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CONSIDERATIONS:
The central concern associated with daylighting is the heat gain that can result when natural light is brought into a home. In our region, this is an especially important concern. During the heating season, the heat gain from natural light can be useful.
Another concern with natural light is the ultraviolet (UV) rays in natural light. When natural light strikes fabrics and some other materials, the UV rays can discolor and weaken the material.
There are simple design strategies and some materials that can facilitate the energy saving advantages of natural light. Light colored interiors and open floor plans are good choices. This approach also augments artificial light efficiency.
Energy efficient lighting is not simply finding the most light for the least wattage or the longest lasting light bulb. Proper sizing of the light to the needs of the location and the tasks that will be performed, called task lighting, is an energy saving strategy.
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The primary goal in daylighting is to bring in indirect light (light that is reflected and not in a direct line to the sun).
To accomplish this, overhangs on windows, which are a primary means of daylighting, must be sized to prevent direct light (light that is in direct line to the sun) from entering except, where desired, in the heating season.
Dividing the height of a wall that needs to be shaded in the summer by four will give the best size for the overhang's projection on south facing windows.
There must be an adequate roof overhang above it to prevent direct solar gain in the cooling season. South facing windows can admit solar gain during the heating season due to the sun's lower angle; fans behind the glass may direct the solar heated air into the living area or into storage. Operable windows in the clerestory also allow heat to escape from the house during the cooling season.
When light enters the clerestory, it illuminates from the highest area in the center of the house.
May require a vertical wall section on the exterior west side to prevent heat gain from the setting summer sun (which occurs in the northwest). A north facing clerestory will not experience solar heat gain in the winter. The glass should be highly insulating in this orientation or employ special insulating strategies to avoid excessive winter heat loss.
Sidelighting from a window is the most common source of daylighting. The amount of light decreases exponentially as you move toward the interior of the room. Light colored walls and ceilings that can reflect light in a diffuse manner will aid the effectiveness of the natural light.
North and south facing windows will provide the best light since the light can be controlled from these sources. Excessive glare and heat gain can occur from east and west orientations. South facing windows can be protected by an overhang while north windows receive direct light for a very short time in the summer.
As a general rule, the depth of a room should not exceed 2 to 2.5 times the height of the window. An overhang needs to be calculated as part of the room's depth when used on the south side. This means that if the window is 5 feet tall and the overhang is 2 feet deep, the depth of the room needs to account for the two feet included in the overhang as part of the room's depth. In this example, 5 feet (height of window) x 2.5 (depth ratio) = 12.5 feet- 2 feet (overhang) = 10.5 feet (room depth).
Horizontally-oriented windows high on a wall will permit the best penetration of light into a room.
Reflected light can be achieved with light shelves and louvered window coverings. The primary drawback is that direct light is needed. Venetian blinds with a reflective coating on the top surface of the slats can be angled to bounce the light to the ceiling. A light colored ceiling will illuminate from the reflected light. A horizontal reflective surface placed near the top of a window (light shelf) will similarly reflect light to the ceiling and deep into a room.
Reflecting direct light to the ceiling reduces glare and provides effective deep penetration of natural light.
Skylights are generally sources of excessive heat gain in our area. They can also cause excessive heat loss in the winter. The following guidelines will help you use skylights advantageously:
If using clear glazing, use a ceiling diffuser at the bottom of the skylight shaft to improve light distribution.
At the minimum, use double glazing.
Provide an interior insulating shade or panel to seal off the skylight shaft from nighttime winter heat loss unless special highly insulating glass is being used.
Use an exterior shading system over the skylight during the summer.
There is a skylight product specifically designed for daylighting purposes. It uses a pipe with a reflective interior to bounce the light down the pipe to a ceiling diffuser from an acrylic dome skylight on the roof. This is a method to bring natural light to a dark interior area of a home without constructing a costly light well (See Resources).
The primary strategy in energy efficient lighting lies in a design that recognizes what will occur in the area to be lit and sizes the lighting to that task. It is also important to consider the quality of the light, which can affect the level of comfort.
In a kitchen, the light requirement over the counter where tasks such as cutting and reading can occur is greater than the light needed for general activities in the room. The counter area should approximate 50-100 foot-candles (a measurement of illuminance) whereas the general light for the kitchen should be 20-50 foot candles. The amount of energy needed to produce that amount of illuminance depends on the distance to the light source. With a shorter distance more illuminance will be available in a defined area which is the reason task lighting can be conserving.
A dining area requires even less illuminance --- 10-20 foot-candles --- while a relaxation/entertainment area needs only 5-10 foot-candles.
A general goal is to minimize the amount of artificial light needed for "background" light (the light needed to pass through or function safely in the house). This can be accomplished with the use of cove lighting. Cove lighting is characterized by the use of ledges, valances, or horizontal recesses that cause the light to be distributed over the ceiling and upper wall. Cove lighting can be provided by low wattage fluorescent lamps or low voltage lights concealed behind a decorative valance in the upper third of the wall. The low wattage fluorescent lamps conserve energy and are long lasting. The reflected light off the ceiling provides even light throughout the room.
