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| Sustainable Building Sourcebook | ||||||
| Chapter: Energy | ||||||
| Natural Daylighting | ||||||
| CSI Numbers: 08800 Glazing, 16552 Ballasts, 16930 Lighting Control Equipment | ||||||
| Introduction: | ||||||
One of the most beneficial, free sources of energy is daylight. Daylight makes a useful contribution to interior luminance when properly utilized, and can be more comfortable than electric lighting by providing a better quality of light. The British Standard Code of Practice for daylighting states, "All occupants of a building should have the opportunity for the refreshment and relaxation afforded by a change of scene and focusÖ Unless an activity requires the exclusion of daylight, a view out-of-doors should be provided irrespective of its quality." Studies have demonstrated benefits in worker productivity and health related to daylight in buildings. Another reason to use daylight in conjunction with appropriate lighting controls is the reduction in building energy use. | ||||||
| At-A-Glance Notes: | ||||||
| Technology: | ||||||
| Many manufacturers offer glazing with high visible light transmission and low shading coefficients (low heat transfer). Many manufacturers also produce dimming ballasts and light-sensing fixture controls. | ||||||
| Suppliers: | ||||||
| There are several Austin-based lighting control manufacturers and many local glazing and window manufacturers. | ||||||
| Cost: | ||||||
| Cost benefits are long-term; there may be additional expense for design fees and the cost of controls. | ||||||
| Public Acceptance: | ||||||
| People generally prefer natural daylight to fluorescent and incandescent lighting. | ||||||
| Regulatory: | ||||||
| B In Austin, there are benefits from following the City's Energy Code-Lighting Budget. Residents may be eligible for a 10 - 30 percent discount on total installed watts for dimming ballasts and lumen maintenance. | ||||||
| Considerations: | ||||||
Decisions made early in the design process that deal with building orientation, form, room depth and height, and the position and size of windows have a significant impact on the level of daylighting and the eventual energy consumption of the finished building. Much research has been done throughout the world on the study of natural illumination and on the development of methods of prediction, so that a variety of design systems are available to the architect for predicting with a reasonable degree of accuracy the quality and quantity of natural interior lighting. A view to the outdoors is an important consideration when placing buildings on the site and positioning rooms in the building. The size of a view window should be proportional to the depth of a room in order to provide an adequate view from a deep room. Glare from direct sunlight and the use of computer terminals are common arguments for excluding daylight from buildings. But people who work at computers a large portion of the day respond very positively to having daylight and window access. Architects need to learn how to design daylight buildings for the benefits of those who use them. Daylighting can be costly to retrofit into existing buildings but should be considered when major building retrofits are in the planning stage, mechanical system replacements are necessary, and when entire roof replacements are needed. | ||||||
| Guidelines: | ||||||
SitingThe long axis of the building should face north and south to maximize available daylight and reduce glare. East and particularly west facing glazing should be eliminated to the extent practical. For good daylighting penetration, the depth of rooms should be kept shallow. Predicting DaylightThe amount of daylight that can penetrate into a space depends on many factors. The key factors are the visible sky angle, the width and depth of the room, the net window area, the visible transmittance of the glass, and the reflectance of the surfaces inside the room. The daylighting factor (DF) is the illuminance at a point indoors, usually on the working plane, expressed as a percentage of the illuminance outdoors. The average daylighting factor is an approximate measure for assessing daylight during the early stages in designing windows and roof monitors. The recommended average DF for ordinary visual tasks is 1.5-2.5%. For moderately difficult tasks, the average DF ranges from 2.5-4.0%. The recommended average DF for difficult, prolonged tasks is 4.0-8.0 percent. The average daylighting factor does not take into account the shape of the room or the shape or height of the window. Deep narrow rooms have a poorer uniformity ratio -- the ratio of the daylighting factor at the back of the room to that at the front, and can seem too dark even if the average daylighting factor is adequate. Reflected light makes a significant contribution to the quantity of light available within a building. The amount of light reflected depends on the reflectance of the surfaces. A white roof surface can reflect considerable daylight into roof monitors and light colored interior finishes can help reflect daylight into the building interior. By diffusing light and eliminating direct beam radiation into an occupied space, potential glare problems can be reduced. Daylighting, Fig. 1
