A skylight is a window located on the roof of a structure to provide interior building spaces with natural daylight, warmth, and ventilation. A skylight can provide your home with daylighting and warmth. When properly selected and installed, an energy-efficient skylight can help minimize your heating, cooling, and lighting costs.
Maximizing a skylight's performance in your home involves three steps.
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.
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 in the United States, 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% 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 can help control solar heat gain.
You'll find that you have several options to consider when selecting the type of skylight to use in your home. When selecting a skylight for energy efficiency, it's important to first consider its energy performance ratings in relation to your climate and home's design. This will help narrow your selection.
A skylight's energy efficiency is dependent upon all of its components:
Even the most energy-efficient skylight must be properly installed to ensure that its energy performance is achieved. Therefore, it's best to have a professional install your skylight.
In addition to following the manufacturer's guidelines when installing a skylight, it's also important to consider slope and moisture control.
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 40° North latitude, is 45° to 55°. 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 skylights. Take the following steps to avoid water leaks:
• Mount the skylight above the roof surface
• Install a curb (a raised, watertight lip to deflect water from the skylight) and flashing
• Thoroughly seal joints
• Follow 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.