What is Daylighting?

Daylighting controls the admission of natural light, direct sunlight and diffused light into a building to reduce electric lighting and saving energy.

Particular attention is given to daylighting in green buildings, where one aim is to maximize visual comfort, improve occupant productivity or to reduce energy use.

By providing a direct link to the dynamic and perpetually evolving patterns of outdoor illumination, daylighting helps create a visually stimulating and productive and healthy indoor environment for building occupants, while directly or indirectly helping to reduce as much as one third of total building energy costs.

A daylighting system consists of technologies and architectural design. The specific practice of designing atriums is an excellent example of a practice that encompasses these considerations.

While not all of these components are required for every daylighting system or design, one or more of the following are typically present:

• Daylight-optimized building footprint
• Climate-responsive window-to-wall area ratio
• High-performance glazing
• Daylighting-optimized fenestration design
• Skylights (passive or active)
• Tubular daylight devices
• Daylight redirection devices
• Solar shading devices
• Daylight-responsive electric lighting controls
• Daylight-optimized interior design (such as furniture design, space planning, and room surface finishes).

Windows

Windows are the most common way to admit daylight into a space.

Their vertical orientation means that they selectively admit sunlight and diffuse daylight at different times of the day and year.
Therefore windows with multiple orientations should be combined to produce the right mix of light for the building, depending on the climate and latitude.
There are three ways to improve the amount of light available from a window
1) Place the window close to a light colored wall
2) Slanting the sides of window openings so the inner opening is larger than the outer opening
3) Use a large light colored window-sill to project light into the room.

Design Considerations for Daylighting

The components of a daylighting system are designed to bring natural light into a building in such a way that electric lights can be dimmed or turned off for a portion of the day, while preventing occupant discomfort or other building loads from increasing. Daylighting should also be as pervasive as possible, while also maximizing privacy wherever possible for indoor building occupants.

Poor daylighting design can cause problems.

For example, direct sun or glare in the eye of a building occupant can harm or irritate, interfering with the occupants ability to see and perform work and should be avoided.

Also, depending on the building construction and prevailing climate, excessive window area could also increase the cooling load in summer or accelerate heat loss in winter, causing an INCREASE in energy expense.

An optimized building orientation is a foundational element of a day lit building design.

Maximizing the amount of south- and north-facing facade area and minimizing east and especially west exposure allows for the easiest controllable daylight fenestration.

Restricting the floor plate depth (north-to-south) also helps to daylight as much floor area as possible, as there are practical limitations to how far one can transmit daylight in side lighting applications.

A classic example of poor daylighting and poor building orientation in terms of design is the United Nations building in New York City. The long building is designed on a north-south axis causing massive solar heat gain during most of the day through the west-facing windows, causing incremental cooling problems and, supposedly, causing major indoor occupant discomfort.

(The image below shows SunPort solar skylights providing ample daylight in this active warehouse with no electric light during the day.)

Design Recommendations

During the design process, the following design strategies should be understood and explored:

• Increase perimeter daylight zones: extend the perimeter footprint to maximize the usable daylighting area.
• Allow daylight penetration high in a space: Windows located high in a wall or in roof monitors and clerestories will result in deeper light penetration and reduce the likelihood of excessive brightness.
• Reflect daylight within a space to increase room brightness: A light shelf, if properly designed, has the potential to increase room brightness and decrease window brightness.
• Slope ceilings to direct more light into a space: Sloping the ceiling away from the fenestration area will help increase the surface brightness of the ceiling further into a space.
• Avoid direct beam daylight on critical visual tasks: Poor visibility and discomfort will result if excessive brightness differences occur in the vicinity of critical visual tasks.
• Filter daylight: The harshness of direct light can be filtered with vegetation, curtains, louvers, or the like, and will help distribute light.
• Building Orientation: Different building orientations will benefit from different daylighting strategies, for example, light shelves which are effective on south facades are often ineffective on east or west elevations of buildings.

Types of Technology

Exterior shading and control devices

In hot climates, exterior shading devices often work well to both reduce head gain and diffuse natural light before entering the work space. Examples of such devices include light shelves, overhangs, horizontal louvers, vertical louvers, and dynamic tracking of reflecting systems

Glazing materials

The simplest method to maximize daylight within a space is to increase the glazing area. However, three glass characteristics need to be understood in order to optimize a fenestration system:

• U-value: represents the rate of heat transfer due to temperature difference through a particular glazing material.
• Shading coefficient: a ratio of solar heat gain of a given glazing assembly compared to double-strength, single glazing.
• Visible transmittance: a measure of how much visible light is transmitted through a given glazing material.

Aperture location

Simple side lighting strategies allow daylight to enter a space and can also serve to facilitate views and ventilation. Typically, the depth of daylight penetration is about two and one-half times the distance between the top of a window and the sill.

Reflectance of room surfaces

Reflectance values from room surfaces will significantly impact daylight performance and should be kept as high as possible. It is desirable to keep ceiling reflectance over 80%, walls over 50%, and floors around 20%. Of the various room surfaces, floor reflectance has the least impact on daylighting penetration.

Integration with electric lighting controls

A successful daylighting design not only optimizes architectural features, but is also integrated with the electric lighting system. With advanced lighting controls, it is now possible to adjust the level of electric light when sufficient daylight is available. Three types of controls are commercially available:

• Switching controls: on-and-off controls that simply turn the electric lights off when there is ample daylight.
• Stepped controls: control individual lamps within a luminary to provide intermediate levels of electric lighting.
• Dimming controls: continuously adjust electric lighting by modulating the power input to lamps to complement the illumination level provided by daylight.