Daylighting

by Gregg D. Ander, FAIA
Southern California Edison

INTRODUCTION

Daylighting is the controlled admission of natural light into a space through windows to reduce or eliminate electric lighting. 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.

DESCRIPTION

In large measure, the art and science of proper daylighting design is not so much how to provide enough daylight to an occupied space, but how to do so without any undesirable side effects. It involves more than just adding windows or skylights to a space. It is the careful balancing of heat gain and loss, glare control, and variations in daylight availability. For example, successful daylighting designs will invariably pay close attention to 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. Benefits of Daylighting

Daylighting has the potential to significantly improve life-cycle cost, increase user productivity, reduce emissions, and reduce operating costs:

• Improved Life-Cycle Cost: At an estimated incremental first cost increase of from $0.50 to $0.75 per square foot of occupied space for dimmable ballasts, fixtures and controls, daylighting has been shown to save from $0.05 to $0.20 per square foot annually [in 1997 $].
• Increased User Productivity: Daylight enlivens spaces and has been shown to increase user satisfaction and visual comfort leading to improved performance.
• Reduced Emissions: By reducing the need for electric consumption for lighting and cooling, the use of daylight reduces greenhouse gases and slows fossil fuel depletion.
• Reduced Operating Costs: Electric lighting accounts for 35 to 50 percent of the total electrical energy consumption in commercial buildings. By generating waste heat, lighting also adds to the loads imposed on a building's mechanical cooling equipment. The energy savings from reduced electric lighting through the use of daylighting strategies can directly reduce building cooling energy usage an additional 10 to 20 percent. Consequently, for many institutional and commercial buildings, total energy costs can be reduced by as much as one third through the optimal integration of daylighting strategies.

As with all energy-efficient design strategies, there are some costs associated with the use of daylighting. Designers must be sure to avoid glare and overheating when placing windows. More windows do not automatically result in more daylighting. That is, natural light has to be controlled and distributed properly throughout the workspace. Also, for cost savings to be realized, controls have to be in proper functioning order. Poor installation, commissioning, or Operations and Maintenance (O&M) practices can all lead to sub-optimum performance.

B. Daylighting Concepts

It is important to appreciate that the daylighting design process involves the integration of many disciplines including architectural, mechanical, electrical, and lighting. These design team members need to be brought into the process early to ensure that daylighting concepts and ideas are carried throughout the project.

1. An awareness of basic visual acuity and performance issues is essential to an effective daylighting design.
   • Veiling Reflections: Veiling reflections of high brightness light sources off specular (shiny) surfaces obscure details by reducing contract. They should be avoided, particularly where critical visual tasks occur.
   • Distribution: Introduce as much controlled daylight as deep as possible into a building interior. The human eye can adjust to high levels of luminance as long as it is evenly distributed. In general, light which reaches a task indirectly (such as having bounced from a white wall) will provide better lighting quality than light which arrives directly from a natural or artificial source.
   • Glare: The aim of an efficient daylighting design is not only to provide illuminance levels sufficient for good visual performance, but also to maintain a comfortable and pleasing atmosphere. Glare, or excessive brightness contrast within the field of view, is an aspect of lighting that can cause discomfort to occupants. The human eye can function quite well over a wide range of luminous environments, but does not function well if extreme levels of brightness are present in the same field of view.
   • Variety: Some contrast in brightness levels may be desirable in a space for visual effectiveness. Dull uniformity in lighting can lead to tiredness and lack of attention—neither of which is compatible with a productive environment. Often times a good daylighting solution will integrate a "blast" of beam daylight in a circulation area for visual interest and to help lead occupants through a building. The human eye is naturally attracted to this bright area and can be useful in guiding people down an otherwise banal corridor.

2. Good daylighting requires attention to both qualitative and quantitative aspects of design. Make sure the combination of natural and artificial sources provides adequate light levels for the required task.
   • 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.
   • For office spaces, the U.S. General Services Administration has interpreted the IES method to recommend a minimum of 50 footcandles on an imaginary desk-height horizontal "work surface." Nevertheless, when used in conjunction with indirect an ambient lighting system and direct task lighting, a high-quality daylighting design can be achieved with ambient lighting levels of 30 footcandles or less.
3. To be effective, daylighting must be integrated with electric lighting design. In particular, daylighting must be coupled with efficient electric lighting controls if net energy savings are to be realized.
   • As part of a daylighting design, consider the use of continuously dimming fixtures controlled by luminous sensors.

