Page E1.2 . 12 December 2001                     
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    Thermal Delight in Courtyards

    continued

    What is thermal comfort in traditional hot-climate courtyard buildings? First, it depends greatly upon the thermal mass of the building for the amount that the temperature fluctuates and by how many degrees, and at what hours it reaches highs and lows.

    Second, it depends on the personal involvement of the building's occupants, whose actions are often called "thermal sailing." This includes watering the courtyard, opening and closing shading devices and windows, and adjusting one's own clothing and activity level.

    And third, it depends on accepting less thermal control than would be possible with today's air conditioning equipment.

    Taming the Climate

    Courtyards represent an attempt to bring the forces of nature under partial control. This is somewhat like trying to regularly feed wild animals; one can never be sure when or how much interaction will result. Expectations of thermal comfort in and around the courtyard might be described as "unpredictable to an acceptable degree."

    As pockets of space that are open to the sky, courtyards intensify some aspects of the climate, such as daylight, and dilute others, such as the wind. The horizontal aperture through which nature enters has several characteristics. In winter when the sun is low in the sky, the result is mostly shade on floor and walls, with only the equator-facing wall receiving much direct sun.

    The horizontal aperture provides extensive exposure to the sky, which is very cold on clear nights. A horizontal aperture thus emphasizes cold in a cold season with long nights.

    It also emphasizes heat in the hot season, because the sun, now high in the sky, can readily penetrate a horizontal opening. Now the floor and several walls are sunny, with only the polar-facing wall remaining a reliably shaded surface. In the tropics, building shadows provide no escape from the sun directly overhead around noon. This heat emphasis is partially offset at night, when clear skies are coldest.

    Thus a courtyard does not guarantee thermal comfort in the surrounding building. In a 1996 study of older courtyard housing and newer row housing in Baghdad, researchers Raid Hanna and Paul Simpson, of the Mackintosh School of Architecture at Glasgow University, found that the newer row houses maintained a lower temperature in July and August.

    The roofs were uninsulated in both housing types; in the courtyard houses, the density of the neighborhood was greater, and the walls were twice as thick. These authors identified access to ventilation as the important variable.

    It seems that for courtyard comfort, the best climates are those with hot, dry days and cool clear nights to radiate heat away to the sky; or those with cool but sunny days at lower latitudes where sun penetration is both easy and welcome.

    It seems that for courtyard comfort, the worst climates are hot and humid, where little wind is available to relieve stuffiness; or cold and cloudy ones at higher latitudes, where little winter sun can penetrate to provide warmth.

    Even in one of the "best" climates, that of Andalucia in southern Spain, there remains the problem of choice of courtyard depth. A deeper courtyard will stay cooler in summer, but exclude sun for warmth in winter.

    It seems that deeper courtyards are generally favored in such climates, perhaps because heating is easier to achieve than is cooling. The inhabitant of a cold-winter courtyard building can burn a variety of fuels (wood, kerosene) in simple heating devices, can wear warmer clothes, can eat hot foods, and can increase activity to cope with the cold.

    In contrast, the inhabitant of a hot-summer courtyard building has fewer low-technology options; sit near a block of ice, turn on a fan (requiring electricity), drink cold liquids. Fewer clothes, or decreased activity, are rarely options that help much (or are realistic). The architecture is therefore arranged to make spaces cooler in the summer, rather than warmer in the winter.

    Avoiding Heat Gain

    Shading, or otherwise avoiding heat gain, is the first rule of thermal comfort in hot-climate courtyard buildings. While the courtyard is by definition open to the sky, there are reasons in addition to the hot sun for at least temporarily filtering this aperture, including dusty wind, and bats or other intruders from above.

    A wide-mesh screen, of chicken-wire, for example, serves to discourage intruders. For one courtyard in Colima, it also becomes a trellis for vines. The vines filter sun and diminish both daylight and wind. But when wind passes through them, they sway and reinforce the impression of air motion.

    A restaurant's courtyard in Oaxaca is covered with a lavender-flowering vine, attracting bumblebees and small birds. It rests just far enough above the tables to allow people to walk comfortably below. The upper leaves absorb the sun, shading the lower leaves, thus keeping them cooler just above one's head.

    (This vine, common in Oaxaca, had many names; my favorite was rompe platos, because, legend has it, if you pick one of the blossoms instead of letting it fall, you'll break a plate. Botanist Tom Ogren suggests it may be thunbergia grandiflora.)

