Skylight Design Considerations

Before selecting a skylight for your home, you need to determine what type of skylight will work best and where to improve your home’s energy efficiency.

Energy Performance

First, it’s a good idea to understand the energy performance ratings of skylights if you don’t already. You can then determine what energy performance ratings you need for your skylight based on your climate and home’s design.

For labeling energy-efficient skylights, ENERGY STAR has established minimum energy performance rating criteria by climate. However, this criteria doesn’t account for a home’s design. Therefore, if you’re constructing a new home or doing some major remodeling, you should also take advantage of the opportunity to incorporate your skylight design and selection as an integral part of your whole-house design-an approach for building an energy-efficient home.

Size and Position

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 percent of the floor area in rooms with many windows and no more than 15 percent 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 leaf-shedding trees or adding a movable window covering on the inside or outside of the skylight. Some units have special glazing that can help control solar heat gain.

Skylight Design Considerations

Skylight Glazing

When selecting a skylight for your home, it’s important to consider what type of glazing you should use to improve your home’s energy efficiency. Based on various skylight design factors-such as its orientation and your climate-you may even want different types of glazing for different skylights throughout your home.

Skylight glazing usually consists of either plastic or glass. Other glazing technologies may also be used for solar heat control.

Plastic Glazing

Plastic glazing is usually inexpensive and less liable to break than most other glazing materials. However, these 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 than polycarbonates, but cost less. Although polycarbonates offer high impact resistance, some yellow with age.

Glass

Glass is usually found on the more expensive skylights. Glass is more durable than plastics and does not discolor. All glass used for skylights must be made of “safety glazing,” a generic term for both tempered and laminated glass. Tempered glass is the most impact resistant. 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.

Solar Heat Control Glazing

Because skylights are located on the roofs, they can result in large amounts of unwanted summer time solar heat gains and winter time heat losses. Manufacturers use various glazing technologies to reduce these impacts. The most common technologies include those also used for window glazing:

  • Heat-absorbing tints
  • Insulated glazing (double-glazed, triple-glazed)
  • Low-emissivity (Low-E) coatings.

Some manufacturers even install a translucent insulation material between several glazing layers to create a more thermally efficient assembly.

Daylighting

Recent “high tech”” developments maximize skylights for daylighting. An “element” on the roof becomes an aperture for collecting sunlight. It may be a sun-tracking, open-sided cylinder; a large lens-like element; or merely a conventional skylight with a mirrored reflector mounted adjacent to it. This aperture may then connect to a mirrored pipe, or “light pipe,” which has 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, relative to equivalent traditional skylights, effectively reduce daytime overheating and nighttime heat loss, but they do not provide views or ventilation.

Ventilation

Skylights can provide ventilation as well as light. Ventilating a building through a skylight opening releases the hot air that naturally accumulates near the ceiling. Ventilating skylights usually open outward at the bottom, some more than others. Some units vent through a small, hinged panel. One design uses a swing-down inner sash with a protected vent strip above. This can reduce the potential for rain or snow entering the room if the vents are open. 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 doors are sometimes called “roof windows.” Roof windows are always located within a few feet of the floor.

Moisture Control

In very cold weather, skylights are often prone to water vapor condensing on the glazing. The accumulation of water may then drip into the room. Better skylights usually have an interior channel to collect the condensate so it can evaporate later. The most thermally efficient skylights are less prone to condensation problems.

Skylight Shapes

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. Their raised design allows light to enter from more extreme angles than flat or warped plane units. This allows more positioning options.

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.