Measure 8.3.1 Install skylights or light pipes.

Skylight Layout

In general, it is better to use a larger number of smaller skylights, rather than one or a few large skylights. There are many examples of horrible daylighting in which someone attempted to illuminate a space with only one big skylight. Figure 6 shows a good distribution of skylights for a space with a tall ceiling. Figure 1 in Measure 8.3.2 shows the interior of this space. Using smaller skylights has several important advantages:

• you can tailor the light distribution within the space more accurately. The skylights do not have to be installed in a regular pattern. In general, the size of skylights, and the spacing between them, should be proportional to the ceiling height. Stated differently, skylights should not have to throw light far to the side.
• an array of smaller skylights provides illumination that is much more uniform than the light from a single large skylight. Installations with large skylights commonly suffer from excess brightness directly below the skylight, accompanied by gloomy dark areas surrounding the skylight.
• less modification of the roof structure is needed. Large skylights require special roof design to carry the roof loads around the skylight. In existing buildings, it is often possible to retrofit small skylights, but not large ones.
• it is easier to avoid leakage problems with small skylights, for the reasons discussed below.

It helps to think of skylights as a class of light fixtures. They must obey the same rules of physics and lighting quality as electric light fixtures.

Compare the skylight installation in Figure 1 of Measure 8.3.2 with the skylight installation in Figure 7. The former provides better distribution of light throughout the space, with less glare. (However, it lacks proper diffusers, as we will discuss.)

Another useful guideline is that the dimensions of the skylight should be a small fraction of their height above the floor of the space. Thus, a large skylight over an atrium may be satisfactory, but not a large skylight over a dining room. The central skylight in Figure 3 is satisfactory because it is high above the floor. However, the daylighting does not penetrate well to the perimeter of the space.

Fig. 5 Inefficient skylight orientation The skylights on this bus garage face north. This avoids glare problems and excessive solar heat gain, but it provides little light in relation to the heat loss from the glazing. Overhead equipment keeps much of the daylight from penetrating into the space. Kalwall Corporation Fig. 6 Good skylight sizing and layout This shows the small fraction of roof area that is needed for typical illumination levels. These skylights are for a gymnasium, and the repetitive layout provides uniform illumination within the space. See Figure 1 of Measure 8.3.2 for the inside. Vistawall Architectural Products

Fig. 7 Gymnasium daylighting with a single large skylight Compare this to Figure 1 of Measure 8.3.2. The barrel roof of this building makes it difficult to install distributed skylights. Fig. 8 Large skylight arrays These illuminate areas of a public library with relatively low ceilings. The areas immediately beneath the skylights receive too much illumination and heat gain in bright daylight, while the light is unable to disperse to the rest of the space. the daylighting does not penetrate well to the perimeter of the space.

Skylights that are large with respect to the ceiling height may be decorative, but they are not optimum for daylighting, and they may be overwhelmingly bright. Figure 8 shows large clusters of skylights installed in a library. The brightness inside the space can be excessive. Figure 9 shows a huge skylight that covers an entire office.

Effective Diffusion is Essential

Skylights generally need diffusion. Direct sunlight through skylights is not suitable for illumination. It is much too intense, it forms localized bright spots, and it shines on the wrong places. Diffusion corrects or reduces these problems by distributing sunlight in a fairly uniform pattern. It also minimizes changes in illumination caused the motion of the sun. Daylighting was abandoned in favor of electric lighting largely because of unsatisfactory illumination resulting from lack of diffusion.

For example, consider a space that has many small skylights. If the skylights are clear, they produce intense bright spots on the floor, surrounded by darkness. See Figure 1 in Measure 8.3.2. As a result of this oversight, the skylights produce no useful illumination, and the electric lights must be turned on to compensate for the glare. On the other hand, if the skylights diffuse the light, it is spread throughout the space.

You can make any surface of the skylight glazing a diffuser by selecting the material for this purpose. Or, you can install separate diffusers either above or underneath clear glazing. It is easier and usually preferable to make the diffuser an integral part of the skylight, unless you are also trying to accomplish passive solar heating.

Diffusion introduces its own set of issues to consider. The main ones are limiting glare, dealing with solar heat gain, and limiting light loss.

Limit Glare

The term “glare,” as we use it here, means an area of intense brightness within the visual field. (Reference Note 51, Factors in Lighting Quality, explains glare in greater detail.) Diffusion has the potential of creating serious glare because it makes skylights look like bright light sources. For example, small skylights are similar to flat-faced fluorescent ceiling fixtures in appearance and light distribution pattern, although they can be significantly brighter.

