Daylighting is the controlled admission of natural light, direct sunlight, and diffused-skylight into a building to reduce electric lighting and saving energy. By providing a direct link to the dynamic and perpetually evolving patterns of outdoor illumination, daylighting helps create a visually stimulating and productive environment for building occupants, while reducing as much as one-third of total building energy costs.
A daylighting system is comprised not just of daylight apertures, such as skylights and windows, but is coupled with a daylight-responsive lighting control system. When there is adequate ambient lighting provided from daylight alone, this system has the capability to reduce electric lighting power. Further, the fenestration, or location of windows in a building, must be designed in such a way as to avoid the admittance of direct sun on task surfaces or into occupants’ eyes. Alternatively, suitable glare remediation devices such as blinds or shades must be made available.
Implementing daylighting on a project goes beyond simply listing the components to be gathered and installed. Daylighting requires an integrated design approach to be successful, because it can involve decisions about the building form, siting, climate, building components (such as windows and skylights), lighting controls, and lighting design criteria.
The science of daylighting design is not just how to provide enough daylight to an occupied space, but how to do so without any undesirable side effects. Beyond adding windows or skylights to a space, it involves carefully balancing heat gain and loss, glare control, and variations in daylight availability. For example, successful daylighting designs will carefully consider the use of shading devices to reduce glare and excess contrast in the workspace. Additionally, window size and spacing, glass selection, the reflectance of interior finishes, and the location of any interior partitions must all be evaluated.
A daylighting system consists of systems, technologies, and architecture. 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).
Since daylighting components are normally integrated with the original building design, it may not be possible to consider them for a retrofit project.
If possible, the building footprint should be optimized for daylighting. This is only possible for new construction projects and does not apply to retrofits. If the project allows, consider a building footprint that maximizes south and north exposures, and minimizes east and west exposures. A floor depth of no more than 60 ft., 0 in. from south to north has been shown to be viable for daylighting. A maximum facade facing due south is the optimal orientation. Deviation from due south should not exceed 15° in either direction for best solar access and ease of control.
With the building sited properly, the next consideration is to develop a climate-responsive window-to-wall area ratio. As even high-performance glazings do not have insulation ratings close to those of wall constructions, the window area needs to be a careful balance between admission of daylight and thermal issues such as wintertime heat loss and summertime heat gain. The American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) offers guidance on these ratios per climate zone in their Standard 90.1 energy code, but these are primarily minimal for thermal performance and do not consider admission of daylight.
A high-performance glazing system will generally admit more light and less heat than a typical window, allowing for daylighting without negatively impacting the building cooling load in the summer. This is typically achieved through spectrally-selective films. These glazings are typically configured as a double pane insulated glazing unit, with two 0.25 in. (6 mm) thick panes of glass that are separated by a 0.50 in. (12 mm) air gap. This construction gives the insulated glazing unit a relatively high insulation rating, or R-value, as compared to single pane glass. A low-emissivity coating is also often part of these high-performance glazing units, which further improves the R-value of the unit.
Types of Technology
Daylighting is an energy-efficient strategy that incorporates many technologies and design philosophies. It is not a simple line item, and can vary tremendously in scope and cost. Many elements of a daylighting implementation will likely already be part of a building design or retrofit (e.g. windows and light fixtures), but a successful daylighting system will make use of the following technology types and construction methods:
- 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. (A related term, solar heat gain coefficient, is beginning to replace the term shading coefficient.)
- Visible transmittance: a measure of how much visible light is transmitted through a given glazing material.
Glazings can be easily and inexpensively altered to increase both thermal and optical performance. Glazing manufacturers have a wide variety of tints, metallic and low-emissivity coatings, and fritting available. Multi-paned lites of glass are also readily available with inert-gas fills, such as argon or krypton, which improve U-values. For daylighting in large buildings in most climates, consider the use of glass with a moderate-to-low shading coefficient and relatively high visible transmittance.
- Aperture location. Simple sidelighting 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.
- Reflectances 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 reflectances 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.
Any of these control strategies can, and should, be integrated with a building management system to take advantage of the system’s built-in control capacity. To take full advantage of available daylight and avoid dark zones, it is critical that the lighting designer plan lighting circuits and switching schemes in relation to fenestration.
Daylighting can be a viable, energy-efficient strategy in almost any climate, including traditionally overcast climates such as those found in parts of the Pacific Northwest. The technology can work in all building types as well, including commercial office buildings, most spaces within a school (i.e. classrooms, gymnasiums, media centers, cafeterias, and offices), retail stores, hospitals, libraries, warehouses, and maintenance facilities. A viable option for most building types and locations, it is important to consider that the architectural response to daylighting differs by building type, climate, and glare tolerability. Daylighting also has the potential to provide significant cost savings.