Measure 8.3.1 Install skylights or light pipes.

Skylight Materials

\Skylights are commonly made from glass, glass composites, plastics, and plastic composites. All these materials can be treated to reduce light transmission and cooling load, either by adding dyes that absorb light or by adding a reflective surface. All glazing materials can be provided with diffusing properties. As discussed previously, the thermal insulation value of skylights can be increased by installing multiple sheets of glazing and by installing translucent insulation between the sheets.

All diffusing materials absorb a significant amount of the entering sunlight because there are multiple reflections within the material. Absorption is greatest with milky diffuser, and lowest with prismatic diffusers. Absorption is also increased by fibers, pigments, and other materials that are embedded in the material.

The advantages of glass include unlimited life, high light transmission, hardness, and rigidity. Glass can be treated to reduce cooling load by selectively absorbing the infrared portion of sunlight. At present, this capability is available only with glass, not with plastic. See Measure 8.1.3 for details. The infrared absorbing surface should be outermost.

The main disadvantage of glass is its vulnerability to breakage, along with the safety hazard that falling glass creates. Glass can be made more resistant to breakage by increasing its thickness, by heat treating it, and by combining it with reinforcing materials. All safety improvements for glass add cost, and they usually add weight.

Plastic materials are much lighter in weight, and they are resistant to shattering, so they pose only a minimal safety hazard. An entire skylight assembly can be molded from a single piece of non-reinforced plastic. Smaller plastic skylights can be molded so that they overlap a mounting curb, providing excellent resistance to water leakage and greatly reducing the cost of the frame. Plastic skylights can easily be fabricated with multiple layers of glazing to improve thermal resistance.

Plastics can be reinforced with fibers of various materials, including glass, to increase strength and service life. The fibers cause some light loss. They also diffuse light, which is useful in most applications. Reinforced plastic is more difficult to mold into compound shapes. It is normally made in flat sheets, which can be curved in one direction.

The plastics commonly used for glazing are acrylics and polycarbonates. Polycarbonates are stronger, but acrylics are more resistant to degradation by the ultraviolet component of sunlight. All plastics deteriorate in strength and light transmission over a number of years. The main causes of deterioration are ultraviolet light, heat, and oxidation, in that order.

The service life of plastic glazing can be extended greatly with additives. Unfortunately, you cannot judge the long-term performance of a plastic material except from manufacturers’ claims, so purchase skylight material from a credible manufacturer. If you buy skylights as prefabricated assemblies, first examine the plastic manufacturer’s data. If possible, investigate actual field experience with the particular products you are considering.

Flat plastic glazing material buckles as it ages. This can be quite noticeable when the material is observed from the outside, but not when looking at the skylight from the interior.

Glass and plastic can be combined in larger skylights to minimize their respective weaknesses. Glass is used for the outer sheet, where it can provide considerable protection to the plastic, while the inner plastic sheet protects against glass breakage. Ordinary window glass strongly absorbs the damaging ultraviolet portion of sunlight, so a plastic material will survive longer if it is installed inside glass. Design or select combination skylights so that the plastic elements can be replaced separately without a great deal of effort.

Skylight Configurations

Skylights are available in almost any configuration that you could want. Skylights made of glass and composite are usually built up from flat sections. Figure 10 shows a sampling. Plastic skylights can be molded into virtually any desired shape. Skylights made of reinforced plastic materials can easily be curved in one direction. Figure 11 shows a sampling of the latter two types.

Fig. 10 Built-up skylights These can be made in virtually any size. Materials that cannot be bent easily, such as glass and fiber-reinforced plastic, are usually made into skylights this way.

Fig. 11 Molded and curved skylights Molded units are limited in size, but they can be ganged together to create a skylight of any size. Individual units can be molded to overlap curbs, an excellent method of avoiding leaks. Large skylights can be made of sheets of reinforced plastic that are curved in one direction.

How to Prevent Water Leakage

Water leakage is a common problem with skylights. Do not select a skylight design that depends primarily on sealants to prevent leakage. The most reliable method of avoiding leaks is to use one-piece molded skylights that overlap the curb. Unfortunately, molded skylights are limited in size.

If a skylight must have joints, design it so that the joints are steeply sloped to shed water. Ideally, no joint or sealant should face uphill, but this condition cannot be met everywhere in built-up skylights. Instead, the frame extrusion should be designed so that it conveys any water that leaks through seals to the outside of the skylight.

Pay attention to the curb on which the skylight is mounted. The curb should be sealed to the roof as effectively as curbs used for installing scuttles or rooftop air handling units. If a curb is installed on a sloping roof, install an eave on the uphill side of the curb to shed water to the sides of the curb.

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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.