According to the U.S. Department of Energy, as a rule of thumb, “a skylight 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 areas for spaces with few windows.”
Furthermore, the actual skylight position can impact daylight and solar heating. For example, skylights on roofs facing North provide cooler illumination. Skylights facing West, provide afternoon sunlight and heat gain. Skylights facing South provide optimal heat gain for winter but unwanted heat gain in summer. One way to counter undesirable heat gains is to choose skylights that have special glazing as mentioned earlier.
No matter how energy efficient a skylight can be, it can be all undone if not installed properly. This is why it is recommended to hire a professional that follows manufacturers guidelines. When it comes to slopes or tilts of skylights installed on a roof, as general rule of thumb is that a low slope will admit more solar heat in summer and less in winter – not a desirable condition to live in. Also it is encouraged to achieve a slope equal to your geographical latitude plus 5 to 15 degrees.
A skylight has two main functions, to provide natural day lighting and ventilation. Usually, skylights are used in both residential and commercial buildings to provide more green friendly spaces. Skylights provide an aesthetically pleasing source of light, save energy costs by substituting the use of electricity to see in darker spaces during the day, and further allow cool air into a space during hot climates. Nevertheless, energy efficiency can be further enhanced by using special tints and coatings to reduce heat gain and loss relative to weather conditions, strategically placing skylights that take into consideration the design of a building or home where wind and solar heat provide both an optimal and desirable source of ventilation and light, and how large the skylight itself may be.
How to choose an energy efficient skylight
When choosing a skylight it is important to remember that it may not be energy efficient at standard. In order to identify an energy efficient skylight look for ENERGY STAR labels based on rating criteria by climate. For example, most of VELUX products are ENERGY STAR qualified in all 50 states, which means they meet criteria set by the Department of Energy and Environmental Protection Agency. However, these ENERGY STAR labels don’t consider the design/layout of your home and building, so it is recommended to hire a roofing or skylight professional to help you locate strategic areas for optimal energy efficiency while you build a new home or plan a remodel.
Energy Efficiency in Product Labeling
Energy Efficiency | Skylights
Where and how to place a skylight for more energy efficiency
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Plastic or glass glazing are the most common type of skylight glazing. Plastic glazing uses acrylics and/or polycarbonates and is more inexpensive, less receptive to breaking but do scratch more easily, become more brittle, and discolored overtime. Additionally, plastic glazing allows more ultraviolet (UV) rays that can damage furnishings if the glazing is not coated with a special film. Glass glazing is more durable than plastics and is more expensive. Skylights glazed with glass can be either tempered or laminated glass. Both offer an impact resistant glazing that minimizes breaking and more safety to indoor occupants. Lastly, skylights with glass glazing do not discolor overtime. If you want to further minimize solar heat and/or reduce winter heat loss look for other glazing technologies which include reflective or spectrally selective coatings, insulated glazing, low-emissivity (low-e) coatings, and heat absorbing tints.
There are many shapes for skylights and each shape can influence energy efficiency in your home or building. For example, arched or domed shaped skylights allows more light to enter from certain angles. Tubular skylights are smaller skylights that can still allow daylight without increasing size and affecting too much architecture. Since tubular skylights have a small surface area, they minimize both heat loss in winter and heat gain in summer.
Skylight glazing and shapes