Passive Heating And Cooling

Passive heating and cooling utilize the heat created by the sun to warm living spaces; prevailing winds and air buoyancy (warm air rises) to provide natural ventilation; and the cooling effect that trees and plants provide through a combination of shade and evapotranspiration to reduce the need for mechanical heating, cooling, or ventilation systems. Passive solar design possibilities are often overlooked during site selection and building design, in urban areas, as the incorrect assumption is that passive solar systems are impractical in a dense, constrained setting. Passive solar gains can be captured in the mass of the overall building, in stairwells, or on the sides of buildings with good solar exposure. Also, the benefits of solar radiation are not limited to locations with a large annual percentage of sunny days, as solar gain can still occur on overcast days or when outdoor temperatures are well below freezing.

Taking advantage of the sun's free heat reduces the need for and the cost of operating conventional heating equipment. Passive solar systems use thermal storage materials such as concrete or masonry that are strategically placed to absorb the heat of the sun during the day and then release it during the cooler nighttime hours. Such systems may require additional building mass, increased insulation, and possibly more involvement by occupants or maintenance staff to open or close window blinds at certain times.

Direct thermal mass is mass that is in the path of sunlight, such as floors adjacent to south-facing windows. Concrete or tile floors are the most commonly used material, with darker materials generally absorbing more heat. Indirect thermal storage is mass that receives little direct sunlight but which can absorb ambient heat during hot parts of the day and release it at cooler times. Indirect thermal mass can be placed anywhere in a building, and can be any color. A thin layer of concrete can be added to upper floors, for instance.

Passive ventilation reduces the need for air-conditioning and fans, and improves indoor air quality by supplementing mechanical ventilation during temperate periods. Strategies include designing window location and floor plans for cross-ventilation; using awnings, louvers, horizontal fins, or trees to block direct sun on windows; and minimizing window exposure to low-angle afternoon summer sun. Recommended passive heating and cooling strategies include the following:

• Optimize the amount of south-facing windows adjacent to thermal mass.

• Orient buildings to capture prevailing winds.

• Place smaller window openings on the sides of buildings that face prevailing winds and larger openings (doors, larger windows) on the opposite sides to increase pressure differentials and facilitate cross-ventilation.

• Provide overhangs or shading devices on the south-side roof lines of buildings to shade windows, doors, porches, and patios from hot midday sun. Overhangs on the east and west sides of buildings are much less effective because of the long sun angle in the morning and evening.

• Consider use of operable exterior blinds or other shading devices on west-facing windows in hot climates.

• Landscape with deciduous trees to provide shade in the hot parts of the year and capture warmth in the winter. Shade is most needed on the west side of a building, which receives sun on hot afternoons.

• Place trellises or arbors on or adjacent to buildings so climbing plants can shade patios, walkways, and windows.

• In warm climates, use exterior cladding and roofing materials with high reflectivity (light-colored) to reduce solar gain.

• Install an a light-colored ENERGY STAR® or Cool Roof to reduce heat transfer into attics or top-floor living units.

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