Daylighting is a measure of the amount of natural light from the sky or reflected off surfaces in the external environment that is experienced inside a space. It is typically captured through windows, skylights, and other glazed openings; daylighting strategies must be carefully designed to gain the maximum benefits of the illuminance available at the project location.
Using daylighting effectively is an important feature of sustainable design because natural lighting can help make the building less reliant on the electrical power typically consumed by artificial lighting, which can reduce the total building energy costs by as much as one-third. Good daylighting design can also help to create a visually stimulating and productive environment, which benefits all of the building’s occupants and users.
Benefits of Daylighting
Daylighting has the potential to significantly improve a building’s lifecycle cost, increase the productivity of occupants and users, and reduce the building’s operating costs and emissions:
- Improved lifecycle cost—installing dimmable ballasts, fixtures and controls to adjust artificial lighting based on daylighting typically adds a small incremental one-time installation cost (estimated at $0.50 to $0.75 per square foot [$5.38 to $8.06 per square meter] of occupied space), but can result in an estimated annual savings of $0.05 to $0.20 per square foot [$0.56 to $2.15 per square meter] (measured in 1997 dollars). This brings a very significant savings over the lifecycle of the building.
- Increased user productivity—research results show that daylight enlivens a space, increases user satisfaction and visual comfort, and leads to improved performance and productivity.
- Reduced emissions—using daylight helps to reduce the use of electrical power for lighting and cooling, thereby reducing greenhouse gas production and slowing fossil fuel depletion.
- Reduced operating costs—artificial lighting typically accounts for 35 to 50 percent of the total electrical energy consumption in commercial buildings. It also generates waste heat, which adds to the loads on the building’s HVAC system. Reducing electric lighting through the use of daylighting strategies can directly reduce the energy needed to cool a building by an estimated 10 to 20 percent.
Daylighting Design Goals
Proper daylighting design requires careful balancing many considerations, including heat gain and loss, glare control, and variations in daylight availability. Successful daylighting designs typically consider:
- Window sizing, placement, and spacing
- Glazing material selection
- Shading devices to reduce glare and excessive contrast
- Light reflecting devices and features to enhance light capture and bounce light to locations where needed
- Color and reflectance of interior finishes
- Location and design of interior partitions and potential light blockers
To qualify for LEED certification, a building’s design must also provide a minimum glazing factor of 2 percent in a minimum of 75 percent of all regularly occupied areas.
Recommended Daylighting Levels
The Illuminating Engineering Society of North America publishes an industry-standard method for determining recommended illuminance levels (expressed in units of footcandles, or fc) for various tasks. The amount of daylighting required in a room depends on the tasks being performed there. The following are some generally accepted lighting-level recommendations in footcandles for different tasks:
- High contrast (ink, soft lead): 50 - 75 - 100
- Low contrast (hard lead): 100 - 150 - 200
- Simple: 20 - 30 - 50
- Moderate: 50 - 75 - 100
- Difficult: 100 - 150 - 200
- Medium, grinding, and so on: 50 - 75 - 100
- Picking, packing, wrapping, labeling: 20 - 30 - 50
- Lobby, corridor, waiting area: 10 - 15 - 20
- Toilets, restrooms: 10 - 15 - 20
- Teller stations, ticket counters: 50 - 75 - 100
- General: 20 - 30 - 50
- Soft pencil (#2), pen, good copies, keyboards, > 8-point type: 20 - 30 - 50
- Hard pencil (#3), phone books, poor copies, < 8-point type: 50 - 75 - 100
- Science laboratories: 50 - 75 - 100
- Inactive: 5 - 7.5 - 10
- Active, large items: 10 - 15 - 20
- Active, small items: 20 - 30 - 50
Daylighting Design Strategies
Design strategies that can be explored to improve daylighting include:
- Increase the perimeter daylight zones—extend the perimeter footprint and wall area to maximize the potential for usable daylighting area.
- Provide daylight penetrations high in a space—locating windows high on a wall (for example, clerestory windows) or in the roof (for example, skylights) brings in light that can penetrate deeper into a space and reduces the likelihood of excessive brightness and glare.
- Reflect daylight within a space to increase room brightness—using properly designed light shelves, designers can improve the overall room brightness while decreasing excessive brightness at the window.
- Slope ceilings to direct more light into a space—sloping ceilings away from the windows and glazing area helps increase the brightness of the ceiling and brings daylight further into the space.
- Avoid direct sunlight and excessive brightness on critical visual tasks—direct sunlight and excessive brightness in the vicinity of critical visual tasks can create user discomfort and poor visibility.
- Filter and soften daylight—using curtains, shades, louvers, natural vegetation or other light filtering devices reduces the harshness of direct light and helps to distribute it more evenly.
Effective daylighting design also depends on the orientation of the building surfaces being designed and the position of the sun. The appropriate combination of daylighting strategies typically varies based on the sunlighting experienced on each building face. For example, light shelves that are typically very effective on south elevations are often ineffective on the east or west elevations of buildings.