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Daylighting Design

Good daylighting design requires a balancing act

Rich Rinka

In recent years, daylight harvesting has become a key element of the sustainability paradigm, in which building components must work interactively to optimize overall energy use, environmental impact and occupant welfare.

Energy Savings

Good daylighting design saves energy in many ways. The obvious one is lighting, which often comprises the bulk of energy consumption in commercial buildings. In addition, electric lights emit heat that forces the air conditioning system to work harder.

Meanwhile, advances in glass and plastic glazing, as well as metal framing materials, have improved fenestration thermal performance characteristics. Spectrally selective low-E glass, for example, transmits visible light waves and reflects infrared heat waves, keeping solar heat gain low, while ensuring bright, light interiors.

Occupant Well-Being

Various studies have proven that people perform better in a daylit environment, showing greater and faster mental activity and decision-making. In retail settings, increased sales and more transactions are the result. Daylit schools have been shown to generate 20 percent faster learning, and daylit offices exhibit 7 percent faster work speed and 10 percent better cognitive performance. Patients also heal faster in daylit health care facilities.

This is because artificial lighting emits primarily long wavelength (reddish) light, while the body’s circadian system governing waking and sleeping patterns prefers bluer light of shorter wavelengths. The latter suppress the natural hormone melatonin while stimulating serotonin, a combination that leads to greater alertness.

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Aesthetics

Daylighting sources may be incorporated into the aesthetic design of a building. It can highlight architectural features, while providing the light needed for interior plant life.

Daylighting Options

In the built environment, daylighting is harvested from two sources: side-lighting (fenestration products installed primarily vertically that face the horizon) and top-lighting (those installed primarily overhead to face the sky). These should be designed to complement one another.

Side-lighting

Side-lighting from windows and doors provides daylight and solar energy along the perimeter of a building. The use of clerestories, light shelves, vertical baffles, sloping the ceilings away from the windows and using high-reflectance paint allow light to travel deeper into a room.

To optimize side-lighting, consider:

  • Orientation (north, east, south, west) with respect to the sun’s path is a critical factor.
  • The amount of daylight available will vary throughout the day depending on the direction the fenestration is facing. External obstructions may reduce access to the available daylight.
  • Shading to avoid excessive glare may be necessary when the sun is low in the sky.

Top-lighting

Because most commercial buildings in the U.S. are two stories or less, they offer an ideal opportunity for top-lighting. Skylights or roof windows deliver natural, even light and do not depend upon the building orientation. Daylight is consistently available throughout the day from both ambient lighting from the sky and direct exposure to the sun. Even in cloudy weather, top-lighting provides excellent daylighting potential all day long.

To optimize top-lighting resources, considerations include:

  • Shading devices and diffusion techniques can be utilized to avoid glare and spread light to wider areas of floor space.
  • Light wells and/or tubular daylighting devices (TDDs) can direct daylight into windowless interior areas such as bathrooms, hallways and kitchens.
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General Design Considerations 

To achieve efficient illumination:

  • Diffuse the light uniformly to avoid hot spots of excessively bright light. Diffusion can be achieved by employing splayed light wells to provide uniform light across the space, as well as adding a diffuser at the bottom of the light well. Glazing with a haze factor (the ratio of diffusely transmitted light to the total transmitted light of a glazing) of more than 90 percent is recommended.
  • Paint interior surfaces with light colors and use high-reflectance paint on the ceilings.
  • Size unit skylights appropriately and space them evenly. Typically, a spacing of one to one and a half times times the ceiling height on-center will provide even light distribution.
  • Consider using automated electric lighting controls. These can be open loop (in which the photo sensor reacts only to the incident daylight levels) or closed loop (in which the sensor looks down on the task area and responds to both daylight and electric light). Control operation ranges from simple on-off to dimming, the latter of which provides the best lighting quality.

The actual layout of windows and skylights depends upon:

  • Height of the daylit space
  • Tasks performed in the daylit space
  • Task lighting requirements of the space (the Illuminating Engineering Society provides recommended illuminance levels for typical tasks that would be applied in daylit spaces)
  • Construction and cost constraints

Rich Rinka serves as American Architectural Manufacturers Association’s (AAMA) technical manager, standards and industry affairs. For more information, visit www.aamanet.org.