Glare is a problem only when the bright surface is within the field of vision. Fortunately, people tolerate bright light sources that are overhead. As with other light sources, the solution to glare is to locate skylights well above the line of sight. In spaces with very tall ceilings, such as gymnasiums and manufacturing plants, the height of the ceiling alone may be sufficient to keep glare within acceptable limits.

If the skylight is installed at the top of a shaft or recess in the ceiling, this keeps the skylight out of normal lines of sight. For this to minimize glare in a space with a low ceiling, the shaft or recess should be at least as tall as the maximum dimension of the skylight. In other words, the recess needs to be taller with bigger skylights.

Locate Diffusers to Minimize or Exploit Heat Gain

The solar heat gain into the space is strongly affected by how the diffuser is installed, and by the characteristics of the diffuser. Heat gain is lowest if diffusion is limited to the outer surface of the glazing. In that case, a large fraction of the heat absorbed by the diffuser itself is carried away be the outside air. Heat gain is greatest if the innermost surface is used as the diffuser.

Heat gain is increased even more by locating the diffuser farther inside the space, so that less of the entering light is reflected back out. To capture solar heat for passive heating, install a separate diffuser inside the space that is made of absorptive material. Measure 8.4.2 explains this arrangement in detail.

Where to Locate the Diffuser if the Skylight is Installed Above a Ceiling Recess

The location of the diffuser matters most when there is a recess or shaft between the skylight and the interior of the space, typically to create a path for the daylight through the roof structure.

The location of the diffuser in a shaft radically affects the light distribution pattern. Installing the diffuser at the bottom of the shaft produces a broad pattern. Installing the diffuser high in a tall shaft produces illumination similar to that of a downlight.

Installing a diffuser higher in the shaft keeps people from seeing it, except when it is more nearly overhead. This reduces the possibility that glare will be a problem.

The surface of the shaft absorbs light. A diffuser installed high in the shaft deflects more of the light toward the shaft surface, making it especially important for the shaft surface to be highly reflective. A specular surface saves more light than a diffuse surface, because it reflects all the light downward. However, a diffuse surface may give better light distribution. It depends on the relative geometry of the skylight, the shaft, and the space.

If you install a separate diffuser, make sure that no sunlight leaks around the diffuser directly into the space. Direct sunlight is an intense source of glare, and it is useless for illumination.

Avoid Nasty Surprises: Heat Loss and Condensation

The heat loss of skylights may be much higher than you expect. Hidden away in the technical literature is the fact that heat loss through glazing is two to three times higher when the glazing is installed in a horizontal or steeply slanted orientation than when it is installed vertically. Thus, the double glazed skylight that you expected to have an R-value of 2 actually has an R-value less than 1.

One of the unpleasant surprises that results from this low thermal resistance is a tendency for skylights to sweat profusely. The condensation can damage or disfigure the surrounding structure. Poorly insulated skylights may drip heavily on the space below. In cold climates, it is worth going to great lengths to limit the heat loss of skylights. These are your possible solutions:

• select multiple glazing. Up to three or four sheets of glazing are practical. Light transmission is reduced somewhat, and weight and cost are increased.
• use a glazing material that includes translucent insulation. Translucent glazing systems are now available that offer R-values as high as 10. These systems combine a plastic or composite glazing material with a layer of translucent insulation, which may be glass fiber or foam. See Measure 8.3.3 for the details. There is a strong compromise between R-value and light transmission, so skylights using this material must be larger than skylights that use conventional glazing.
• install movable insulation, which can greatly reduce heat loss without reducing light transmission. Movable insulation is challenging to design and to install. For an introduction to movable insulation, see Reference Note 47, Passive Solar Heating Design.Super Sky Products, Inc.

All skylight frames should include gutters to catch condensation that flows off the interior surface of the glazing. This is important to keep the condensation from rotting or disfiguring the structure around the skylight.

The gutters should be large enough to hold all the condensation until it can evaporate back into the space.

Fig. 9 An illustrative example Analyze this skylight installation, which illustrates most of the issues of daylighting. How suitable is the skylight for the activities? How appropriate is the glazing area from the standpoints of glare, solar heat gain, and conductive heat loss? How effective are the controls for the electric lights? What should be the transparency of the glazing? Should diffusers be used? Internal shading? How would these issues be affected by the location of the building? Overall, is this effective daylighting or primarily an esthetic feature?

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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.