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Daylighting

[dey-lahyt]

Daylighting is the practice of placing windows, or other transparent media, and reflective surfaces so that, during the day, natural light provides effective internal illumination.

Within the overall architectural design of a building, particular attention is given to daylighting when the aim is to maximize visual comfort, productivity, or to reduce energy use. Energy savings from daylighting are achieved in two ways--either from the reduced use of electric lighting, or from passive solar heating or cooling.

Electric lighting energy savings can accrue because occupants choose not to switch their lights on, or because an automatic lighting control system ("photocontrol system") switches the lights off or dims them to a lower level.

In passive solar technique, buildings are designed such as to account for local climate, in particular the luminance of the sky. For instance, in cooler parts of the globe with largely overcast skies, a house will be designed with minimal windows on the polar side but more and larger windows on the equatorial-side. This is because there is no direct sunlight on the polar-side wall of a building from the autumnal equinox to the spring equinox in parts of the globe north of the Tropic of Cancer and in parts south of the Tropic of Capricorn. Equatorial-side windows receive at least some direct sunlight on any sunny day of the year, so they are effective at daylighting areas of the house adjacent to the windows. One disadvantage of relying on conventional window space for daylighting is that, especially during mid-winter, it tends to be highly directional light that casts deep shadows. This may be partially ameliorated through light diffusion and somewhat reflective internal surfaces.

Windows

Windows are the most common way to admit daylight into a space. Their vertical orientation means that they selectively admit sunlight and diffuse daylight at different times of the day and year. Therefore windows on multiple orientations must usually be combined to produce the right mix of light for the building, depending on the climate and latitude. There are three ways to improve the amount of light available from a window.

Place window close to a light colored wall.
Slant the sides of window openings so the inner opening is larger than the outer opening.
Use a large light colored window sill to project light into the room.

Different types and grades of glass and different window treatments can also affect the amount of light transmission through the window.

Light reflectors

Once in extensive use in office buildings, the adjustable light reflector is seldom seen, having been supplanted by a combination of other methods in concert with artificial illumination. The reflector found favor where the choices of artificial light provided poor illumination compared to modern electric lighting.

Light shelves

An effective way to enhance the lighting from windows on the equator-facing side of a structure is to place a white or reflective metal light shelf outside the window. Usually the window will be protected from direct summer season sun by a projecting eave. The light shelf projects beyond the shadow created by the eave and reflects sunlight upward to illuminate the ceiling. This reflected light can contain little heat content and the reflective illumination from the ceiling will typically reduce deep shadows, reducing the need for general illumination.

In the cold winter, a natural light shelf is created when there is snow on the ground. As the outside temperature drops below freezing, moisture in the atmosphere precipitates out, often in the form of snow (or freezing rain). This makes the ground highly reflective, and the skies have few clouds. Low winter sun (see Sun path) reflects off the snow and increases solar gain through equator-facing glass by one-to-two thirds, brightly lighting the ceiling of these rooms. Glare control (drapes) may be required.

Skylights

Skylights are often used for daylighting. They are horizontal windows placed at the roof of buildings. Skylights admit more light per unit area than windows, and distribute it more evenly over a space. They can therefore be a good choice when daylight is being used to illuminate a space. In order to maximize efficiency, skylights are best placed to the back or front of the building's roof (depending on the sun), so that it floods a light colored wall rather than shining on the center of the room. The reflection from the wall increases the light flowing into the room. The optimum number of skylights (usually quantified as "effective aperture") varies according to climate, latitude, and the characteristics of the skylight, but is usually 1-10% of floor area. The thermal performance of skylights is affected by stratification, i.e. the tendency of warm air to collect in the skylight wells, which in cool climates increases the rate of heat loss. During warm seasons, skylights also can cause internal heat problems, which is usually treated by placing a shade over the skylight, or by opening it if it is openable.

The amount of light skylights deliver peaks around midday, when the additional light and heat it provides is least needed. Some skylight designs use domed or pyramidal shapes along with prismatic or other light-redirecting glazings to achieve more even light levels through the course of a day. Poorly constructed or installed skylights may have leak problems and single-paned ones may weep with condensation. Using skylights with at least two panes and a heat reflecting coating will increase their energy efficiency.

Light tubes

Another type of device used are light tubes, also called solar tubes, placed into a roof and admitting light to a focused area of the interior. These somewhat resemble recessed light fixtures in the ceiling. They do not allow as much heat transfer as skylights because they have less exposed surface area. It is also easier to retrofit light tubes into existing buildings, especially those with deep roof constructions.

Clerestory windows

Another important element in creating daylighting is the use of clerestory windows. These are high, vertically-placed windows. They can be used to increase direct solar gain when oriented towards the equator. When facing toward the sun, clerestories and other windows may admit unacceptable glare. In the case of a passive solar house, clerestories may provide a direct light path to polar-side (north in the northern hemisphere; south in the southern hemisphere) rooms that otherwise would not be illuminated. Alternatively, clerestories can be used to admit diffuse daylight (from the north in the northern hemisphere) that evenly illuminates a space such as a classroom or office.

