Sustainable Building Technical Manual - Etn-presco.net

Passive Solar Cooling❑ Design buildings for cooling load avoidance.Minimization of cooling loads should be carefully addressed for both solar building andconventional energy-efficient building design. Design strategies that minimize the needfor mechanical cooling systems include proper window placement and daylightingdesign, selection of appropriate glazings for windows and skylights, proper shading ofglass when heat gains are not desired, use of light-colored materials for the building envelopeand roof, careful siting and orientation decisions, and good landscaping design.❑ Choose one or more shading strategies.– Install fixed shading devices, using correctly sized overhangs or porches, or designthe building to be “self-shading.” Fixed shading devices, which are designed into abuilding, will shade windows throughout the solar cycle. They are most effective onthe south-facing windows. The depth and position of fixed shading devices must becarefully engineered to allow the sun to penetrate only during predetermined timesof the year. In the winter, overhangs allow the low winter sun to enter south-facingwindows. In the summer, the overhangs block the higher sun.– Plant trees and/or bushes to shade the windows at the right time of day and season(see Chapter 7, “Site Materials and Equipment”). Deciduous vegetation is often anattractive and inexpensive form of shading, because it follows the local seasons, notthe solar calendar. In the warm south, where more shading is needed, trees leaf outearlier, while in the cold north, where solar heat is beneficial late into spring, treeswait until the weather warms up before they leaf out. Trees can be strategicallyplanted on east and west sides to block the rising and setting sun. Bushes can bepositioned to block undesirable low sun angles from the east or west, and deciduousvines trained to grow over trellises make easily controlled shading systems.Evergreen trees trimmed so that their canopies allow low winter sun underneath butblock the high summer sun can be very effective. Properly placed vegetation canalso guide air flows toward buildings for natural ventilation and can block cold winterwinds. Vegetation and groundcover also contribute to evaporative coolingaround a building.Vegetation used for shading should be properly located so as not to interfere withsolar gain to buildings in winter. Deciduous trees can reduce winter solar gain by20 percent or more and should not be placed in the solar access zone. Also notethat trees require maintenance, pruning, watering and feeding. As they grow theychange their shading pattern, and they can be damaged or killed, leaving thebuilding exposed.– Consider awnings that can be extended or removed. Movable awnings are an oldtradition and an excellent solution to the variation between seasons and the solaryear. When rolled out in the summer, they not only provide deep shade but alsolend a colorful touch to a building’s facade. When rolled up in winter, they allowmore sun into the building and avoid snow loads and/or excessive weathering.– Consider exterior roll-down shades or shutters. An enormous variety of verticalshading devices are readily available. Wooden shutters are the most traditional. Alsoavailable are many exterior-grade fiberglass and plastic fabrics that cut out a significantamount of sunlight but still allow a clear view through the window. However,they do not prevent the glare problems caused by low-angle sun. Opaque steel orplastic roll-down shutters have proved reliable and long-lasting. Although expensive,they can also provide additional storm and vandalism protection.– Limit east/west glass. Glass on these exposures is harder to shade from the easternmorning sun or western evening sun. Vertical or egg-crate fixed shading workswell if the shading projections are fairly deep or close together; however, thesemay limit views. North-facing glass receives little direct solar gain, but does providesdiffuse daylight. 2