In rooms that receive natural light, there can be a frequent varying requirement for artificial light according to how bright it is outside. A fluorescent light with a "daylighting" ballast will vary in light output according to the light needs of the room. The fluorescent light, in this case, is conserving in its light output per wattage and the daylighting ballast is conserving by dimming the light output if natural light levels or other light sources are providing adequate light.
In outdoor lighting, photocells will respond to daylight levels and activate the outdoor light operation when darkness arrives.
To be most conserving in an outdoor application, combine the photocell with an infrared or ultra-sonic detector (see following). The photocell activates the lighting circuit for nighttime operation and the sensor turns on the light only when needed by responding to the presence of people.
In rooms that do not have natural light, an occupancy sensor can prove highly conserving when connected to background lighting. The sensor will operate the lights only when people are in the room.
There are two general types of occupancy sensors: infrared and ultra-sonic. Ultra-sonic sensors are best in rooms with partitions or dividers. Infrared sensors are better for open areas. Some sensors include both features. Integrated units that include the sensor and relay in a single housing that fits into a standard electrical box are priced more reasonably for residential applications.
Timers turn lights on and off at predetermined times.
Photocells respond to light levels and do not need rescheduling.
Dimmers will reduce the light level and will only save energy when used consistently.
Natural light when brought into a home in a non offensive way - not too bright or glaring or too hot - contributes to the comfort within a home. The selection of light bulbs for artificial lighting should therefore attempt to combine comfort and energy efficiency.
Energy efficient lighting usually means the conversion to fluorescent bulbs, either tubes and/or compacts (called "PL's").
About four times as much of the input energy in fluorescents is converted to visible light as with incandescent lights. Most people do not associate fluorescent lighting as being comfortable light due to experiences with fluorescent lighting in workplaces. However, there are differences in the light output and ballasts available from different fluorescent light bulbs.
The following guidelines will help in selecting the best fluorescent light bulbs:
Select lights with a mid range temperature. This is given in the color temperature K (for Kelvin) shown for lighting products. A range of 3500-5000K is best. This will be the preferable light for a residence. A Color Rendering Index (CRI) is often the only information available for compact fluorescents. Select a CRI closest to 100, which is the best.
Select fluorescent bulbs that have an electronic ballast. With compact fluorescents, select a ballast base that is separate from the bulb. The ballast will outlive the bulb allowing the owner to change the bulb only.
Compare the rated life of lights with the above features and select the longest rated life.
The efficiency of lights or (efficacy) is measured by the amount of lumens created per watt of electricity expended. For example, fluorescent lights may range from 22-85 lumens per watt.
Triphosphers or multiphosphers lights give off the most light. (Phosphers are the glowing and light creating aspect of fluorescents.) These are sometimes referred to as "deluxe" lights.
These resemble incandescents in appearance and in compactness with concentrated and directed light output. However, they operate similarly to fluorescents. These lamps are most suitable for outdoor/security lighting.
Mercury vapor lamps are the least efficient HID lamp. Metal halide HID lamps are about 50% more efficient with better color rendition.
Sodium in HID lamps gives greater efficiencies. High pressure sodium has a high first cost and poor color rendition. Low pressure sodium is the most efficient and has poor color rendition. It is suitable where the light is needed for visibility only such as in security lighting.
Reduced wattage lamps are offered by several manufacturers. These use approximately 15% less power due to improved manufacturing technology.
Incandescent lamps with reflectors have improved light output. Reflector (R) lamps are one type. Ellipsoidal reflector (ER) lamps outperform R lamps due to the way the light is reflected out of the lamp. Parabolic aluminized reflector (PAR) lamps are available with improved performance due to reflector redesigns. PAR lamps are suitable for exterior applications.
Halogen lamps will consume about 60 % less power than incandescents and provide longer service. Some are made for low voltage applications.
Diodes and thermisters are electronic components that can be added to incandescent lamps.
Diodes are wave rectifiers reducing 60 hertz AC cycles in half. The results are similar to using a dimmer. Power consumption is reduced by 42% but light output is reduced by 70%. Lamp life is typically extended but color rendition is poor and the economics is generally unfavorable.
Thermisters limit the inrush current and reduce the voltage. Power consumption is reduced 2-4% and light output drops 7-16%. Life is increased 2-21/2 times.
The SunPipe Company
P. O. Box 2223
Northbrook, IL 60065
(708) 272-6977
daylighting skylight product
Illuminating Engineering Society of North America
345 E. 47th St.
New York, NY 10017
(212) 705-7925
International Dark-Sky Association gives tips and examples of good outdoor lighting, so that we can continue to see the stars at night!
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Sustainable Building Sourcebook web version copyright Sustainable Sources 1994-1999.