Light shelves, mirrored louvers, and prismatic glazing are examples of innovative systems that can assist in reducing glare, increasing light levels, redirecting daylight further into the room, and improving the uniformity of daylight within the room. Daylighting, Fig. 2
Controlling sunlightSolar control is in a delicate balance with thermal control. Obtaining balance requires an evaluation of glazing, surfaces, heat and light transmittance, and shading devices. Shading devices include external shades or overhangs, deciduous trees, internal blinds, and solar control glazing. External shading is the most effective means of intercepting unwanted solar heat gain before it enters the building. A shading device is more effective than using tinted or heat-reflecting glass. Keep in mind that all permanent shading devices reduce sunlight, especially during winter when skies are overcast. Deciduous trees are excellent shading devices for south facing exposures. Internal blinds are effective means of allowing diffused daylight into a space if they are properly adjusted. Their greatest weaknesses are that they do not keep solar heat out, and if fully closed, they block available daylight. Spectrally selective glazing is available in a wide range of choices and should be selected based on orientation and application. It is designed to admit the visible portion of the sun's electromagnetic spectrum while severely restricting the infrared and ultraviolet radiation. In the Austin area, if the glazing has no shading, select glazing with visible light transmittance (VT) of at least 75 and a low solar heat gain or shading coefficient (SC) of 50 or less. VT and SC are listed in glazing manufacturers" technical data. Electric lighting controlsAs bright and pleasant as daylighting features may be, there can, of course, be no significant energy savings without control of the electric lighting system. The combination of daylight without heat gain and the appropriate lighting controls is the best solution. See figure 19.) The use of T-8 fluorescent fixtures driven by dimmable electronic ballasts that permit continuous dimming can dim light output down to 10 percent while saving up to 80 percent in energy. The connection in the electronic dimming ballast is another hallmark in lighting innovation. The ballast receives its operating signal via control wiring that snaps in with a standard RJ-11 phone jackóit's that simple. A closed-loop control system dims the fluorescent fixtures in response to available daylight. For each zone, a ceiling-mounted photosensor reads the illumination level within a 60-degree cone of vision. Each sensor connects directly to a group of several fluorescent fixtures by control wiring with RJ-11 connections. The sensor dims its group of fixtures in response to the available daylight. Lighting control strategies that employ daylight switching can annoy occupants with too frequent switching and lead to overriding or disarming of controls completely. Control systems should take into account the different patterns of occupancy and make the best use of controls, rather than simply aiming to minimize energy consumption. The installation of a control system that dims electric lights in a gradual fashion will help to avoid frequent switching. One important consideration is that individual occupants should have control of their own local task lights. The ownership of spaces has a major influence on the best choice of control method. Daylighting, Fig. 3
Appropriate design toolsOne of the fastest, easiest, and least expensive methods of daylighting analysis is with a physical model. A physical model will also enable photography for qualitative analysis, comparative evaluation of mathematical models, and use in architectural renderings and presentations. Computer tools are capable of providing more detailed analyses than physical models and have the potential to expand the envelope of testing by providing yearly analysis of daylighting performance and control. Various daylight modeling software programs are available for use in evaluating the daylighting strategies. The results of the daylighting models are then input into a building energy simulation package in order to compare design options and analyze energy consumption. This adds to design costs but can reduce construction costs and long-term operation and maintenance costs. | ||||||
| Resources: | ||||||
| Professional Assistance: | ||||||
See "Lighting Consultants" in Yellow Pages Powell Engineering |
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| Components/Materials/Systems: | ||||||
See "Light Bulbs and Tubes" in Yellow Pages. Leviton Lighting Division ODL Inc. Solatube So-Luminaire Daylighting Systems Corp. Southwall Technologies (Heat Mirror) Sun Star Tubular Skylights The Sun Tunnel SunScope Natural Light System VELUX America, Inc. Viracon |
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| General Assistance: | ||||||
Illuminating Engineering Society of North America US Environmental Protection Agency ENERGY STAR Program Modeling Software\ADELINE Lightscape Lumen Micro Radiance |
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