High Performing Buildings - Spring 2008 - (Page 37) deep shelf. Another option is to incorporate blinds between the glass to intercept this problematic light and reflect it up to the ceiling. When sizing the amount of glazing required, account for the loss in transmission due to blinds between the glass. Most blinds between the glass can be closed if desired. However, if the space does not need this option, the blinds’ position can be permanently set to angle up to the ceiling. • E N E R G Y R A M I F I C AT I O N S A daylighting strategy that is not typically superior to electric lighting will create a negative energy situation. Insufficient daylight results in the lights being turned on, meaning heat is produced from the lights as well as from the sunlight. If designed correctly, a daylighting strategy can reduce electricity for lighting and peak electrical demand, cooling energy and peak cooling loads, maintenance costs associated with lamp replacement, and electrical service to the building. Maximize south glazing and minimize eastand west-facing glass. The accompanying charts indicate the amount of thermal gain on different flat surfaces without a south overhang. To maximize winter heat gain and minimize summer heat gain, use southfacing apertures. It is easy to add glazing and achieve a high daylighting contribution. The key is implementing glazing that provides superior daylighting for 60% to 70% of the time without increasing cooling loads during peak. The best design can be determined by simulating the varying glazing amounts and overhang lengths during peak cooling times. Because of the sun’s low azimuthal angles that hit the east and west elevations, it is difficult and can be expensive to produce a controlled daylighting solution using east- and west-facing glazing. Avoid uncontrolled horizontal skylights. Skylights provide the least daylighting benefits from an energy perspective. More than twice the heat will enter into a building through a flat skylight in the summer than in the winter, just the opposite of what is desired. If the size of a flat skylight is optimized to reduce heat during the summer, it will not provide the necessary daylight during the rest of the year and can become problematic from a direct beam standpoint. Although skylights can be designed with internal tracking louvers and produce good quality daylighting, it is difficult to justify their use when considering cooling peak loads. Only in a few areas of the country, where the climate is mild and sky conditions are optimal, should skylights be considered a better energy choice than roof monitors or lightshelves. Reduce cooling loads. In the warmer months, cooling loads can be reduced by providing just the right amount of daylight to meet the footcandle objective. When the electric lights are off, the cooling load is less because daylight can produce the same lumens as fluorescent fixtures but with only half the heat. To achieve these cooling reductions, use south- and north-facing strategies with automated dimming. Consider passive solar gain to offset winter heat previously provided by lights. Good daylighting strategies result in the lights being out the majority of the time year-round. This means that the winter heat typically produced by lights is significantly less. South-facing, vertically placed daylighting strategies naturally increase heat entering into the space during the winter months and can compensate for the heat that was provided by lights. Select the right glazing. It is important to minimize the size and maximize the visible light transmission of daylighting apertures. Windows should be made of high-quality construction, incorporate thermal breaks, and include the appropriate glazing for the application. Make a clear distinction between glazing for views and/or ventilation and glazing for daylighting. For daylighting windows, clear glass typically has an advantage over glazing with a low-e coating. Because a 10% to 40% reduction in visible light transmission characteristics of most low-e coatings exists, 10% to 40% more glass is required to attain the same daylighting benefit. When comparing the thermal benefits of low-e coatings to the benefits of visible light transmission, consider the cost of lightshelf or roof monitor components that would be added if more glazing is required. However, wherever low view glass windows are incorporated, low-e coating should be used to improve comfort and save energy. Increase the performance of the lightshelf by implementing a sloped ceiling from the top of the lightshelf glass downward to the back, north wall. In my experience, this improves reflectance and can reduce glazing up to 10%. By sloping the ceiling from the outside wall to the back of the space, it is often possible to encroach into the ceiling cavity above the window area and gain needed space for mechanical systems on the north side without increasing floor-to-floor dimensions. The benefits of the sloped ceiling are apparent when comparing a room with a flat, 10 ft ceiling to one with a ceiling that is 11 ft 4 in. at the lightshelf and 9 ft at the back of the space. North-Facing Transom Glazing In spaces located on the north side of the building, high transom glazing can be an effective strategy, specifically in narrow rooms or large spaces, when used in combination with south-side lightshelves or roof monitors. High north-facing transom glazing can provide good daylighting in spaces that are not too deep without the problem of direct beam Spring 2008 HIGH PERFORMING BUILDINGS 37
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