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Green & Sustainable Construction

An Overview of Daylighting

Daylighting involves designing buildings for optimum use of natural light and provides numerous benefits over artificial lighting:

  • Daylight allows us to dim or switch off artificial light, resulting in energy and cost savings.
  • Daylight admits less heat into a space than artificial light per unit of light. Incandescent lamps are essentially electric heaters that happen to give off a little light, and fluorescent lights also introduce heat into a space, exacerbating cooling loads.
  • Occupants of daylit spaces are certainly happier, and evidence shows that they are more productive. It can be disturbing for people to work in windowless cubicles with no awareness of the weather and no connection to the outdoors.

Daylighting is more than just having windows. It is admitting natural light into the space, but it also includes controlling and distributing light for uniform lighting levels, avoiding glare and reflections, and controlling artificial light to achieve energy and cost savings.

Daylight is very bright compared to the light we need in a built environment. As a result, small apertures in building walls and roofs are sufficient to meet daylighting goals.

Daylighting works best in a task-ambient lighting strategy, where daylight is used to maintain a low ambient light level everywhere, and task lights (such as desk lamps) are used to provide a higher light level only when and where needed.

Designing Buildings for Daylighting

To achieve daylighting goals, the designer must be involved early in the programming phase of a project, when the relationships between spaces are being laid out.

In general, daylight cannot be expected to penetrate more than 15–20 feet into a room from a perimeter window. Overhead skylights can provide light in areas farther from walls, but only on the top floor or in single-story buildings. Several devices have been invented to project daylight deep into the core of a building, including window reflectors, light pipes, and fiber optics.

A building design that puts occupants in the proximity of perimeter windows results in high-quality daylighting and high occupant satisfaction (by providing a visual connection to the outdoors). This requires a more articulated plan, which may increase wall heating and cooling loads, but can also fit well with natural ventilation and passive solar heating objectives.

Historic buildings have provided a good lesson in effective daylighting, with designs such as the double-loaded corridor (two rows of rooms separated by a corridor, so that every room has an exterior wall). Premium daylight is available to all rooms through the outside wall, and the lower light level required in the hall is “borrowed” through transom windows between the hall and the room. Often these windows are high (above the doors) and translucent to provide privacy. While a double-loaded corridor with room windows facing north and south would be best, it can be configured around a courtyard, an E shape, or an infinite variety of other shapes.

Potential Concerns

It is important to discuss potential pitfalls related to daylighting so that they may be avoided.

  • Brightness: The pupil of the human eye constricts in response to bright light. If brightness is not uniform throughout the room, this constriction makes it hard to see in the darker areas, requiring people to install and use even more artificial light.
  • Computer Use: Computer screens, increasingly common in all environments, are best viewed in low ambient light levels. Surrounding sources of light are reflected to the eye from the surface of a computer screen, causing an annoying veil over the image on the screen. Computer workstations can be oriented to avoid reflections of windows and lighting fixtures. Fixtures with sharp cut-off angles may be specified so that reflections of light sources do not appear on computer screens.
  • Thermal Discomfort: The temperature a person feels is a combination of the surrounding air temperature and radiant heat gain. Radiant heat gain from sunlight is intense, and direct exposure to sunlight indoors almost always causes thermal discomfort. Direct sun may be acceptable in circulation spaces (such as an atrium or hallway), but should be avoided on all workstations.

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