Often, clerestory windows also shine onto interior wall surfaces painted white or another light color. These walls are placed so as to reflect indirect light to interior areas where it is needed. This method has the advantage of reducing the directionality of light to make it softer and more diffuse, reducing shadows.

Sawtooth Roof

Another roof-angled glass alternative is a "sawtooth roof" (found on older factories). Vertical roof glass faces away from the equator side of the building, to capture diffused light (not harsh direct equator-side solar gain). The angled portion of the glass-support structure is opaque and well insulated with a cool roof and radiant barrier. The sawtooth roof lighting concept partially reduces the summer "solar furnace" skylight problem, but it still allows warm interior air to rise and touch the exterior roof glass in the cold winter, with significant undesirable heat transfer.

Solarium

In a well-designed isolated solar gain building with a sun room, solarium, greenhouse, etc., there is usually significant glass on the equator side. A large area of glass can also be added between the sun room and your interior living quarters. Low-cost high-volume-produced patio door safety glass is an inexpensive way to accomplish this goal.

The doors used to enter a room, should be opposite the sun room interior glass, so you see outside immediately when you enter most rooms. Halls should be minimized and open spaces used instead. If a hall is necessary for privacy or room isolation, inexpensive patio door safety glass can be placed on both sides of the hall. Drapes over the interior glass can be used to control lighting. Drapes can optionally be automated with sensor-based electric motor controls that are aware of room occupancy, daylight, interior temperature, and time of day. Passive solar buildings with no central air conditioning system, need control mechanisms for hourly, daily, and seasonal, temperature-and-daylight variations. If the temperature is correct, and a room is unoccupied, the drapes can automatically close to reduce heat transfer in either direction.

To help distribute sun room daylight to the sides of rooms that are farthest from the equator, inexpensive ceiling-to-floor mirrors can be used. They are particularly useful on west walls, where you would prefer to have no windows at all.

Building codes require a second means of egress, in case of fire. Most designers use a door on one side of bedrooms, and an outside window, but west-side windows provide very-poor summer thermal performance. Instead of a west-facing window, you can use an R-13 foam-filled solid energy-efficient exterior door. It may have a glass storm door outside. Opening the inner door allows light in. East/west glass doors and windows should be fully shaded top-to-bottom, or a spectrally-selective coating can reduce solar gain by more than three quarters. Exterior shade is better.

Translucent Insulation

Aerogel is a new alternative to inefficient windows, if all you need is daylight in a room. It has the lowest density of any solid material. It is translucent and has excellent insulating properties, but limited structural strength. Aerogel can be placed between two pieces of structurally-strong translucent fiber-reinforced plastic to create a translucent wall that allows 20% of available light to come through, but has an R-20 resistance to undesirable heat transfer. Images cannot be clearly seen through it, but diffuse daylight does pass through, without sacrificing good passive solar building thermal performance. Transparent insulation can be used effectively on many vertical walls, or on a sawtooth roof, to provide low-cost natural daylight with reduced heat transfer over window glass. Exterior-or-interior shading, shutters, drapes, or shade screens may be useful in some applications.

Fiber Optic Concrete Wall

Another way to make a secure structural concrete wall translucent is to embed optical fiber cables in it. (See Concrete casts new light in dull rooms) Daylight (and shadow images) can then pass directly through a thick solid-concrete wall. The only drawback is an inability to put insulation on either side of such a fiber-optic concrete wall. One possibility is to insulate it with aerogel after concrete wall construction, for natural daylight with the highest-possible structural security and NO glass windows.

Hybrid Solar Lighting

Oak Ridge National Laboratory (ORNL) has developed a new alternative to skylights called Hybrid Solar Lighting, which uses a roof-mounted light collector, large-diameter optical fiber, and modified efficient fluorescent lighting fixtures that have transparent rods connected to the optical fiber cables. Essentially no electricity is needed for daytime natural interior lighting.

2006-2007 field tests of the new HSL technology were promising, but the low-volume equipment production is still expensive. HSL should become more cost effective in the near future. A version that can withstand windstorms could begin to replace conventional commercial fluorescent lighting systems with improved implementations in 2008 and beyond. The U.S. 2007 Energy Bill provides funding for HSL R&D, and multiple large commercial buildings are ready to fund further HSL application development and deployment.

At night, ORNL HSL uses variable-intensity fluorescent lighting electronic control ballasts. As the sunlight gradually decreases at sunset, the fluorescent fixture is gradually turned up to give a near-constant level of interior lighting from daylight until after it becomes dark outside.

HSL may soon become an option for commercial interior lighting. It can transmit about half of the direct sunlight it receives.