Optimal Window-To-Wall Ration Depends, Sanders Tells GANA Conference Attendees
September 1, 2010

Dr. Helen Sanders of SAGE Electrochromics gave a presentation on the impact on energy savings from daylighting during last week’s Fall Conference for the Glass Association of North America. Sanders talked first “about the industry’s conventional view of the impact of glass on energy performance of the building … Generally what you see is increased energy usage as window-to-wall-ratio increases,” she said. “The conventional view of glass is more glass is not so good in terms of energy performance.”

However, she noted, “There are ways you can have more glass without an energy penalty. You do it by harvesting the natural daylight and turning off the electric lights.”

According to Sanders, continuously dimming lighting controls are essential in maximizing the energy efficiency of glass. One graphic she presented showed a study that found that a 60-percent window-to-wall-ratio provided a better energy performance than having no windows at all, when appropriate daylighting controls were used.
Sanders also pointed out that the optimal window-to-wall-ratio depends. The point she stressed was that the one-size-fits-all approach outlined by the updated ASHRAE 90.1 doesn’t necessarily provide energy savings. Sanders suggested that designers consider the impact of various factors ranging from orientation and building design to climate zone to window performance and use of exterior shading. She also commented that small windows (less than 30-percent window-to-wall ratio) can be worse for glare than strip windows, as the eye struggles when there is a contrast between high and low light, whereas it might adapt to one or the other. If the prediction made during another presentation, that punched openings will become more common than strip window in low-rise office buildings, comes true those buildings might see increased energy usage as occupants pull the blinds against the glare and turn on the lights.

Sanders offered several design principles for her audience to propose to their clients:

  • Putting windows higher up -- to allow deeper daylight penetration;
  • Windows with higher visible transmittance – to admit more daylight (glare needs to be managed properly);
  • Overhangs – to cut off undesired direct sun that causes glare;
  • Lightshelves – to bounce direct sun into the space;
  • Sloped ceilings – to allow deeper daylight penetration;
  • Dynamic response for glare – blinds/shades or dynamic glazing; and
  • Continuous windows – instead of punched openings to avoid glare due to luminous contrast between the wall and window.

Sanders ended her presentation with a cautioning note. “We have to bear in mind that things are changing rapidly in the lighting arena too,” she said. She pointed out that the lighting industry is likewise finding ways to reduce energy usage so that in some years’ time the amount of money saved with dimmable lighting controls is going to diminish. “While it’s an appropriate argument to use now, we’ve got to start augmenting that message,” she said.

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