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Jack Sargent, P.E., LEED® AP,
San Francisco Associate Principal
In creating sustainable building environments, the use of natural light, daylighting, as a resource can enhance the functionality and aesthetics of a space, and provide lighting at no cost. Of course, being near a window or having a "window office" has long been a coveted benefit in the working world. People feel more comfortable when they have a sense of what is going on outside of the building. To optimize this effect, there are many considerations in designing for daylighting. As natural lighting is variable, methods of allowing the users of a space to control the quality and quantity of light entering the interior environment are important.
One of the first steps in using daylighting is to determine how much direct and indirect light is available. The quantity and quality of light is affected by the exterior environment, the means of transmitting the light to the space, and the characteristics of the interior environment. The exterior considerations, such as landscaping adjacent buildings or hills, and sky conditions, quickly bring to the forefront that daylighting is site and time specific, affected by variations in season, time of day and geographical location. Computer modeling of a space allows accurate prediction of the lighting levels under different conditions. Guidelines, such as designing the window height to be at least one half the depth of the room, and the glass area to be at least 25% of the floor area, to achieve penetration and uniformity of natural light in the space, are also useful. The inherit variability of natural light is part of the appeal. However, direct sunlight typically does not provide appropriate lighting for performing a task. To avoid problems from glare or excessive light levels, means of control are important to providing a comfortable and functional environment.
The amount of natural daylighting also significantly affects the thermal comfort of a space. Selecting the right transmission qualities of glass can help control the heat and light entering a space. Interior shading devices, such as blinds and shades, may control the light but can allow heat to enter the space. These effects underscore the importance of controlling the amount of daylight entering a space. This control can be achieved by both passive and active means.
Passive means of controlling daylight include the architectural configuration and orientation of the building, and the design of fenestrations and shading devices as well as automatic controls. Landscaping can have a significant affect on the daylighting entering a space and controlling the entry of the light into the interior environment. Deciduous trees provide an attractive and natural positive affect on daylighting. In seasonal climates the trees provide shade in the summer, helping to keep the building cool. In winter when the leaves have fallen, the additional light entering the building provide needed heating. Automatic dimming devices that adjust the light level in a space can also be used. These devices sense the amount of light in the space and reduce the level of electric light appropriately, thus saving energy and reducing the variability of the light levels. Glumac has found the use of "bottom up" type shades at perimeter windows to be a very cost effective way to control glare at eye level, while allowing useful light in near the ceiling. By adding shades and dimming control options, occupants can actively adjust light levels in their environment to maximize comfort and satisfaction.
Having a lighting professional involved in the early stages of design can reduce project costs and allow for an integrated system of daylight and electrical lighting. Lighting designers and engineers can assist in the design process by helping the architect understand the impact that choices, such as window size and materials, have on lighting and thermal characteristics of the space. There are factors that are quantifiable but due to the many variables, human perception, weather, and personal taste, there is a great opportunity for engineers and architects to collaborate to achieve the best results when using daylighting.
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Summer Solstice |
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