Between open floor plans, window walls, and outdoor living, homeowners seem to be trying to satisfy a craving for daylight. “Humans are phototropic; we enjoy light,” says Velux director of sales Stephan Moyon. “We sit by the restaurant window, we want a corner office, retail customers in a daylit environment will spend more time there and therefore spend more money.”

And while windows are the obvious choice to naturally light a home, evidence suggests that skylights provide not just more, but better daylight than vertical windows.

A recent study by the Danish Building Institute found that the daylight factor (DF) for skylights was twice that of vertical windows and more than three times the DF for dormer windows (see renderings below). “Skylights better penetrate the core of a space, so you have better light quality in a room,” Moyon says. Using skylights and vertical windows together can actually reduce the overall square footage of glass in the building envelope, thereby improving energy efficiency.

Dormer windows (left) offer a 0.85 daylight factor (DF), followed by vertical windows at 1.41 DF. By helping to illuminate the center of a space, skylights showed a 2.86 DF.
Dormer windows (left) offer a 0.85 daylight factor (DF), followed by vertical windows at 1.41 DF. By helping to illuminate the center of a space, skylights showed a 2.86 DF.

Velux’s Daylight Visualizer modeling program shows that even at a 30% window-to-floor area (WFA), the model only reaches a 2.5 DF. The light is “struggling to reach the core of the space,” Moyon says. “The alternative is to combine skylights with vertical windows to help daylight reach the center of the room.” When skylights are added to the model, the WFA drops dramatically to just 5%, while the daylight factor increases to 3.

Velux recently contracted consultancy Group14 Engineering to look independently at the energy cost impacts of achieving minimum daylight levels in a single-story home. Using Department of Energy software, the group mocked up 126 models of skylight and window orientations and matched the models with climate data.

The results were interesting:

  • Skylights reduced overall fenestration from 20% WFA to 12% WFA while maintaining a DF of 5.
  • Less glazing indicated lower annual heating and cooling costs in 98% of the 108 models with skylights.
  • In warm climates, the most savings came from reduced solar heat gain.
  • Cool, dry, or mixed climates generated higher cost savings, especially with south-facing skylights.


Glossary


  • Daylight Factor (DF): The ratio of internal light to external light. A DF of 2 is considered minimum before artificial lighting is required. In many European countries, 5 is considered the minimum.
  • Window-to-floor area ratio (WFA): The square footage of window glass divided by the square footage of the floor. California code only allows for WFA of 20% or less.