Measure 8.3.1 Install skylights or light pipes.

Energy Saving Potential

In single-floor buildings, skylights may provide a large fraction of illumination requirements. Sunlight is so intense that skylights can provide virtually any illumination level that is required. Of course, artificial lighting is still needed at night.

Sunlight has a better ratio of light to heat than any type of electric lamp. Therefore, if the light from skylights is distributed efficiently and the skylights are not oversized, they may not substantially increase the cooling load. However, this ideal is difficult to achieve.

Skylights can provide significant passive heating during cold weather. This advantage is offset by conductive heat loss at night. In all but the coldest climates, there is a net heat gain if the skylights are located so that they collect the maximum amount of sunlight. On the other hand, skylights that face away from the sun may suffer a net heat loss even in relatively mild climates.

Surface Area Required for Daylighting

Outdoor sunlight from a clear sky produces an illumination of about 60,000 lux, most of which comes directly from the sun. Using this fact, you can easily calculate the fraction of the ceiling area that needs to be converted to skylight. (For a quick introduction to “lux” and other measures of light intensity, see Reference Note 50.)

For example, consider an application that needs an illumination level of 500 lux. To account for losses in reflection and diffusion within the skylight assembly, assume that 40% of the sunlight entering the skylight makes its way into the space. Thus, on a bright day, about 2% of the ceiling area needs to be skylights. To compensate for low sun angles, hazy conditions, dirty skylights, etc., double this to about 4%. To account for average cloudy conditions, increase this to 10% or 15%.

The installation in Figures 1 and 3 have skylights that are sized for approximately the latter percentages of ceiling area. The installation in Figure 2 has a much higher percentage, with the result that the glazing must be darkly tinted to avoid glare. The unusual installation in Figure 4 has a variable amount of skylight area.

These figures assume that the skylights are installed where they remain exposed to direct sun throughout most of the day. In some older industrial buildings, large skylights were installed facing north to avoid glare. This unnecessary practice is still followed in some new buildings, as shown in Figure 5. This greatly increases the glazing area required, which also increases heat loss in cold weather. To keep skylights as small as possible, install them so they face the sun as much as possible. Control glare with diffusion and careful space layout, as discussed below.

In predominantly warm climates, select the skylight area to give the best compromise between savings in lighting energy and extra cost for cooling energy. If the space is air conditioned, design the skylights for a clear sky. This means, keep them small.

In predominantly cold climates, the balance usually shifts toward larger skylights. This makes it more important to select the skylights for low conductive heat loss. If you want to exploit passive heating, the skylights must be much larger, and the whole arrangement Super Sky Products, Inc.

Fig. 2 Large, attractive skylight This is a large amount of glazing area in relation to the illumination requirement.

Fig. 3 Skylighting for a variety store The skylight consists of a small area of translucent panels surrounding the cupola. The lighting is effective for the colorful merchandise. The geometry of the skylight does not extend sufficient daylight into the ends of the space, so artificial lighting is needed there. Daylighting of this space was later abandoned, for unknown reasons.

becomes expensive and elaborate. For more about passive heating, see Reference Note 47, Passive Solar Heating Design.

Use the minimum total skylight area that you need to provide good illumination, and if appropriate, passive heating. This is because surface area increases heat loss, cost, and structural problems. Therefore, make all skylights as transparent as possible, subject to the need for diffusion, multiple glazing, reinforcing fibers in plastic material, etc. Do not use skylight materials with tints, and do not use skylight materials that reflect sunlight.

Efficient distribution of daylighting within the space is as important as the skylight area. Skylights should deliver their light where it is needed, and they should avoid creating visual problems. You achieve these characteristics by effective layout of the skylights, and by using diffusion. We will cover these two topics next. 

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These excerpts shows the level of detail and accuracy that you'll find in the Energy Efficiency Manual by Donald Wulfinghoff. This 1500-page manual offers the most comprehensive coverage of energy-saving measures for buildings ever assembled under one cover. For more information and to view the complete table of contents, find the Energy Efficiency Manual in the Oikos Bookstore.