    Shading cloth is a narrow-mesh material gaining wide usage, available in varying degrees of sun blockage (90 percent is quite common). It still allows a small percentage of direct sun through, so there are still patterns of sun and shadow below it.

    The amount of light and heat is greatly reduced, yet the view through it surprisingly clear. A restaurant at the edge of the tropics in Salta, Argentina benefits from the sparkle of direct sun, but gets little of its heat.

    Is the shade from an overhead fabric cover a toldo preferable to shading by plants? In its open position, a toldo is folded against one wall. Like a large tree, the toldo casts shade over the whole courtyard; unlike a tree, it is swept away in the early evening to facilitate both ventilation and cold-sky radiation, all night long.

    The toldo changes the microclimate, providing even more control of this highly managed landscape. With its daytime shade, it favors the more delicate ferns and vines.

    A key characteristic of the toldo is its degree of translucence. The more translucent this cover, the more diffused is the light cast over the entire courtyard, rather than the unshaded contrast of bright sunlight and deep shadow. This diffused light is influenced by the color of the toldo.

    Physiologically, the translucent toldo favors plant growth and provides evenly distributed daylight to the arcades. Psychologically, an orange or yellow toldo connotes heat while flattering the human complexion; a blue or green toldo connotes "coolth" while rendering human complexions cadaverous.

    The most neutral toldo is white. From a maintenance viewpoint, the translucent toldo will show the silhouette of dirt and debris on its surface.

    An opaque toldo casts a deep shade, leaving a ring of daylight at its edges, and a grid of small elliptical sunspots across the courtyard. This is due to the grid of grommets, provided so that the horizontal toldo will not collect water and collapse in a rain. storm.

    Besides discouraging plant growth, the opaque toldo absorbs sunlight, becoming a huge radiant heater. The closer this hot surface is to the floor, the greater the daytime discomfort. Yet the opaque toldo is fairly common, probably because it doesn't show accumulated dirt and debris as does the translucent one.

    As typically applied, both the translucent and opaque toldo nearly fill the sky opening, all but eliminating the possibility of ventilation while they are closed. Another impact is increased reverberation; any toldo will be more reflective of courtyard sounds than is the open sky.

    Besides shading and ventilation, other courtyard strategies for summer cooling include evaporation and the use of massive building materials.

    John S. Reynolds is a professor emeritus of architecture at the University of Oregon and active with the American Solar Energy Society and the Society of Building Science Educators. The recipient of numerous fellowships and grants, his research has focused on the relationship between environment, inhabitants, and energy use.

    This article is excerpted from Courtyards: Aesthetic, Social, and Thermal Delight copyright 2001, available from John Wiley & Sons and Amazon.com.

     

    AW

    ArchWeek Image

    The toldo is the moveable fabric cover over hot-climate courtyards. Here, a translucent white toldo is arrayed in strips at Oaxaca's Museum of Graphic Arts.
    Photo: John S. Reynolds

    ArchWeek Image

    This Seville, Spain courtyard by architect Jaime Lopez de Asiain, is shaded by velas (sails) to keep out the rain.
    Photo: John S. Reynolds

    ArchWeek Image

    An opaque toldo casts a very dark shade.
    Photo: John S. Reynolds

    ArchWeek Image

    A translucent blue toldo, is left open at one end, allowing a contrast between the patch of sun and the cool-colored shade. Calle Fernando Colon, Cordoba.
    Photo: John S. Reynolds

    ArchWeek Image

    Combinations of air temperature and relative humidity marked by dashed lines can produce comfortable conditions given sufficient ventilation.
    Image: John S. Reynolds, based on a diagram by Murray Milne and Baruch Givoni

    ArchWeek Image

    In the zone marked "high mass," these outside conditions can still produce comfort if highly massive construction is skillfully employed.
    Image: John S. Reynolds, based on a diagram by Murray Milne and Baruch Givoni

    ArchWeek Image

    In the zone marked "evaporation," hot, dry outside conditions can produce comfort if evaporative cooling is available.
    Image: John S. Reynolds, based on a diagram by Murray Milne and Baruch Givoni

    ArchWeek Image

    Courtyards: Aesthetic, Social, and Thermal Delight, by John S. Reynolds.
    Image: John Wiley & Sons

     

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