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MAY/JUNE 2008 SPECIAL SECTION:
The Architect's Guide to
Passive Solar Heating and Cooling |
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| Vertical Ventilation for Passive Cooling |
Hot air rises. So use it to drive natural ventilation. |
By Virginia B. Macdonald, FAIA
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Vents can be used as design elements. In this house the bench ducts air from a cool crawl space. Decorative grates can be set directly into flooring tiles but should be backed with filters. |
Hot air rises. By taking advantage of the natural vertical flow of air, it is possible to regulate the temperature of any enclosed space, making it cool or warm as needed. With nothing more than good natural design, a home, office or business space can be kept comfortable without the use of expensive and energy wasteful air conditioning.
Vertical ventilation is a natural cooling method. It uses only the sun for energy, and the air that already surrounds a building, and thus makes no demands on the world’s fossil fuel supplies or other non-sustainable resources.
For effective passive solar ventilation, the building should act as a thermal chimney, always allowing warm air to move up and out. For example, in a chimney the air flow is always warm to cool, but unheated air moves languidly, if at all. By lighting a fire in the stove or fireplace, the air warms and moves faster because of the confines and height of a chimney. In buildings, the controlled use of sunlight can stimulate airflow in the same way.
Any building can be designed to function as a chimney. The heat of the sun entering a building, whether from skylights or, in southern latitudes, from other glazing, accelerates upward air movement. Cool air is thus drawn in from below. The difference in temperature between the warm air at the top of the column and the cool air at the bottom is called the Delta T. Warm air holds moisture better than cool air does, so that as warm air expands, rises and vents, humidity is also removed from the interior space.
Only three things are needed to make the system work:
• An intake area near ground level or under the floor if there’s a crawl space, where air is cool.
• A sunlight source (spectrally selective skylights or windows) to create at least 10° F (5.6° C) of Delta T.
• Vents near the top of the building to let warm air escape.
The upper vents must be designed so that escape is always possible, regardless of wind direction, since the rising interior air won’t fight its way out even against a light breeze.
The most efficient way to achieve Delta T and incorporate out-vents is the venting skylight. The vents can be hidden and protected from outside weather factors like rain or snow. The shape of the vent should present minimum resistance to outward air flow.
Early in the design process, the architect should give consideration to the orientation of roof planes containing skylights, with respect to the best alignment to the sun’s path. Skylights with a good light-to-solar-gain ratio can accelerate vertical air movement by warming the interior.
An obvious benefit of skylights is daylighting. In practice, skylight area totaling about 16 percent of floor area provides good overall light.
Cool air can enter through low in-vents in a variety of arrangements. Buildings with concrete slab floors can have low wall vents. Buildings with raised floors supported by foundation walls can have floor vents and/or wall vents. Vents should be screened against insects and leaves, sheltered from rain and ramped for rain run-off. In temperate climates a hinged cover can be provided on the inside to shut off or regulate airflow.
In warm and temperate climates, low wall vents are most effective when located on the shady side of a building or when well-shaded by trees or shrubs. Heat gun data show that good shading by plants provides air averaging 9° to 11° F (5° to 6° C) cooler than air with no shading. A well-shaded hopper window can function as an in-vent.
Out-vents should be situated at the highest possible point of the interior space. If there’s a ceiling, there must be a way around or through it so that the rising warm air can exit. In temperate climates, out-vents need to be designed so that they can easily be sealed (and insulated) against winter winds.
Open selected vents for greater cooling, or close some vents to capture warm air. When the system is designed properly it will work continuously night and day.
Open windows and doors halfway up the air column can disrupt efficient vertical airflow. To prevent that, the building may be designed with fixed glazing in the exterior walls (this alone can save 40 percent on the cost of windows). Local codes often mandate an emergency fire exit from each bedroom, a requirement sometimes met by providing operable windows. Where operable windows are mandatory, occupants may be educated about their efficient use, depending on wind and weather conditions.
A steep roof with no ceiling inside will improve the effectiveness of vertical ventilation. Strategic use of diagonal walls can direct airflow within the structure. Under a high roof, an open floor plan with few right-angle walls looks even more spacious and inviting by creating sight lines that can stretch from one end of the house to the other. The result can be the large open interior spaces that are popular with many homebuyers today.
Vertical ventilation can function in multistory buildings, with additional design considerations. Multistory buildings should be situated so that one side is shaded all day while the other side faces the sun most of the day. In-vents should be placed low on each floor on the shady side, while out-vents are placed high on the sunny side. A stairway running the full height of the building on the sunny side can act as a chimney, providing an exit route for warm air. Out-vents placed at the top of glass walls on each floor can also provide the needed ventilation. Air movement is accelerated if the building is long and narrow, with one of the long sides facing the sun most of the day.
Every site is unique. Buildings should be oriented and designed to take full advantage of sun, wind and landscape features.
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This article was adapted, with author’s permission, from the book Heating or Cooling Your Building Naturally: Solar Architecture Solutions by Virginia Macdonald, FAIA (2005) ISBN 0-9774561-0-2. The book is available to ASES Professional Members, for $20, at asespubs.org.
About the author: Virginia Macdonald, FAIA, has had a long and distinguished career in her adopted state of Hawai’i, designing buildings that are notable for their innovative use of environmentally sensitive techniques of natural lighting and ventilation. In 1999, she was honored by her peers with a Fellowship in the American Institute of Architects. In presenting the award, the AIA cited her “Passive solar ventilation designs that creatively combine science and architecture and produce major economic and environmental benefits.” Today she lives in California and remains a committed advocate ofgood design using environmentally friendly techniques.
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May/June issue: pubs@ases.org.
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