minimize absorption of light. Use high-reflactance interior finishes. Maximize the ceiling height near windows to allow high windows and slope ceiling down toward the rear to minimize interior surface area.
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Sources; Architectural Lighting by M. David Egan and Victor W. Olgyay