What is a


Simply put, it’s a window installed on a roof that provides daylight and, in some cases, ventilation to the building below.

But, believe it or not, there’s more to the way a skylight is made and how it functions – even more than you may think.

velux skylights
Operation and Use

Most skylights are installed to provide daylight and/or ventilation. And today, previous skylight-related problems like daytime heat gain and nighttime heat loss are being combated with recent developments in design, including sun-tracking, open-sided cylinders, large lens-like elements, and mirrored reflectors.

Skylights now may even come connected to a mirrored pipe, or “light pipe,” with a diffusing lens that mounts on or is recessed into the ceiling of the room below. These have become known as tubular skylights, but these designs typically do not provide views or ventilation.

Speaking of ventilation: Ventilating a building with a skylight that opens releases the hot air that naturally accumulates near ceilings. Ventilating skylights usually open outward at the bottom, while some units vent through a small hinged panel. Manually operated venting skylights are opened with a pole, chain or crank.

Automated skylights with electric motors or pneumatic devices are also available, and some models even incorporate moisture sensors, so the skylight automatically closes when it rains. Larger skylights that can be used as emergency exits are sometimes called “roof windows,” but to qualify as an emergency exit, they must be located within a few feet of the floor.

Skylight Design


When it comes to choosing a skylight to install, your customers want options. In order to choose the best option, it’s important to consider three main factors: the selection, glazing, and shape of the skylight.

1 – Selection

Skylights work best when they’re selected based on the local climate and building design. The physical size of a skylight will greatly affect the illumination level and temperature of the space below. Skylight should never cover more than five percent of the floor area in rooms with many windows, and no more than 15 percent of the room’s total floor area in rooms with fewer windows.

To maximize daylighting as well as passive solar heating potential, consider the skylight’s position on the roof:

  • Skylights on north-facing roofs provide fairly constant but cool illumination.
  • Skylights on east-facing roofs provide maximum light and solar heat gain in the morning.
  • Skylights on west-facing roofs provide afternoon sunlight and heat gain.
  • Skylights on south-facing roofs provide the greatest potential for passive solar heat gain in the winter, but often allow unwanted heat gain in the summer.

Unwanted solar heat gain can be prevented by installing the skylight in the shade, or by adding a shade or blinds to the skylight.

2 – Glazing

Typically, skylight glazing is either plastic or glass, but other glazing technologies may be used for solar heat control. And depending on expected performance, you may even opt for different types of glazing for different skylight locations throughout your building.

Acrylics and polycarbonates are the most commonly used plastic glazing, but more expensive skylights are usually glazed with glass. To comply with building codes, glass used for skylights must utilize “safety glazing” for both tempered and laminated glass to keep it from breaking into large, sharp pieces.

3 – Shapes

Skylights are installed in a variety of shapes and sizes. The most common shapes include rectangular, oval, circular, triangular, diamond, multi-sided and tubular.

Author: Kat Wiseman, WDMA PR Committee member and content writer for VELUX America.

Understanding Window, Door, And Skylight Certification Webinar

The Understanding Window, Door & Skylight Certification webinar is designed to help architects and remodelers understand the benefits of window, door and skylight certification and how certified products can enhance their projects.