C. Design Recommendations

A number of design strategies should be understood and explored during the design process. These strategies are briefly described below.

• 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 form 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.
• Understand that different building orientations will benefit from different daylighting strategies; for example light shelves which are effective on south façades are often ineffective on the east or west elevations of buildings.

D. Materials and Methods of Construction

1. Exterior Shading and Control Devices: In hot climates, exterior shading devices often work well to both reduce heat 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 or reflecting systems.
2. 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, Shading Coefficient, and Visible Transmittance.
    
   • U-value represents the rate of heat transfer due to temperature difference through a particular glazing material.
   • Shading Coefficient (SC) is a ratio of solar heat gain of a given glazing assembly compared to double-strength, single glazing. [NB: A related term, Solar Heat Gain Factor (SHGF), is beginning to replace the term Shading Coefficient.]
   • Visible Transmittance (Tvis) is 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 large buildings in most climates, consider the use of glass with a moderate-to-low SC and relatively high Visible Transmittance.

3. Aperture Location: Simple sidelighting strategies allow daylight to enter a space and can also serve to facilitate views and ventilation. A rule-of-thumb is that the depth of daylight penetration is about two and one-half times the distance between the top of a window and the sill.
4. Reflectances of Room Surfaces: Reflectance values for 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.
5. 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/off controls simply turn the electric lights off when there is ample daylight.
   • Stepped controls—provide intermediate levels of electric lighting by controlling individual lamps within a luminaire.
   • 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. The following figure shows several control schemes.

Fig. 1

6. Other Lighting Control Systems: In addition to daylight controls, other electric lighting control strategies should be incorporated where they are cost effective, including the use of:
   • Occupancy controls—Using infrared, ultrasonic or micro-wave technology, occupancy sensors respond to movement or object surface temperature and automatically turn off or dim down luminaires when rooms are left unoccupied. Typical savings have been reported to be in the 10 to 50 percent range depending on the application.
   • Timers—these devices are simply time clocks that are scheduled to turn lamps or lighting circuits off on a set schedule. If spaces are known to be unoccupied during certain periods of time, timers are extremely cost effective devices.

E. Analysis and Design Tools

Physical Modeling

The physics of illumination are such that light behaves exactly the same way in a scaled model as it does in a full-size room. Physical models can be built inexpensively and at various stages of the design process. A number of issues can be accomplished with physical models.

• Photographs of the model interior can be taken to record and study various design alternatives;
• The effect of different glass areas and locations can be studied;
• Using photometers, illumination levels resulting from different daylighting schemes can be compiled and used to project energy savings.

Calculation Tools

Over the past 50 years many daylighting calculation tools have been developed. These include hand methods, nomographs, and computer models to simulate both daylighting design and its impact on the overall thermal performance of buildings. The following listing briefly describes some of the available tools.

University of Washington Graphic Daylighting Design Method (1980)
Description: Determines daylight patterns for a room based on the proportions of the window openings, providing illumination level, distribution, and gradient. For more information see: Mechanical and Electrical Equipment for Buildings, 10th Edition by B. Stein, J. Reynolds, W. Grondzik, and A. Kwok. New York, NY: John Wiley & Sons, Inc., 2006. or Inside Out, Second Edition by G. Z. Brown et al. New York, NY: John Wiley & Sons, Inc., 1992.
Availability: University of Washington, Department of Architecture
Contact: Professor Marietta Millet

Computer Software

APPLICATION

Among the primary types of buildings that can benefit from the application of daylighting are administrative buildings (e.g. offices), educational buildings (e.g. child development centers), storage facilities (e.g. warehouses), and maintenance facilities.

Case Studies

RELEVANT CODES AND STANDARDS

Federal Mandate

Federal Agencies

ADDITIONAL RESOURCES

WBDG

Design Objectives

Aesthetics, Productive, Sustainable

Products and Systems

Section 07 92 00: Joint Sealants, Building Envelope Design Guide: Fenestration Systems, Glazing, Windows, Curtain Walls, Sloped Glazing, Atria Systems

Publications

Daylighting Performance and Design, 2nd Edition by Ander, G. D. New York: John Wiley & Sons Inc., May 2003. This is an excellent source for daylighting design issues, tutorials on calculation technologies, and has many case studies documented.

Organizations and Associations

Updated: 03-14-2007

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