TIPS & BENEFITS
Skylight Design Considerations
It's a good idea to understand the energy performance ratings of skylights so you can select your skylight based on the local climate and your home's design. For labeling energy-efficient skylights, ENERGY STAR® has established minimum energy performance rating criteria by climate. These criteria don't account for a home's design, however, so if you're building a new home or planning a major remodel, take advantage of the opportunity to incorporate your skylight design and selection as part of your whole-house design.
The physical size of the skylight greatly affects the illumination level and temperature of the space below. As a rule of thumb, the skylight size should never be more than 5% of the floor area in rooms with many windows and no more than 15% of the room's total floor area for spaces with few windows.
You should also consider a skylight's position if you want to maximize daylighting and/or passive solar heating potential. Skylights on roofs that face north provide fairly constant but cool illumination. Those on east-facing roofs provide maximum light and solar heat gain in the morning. West-facing skylights provide afternoon sunlight and heat gain. South-facing skylights provide the greatest potential for desirable winter passive solar heat gain than any other location, but often allow unwanted heat gain in the summer. You can prevent unwanted solar heat gain by installing the skylight in the shade of deciduous (leaf-shedding) trees or adding a movable window covering on the inside or outside of the skylight. Some units have special glazing that help control solar heat gain.
Skylight glazing is usually either plastic or glass, although other glazing technologies may be used for solar heat control. Depending on the performance you expect from a skylight, you may choose different types of glazing for different skylight locations throughout your home.
Plastic glazing is usually inexpensive and less liable to break than most other glazing materials. However, plastic surfaces scratch easily, and they may become brittle and discolored over time. Many plastics also allow most of the ultraviolet (UV) rays in (unless the glazing is coated with a special film), which increases fading damage to furnishings. Acrylics and polycarbonates are the most commonly used plastic glazing. Acrylics are weaker but less expensive than polycarbonates. Although polycarbonates offer high impact resistance, some yellow with age.
More expensive skylights are usually glazed with glass. Glass is more durable than plastics and does not discolor. Glass used for skylights must be "safety glazing," a generic term for both tempered and laminated glass. Tempered glass is the most impact resistant, and laminated glass is fabricated with a thin layer of plastic embedded near the center of the glass. Both keep the glass from breaking into large, sharp pieces. Skylights are often made with a tempered glass on the exterior side and a laminated pane on the interior side. This arrangement gives maximum impact resistance while protecting occupants from falling shards of glass.
Skylights are located on the roof, so they can result in unwanted summertime solar heat gain and wintertime heat loss. Manufacturers use various glazing technologies to reduce these impacts, including heat-absorbing tints, insulated glazing, and low-emissivity (low-e) coatings. Some manufacturers even install a translucent insulation material between several glazing layers to create a more thermally efficient assembly.
Operation and Use
Most homeowners install skylights to provide daylighting and/or ventilation. Recent developments in skylight design use sun-tracking, open-sided cylinders; large lens-like elements; or mirrored reflectors mounted adjacent to a conventional skylight to provide daylighting without daytime heat gain or nighttime heat loss. Such a skylight may connect to a mirrored pipe or "light pipe" with a diffusing lens that mounts on or is recessed into the ceiling of the room below. Most tubular skylights have this feature. These skylight designs do not, however, provide views or ventilation.
Skylights can provide ventilation as well as light. Ventilating a building with an operable skylight releases the hot air that naturally accumulates near the ceiling. Ventilating skylights usually open outward at the bottom, and some units vent through a small, hinged panel. Skylights may be opened manually with a pole, chain, or crank. Automated units with electric motors or pneumatic devices are also available. Some models incorporate moisture sensors to automatically close the skylight when it rains. Larger skylights that can be used as emergency exits are sometimes called "roof windows" and are located within a few feet of the floor.
Skylights are available in a variety of shapes and sizes. The most common shapes include rectangular, circular, oval, diamond, triangular, multi-sided, and tubular.
Non-rectangular units usually use plastic glazing, but higher quality ones use glass. The glazing can be flat, arched, domed, pyramidal, or "warped plane"—flat on the low side and concave in section on the high side. Of these, the pyramidal, arched, and domed shapes offer flexibility for positioning, because their raised design allows light to enter from more extreme angles than flat or warped plane units.
The slope or curvature of the glazing also helps to shed moisture and leaves. These skylight designs also do not require the additional framing needed to slope a flat skylight for proper drainage on flat or low-slope roofs.
Tubular skylights are smaller than most other skylights. They consist of roof-mounted light or solar collectors, which increase their daylighting potential without the need to increase their size. Because the rooftop solar collector has a small surface area, tubular skylights minimize heat loss in the winter and heat gain in summer. Their small size also minimizes their impact on a home's architecture.
Even the most energy-efficient skylight must be properly installed to ensure that it achieve its energy performance, so it's best to have a professional install your skylight. In addition to following the manufacturer's guidelines, it's also important to consider slope and moisture control during installation.
The slope or tilt of the skylight affects solar heat gain. A low slope will admit relatively more solar heat in the summer and less in the winter, exactly the opposite of what is desirable.
As a general rule of thumb, you want to achieve a slope equal to your geographical latitude plus 5 to 15 degrees. For example, the optimum slope for a south-facing skylight in Columbus, Ohio, at 40o north latitude, is 45o to 55o. At least one skylight manufacturer makes a prefabricated, tilted base that increases the angle of a skylight above the roof.
Water leaks are a common problem with improperly installed skylights. Avoid water leaks by:
- Mounting the skylight above the roof surface
- Installing a curb (a raised, watertight lip that helps to deflect water away from the skylight) and flashing
- Thoroughly sealing joints
- Following the manufacturer's guidelines.
It is also prudent to apply a layer of sheet waterproofing over the flanges/flashing of the skylight. This is generally installed under the finish roofing material as an aid in protecting against ice dams. Avoid water diversion devices such as roof crickets or diverter strips, as they often create more problems than they solve.
Article Source: Energy. gov