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Daylighting Design Strategy
Because daylight produces less heat per unit of illumination than many artificial lights, daylighting may reduce cooling costs when it replaces artificial lighting. As part of a passive solar heating system, sunlight can also provide supplementary building heat. If improperly designed, however, glazed areas that allow daylight into a building also contribute to heat loss in the winter and undesirable heat gain in the summer, leading to heating and cooling costs that can offset savings from reduced lighting costs. Daylighting designs must be carefully analyzed to ensure that they reduce artificial lighting needs without increasing cooling or heating requirements.

The analysis must take into account a building’s orientation with respect to the sun, the path of the sun at various times of the year. It involves some understanding of how a given glazing system transmits visible light:

Visual transmittance (TV) is a measure of the portion of visible light that passes through a window. Glazing systems with high TV values (0.7 to 0.9) provide good natural light.

Solar heat gain coefficient (SHGC) and shading coefficient (SC) are measures of a glazing system’s net solar gain. Systems with high SHGCs (0.7 to 0.9) provide significant solar gain; those with values in the 0.2 to 0.4 range provide little solar gain. To reduce heating in northern climates, select the highest SHGC you can find so that winter solar gains can offset heating needs. In central climates, with significant air conditioning costs or summer overheating problems, look for SHGC values of 0.40 or less. A low SHGC is the most important window property in warm climates.

The light-to-solar gain ratio is TV divided by SHGC, and is an index of how much light a system provides in proportion to the solar gain produced. Systems with an LSG ratio greater than 1 provide more heat than light.

U-value expresses how much energy a glazing system transfers by conduction and convection. In general, select windows with U-values of 0.40 or less.

R-value is the insulating value of a material. Single-glazed windows have R-values of 1; double glazed panels, about 2. In laboratory tests, researchers are creating window systems with R-values of between 6 and 10. These are multiple-pane systems with two low-e coatings and interior air spaces filled with an inert gas, like argon, that conducts heat less than air.

In some climates, U-value should take precedence over the Solar Heat Gain Coefficient when selecting a glazing infill. It costs more to air-condition a space than it does to heat it. In warm climates you should choose an infill based on its SHGC. The lower the coefficient the less heat allowed by the infill. In cold climates you should choose an infill with a low U-value. A low U-value will slow the loss of air you’ve paid to heat by slowing heat transfer through the material.

It is important that designers consider vision glazings and daylighting glazings differently because they perform very different functions. Vision glazings typically use lower transmittances to provide comfortable views to the outside. Daylight glazings, because they are used to provide interior illumination, generally have a much higher visible transmittance than vision glazings. As a general rule, select a vision glass with a visible transmittance between 20 and 30 percent, and a daylighting glass with a visible transmittance of 50 to 60 percent.

Glazing may be clear, tinted, coated or filmed. Windows may be single-or multiple paned, and multiple-paned systems can be vacuum-sealed or filled with either of several inert gases. Glazing materials that selectively control the spectral aspect of solar radiation are now commonplace, and additional energy savings can be obtained by controlling the spectral characteristics of glazing with the new materials.

Low-emissivity coatings suppress infrared radiation, resulting in additional thermal insulation. Modified low-e coatings can reject unwanted heat gain due to solar infrared. In cold climates, low-e glazings have the effect of keeping warmth in the building during winter. A typical pane of single glass has a U-value of about 1.0 BTU/hr-sq ft; a typical low-e glass a U-value of about 0.63.

To optimize the response of glazing systems to unwanted radiation, researchers at the Berkeley National Laboratory, and elsewhere, are experimenting with “electrochromic” materials, whose optical properties change with the injection of light ions. A number of other options—photochromics, reflective hydrides, liquid crystals, thermotropics and suspended particle displays—all of which have peculiar characteristics that change in response to varying heat or light, are being studied to achieve the same result. The application of electrochromic technology is still limited, but may play an important role in glazing design in the not-too-distant future.