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Green & Sustainable Construction

Cooling Load Advoidance

Since heat sources internal to the building, such as lighting and computers, are often constant throughout the year, the peak cooling load and the size of the air conditioning system required to meet this peak are often determined by solar heat gain on the building envelope.

On a national average, space cooling represents 10% of annual energy use in residential buildings, and 12% in commercial buildings. In commercial buildings, 33% of the cooling load is due to solar heat gain through the windows (of the remainder, 42% is due to heat from lights, 18% to heat from equipment, and 7% to heat from the people inside).

Since the sun cuts a high arc across the sky in summer, a building with small east and west dimension is recommended for cooling load avoidance, as it is for solar heating in winter, when the sun cuts a much lower arc to the south. In the summer, the sun is at a maximum on the roof and on the west façade, which is why these faces deserve the most attention regarding strategies to reduce solar heat gain.

While solar heat gain on well-insulated opaque surfaces is negligible, the size and orientation of windows is key. Solar heat gain on west-facing windows is at a maximum on summer afternoons, so the size of these windows should be no more than what is required to take advantage of an important view or to meet daylighting goals. Windows on the south side are benefi cial for winter heat gain, and an overhang over them blocks the sun when it is higher in the sky in summer. An overhang can be designed to provide shade in summer and sun in winter, but only on the south side. On the north side, an overhang is never needed, and on the east and west sides is not effective due to low sun angles in the morning and at night.

Solar heat gain can also be controlled by careful selection of windows glazing properties. Glazing with a low solar heat gain coefficient (SHGC) attenuates solar heat gain. The low SHGC is achieved by absorbing the energy in the tint of the glass or reflecting it with a surface coating. Reflection is the most direct way to reject solar heat, since some of the light absorbed in the tinted glass will be re-radiated or convected into the room air. If a clear appearance is desired, or if a high visible transmittance is required to meet daylighting goals, a selective glazing is recommended. Selective glazing screens out the infrared and ultraviolet portions of the solar spectrum, but allows much visible light to pass. A double-pane assembly of selective glazing typically has an SHGC of 0.35.

Occupant comfort may be improved by the use of shades and blinds to block the sun. However, once solar heat makes it through the window glass, it must be removed by the building mechanical system, with associated energy cost and environmental impacts. In other words, blinds and drapes only stop the heat flow after the heat is already in the house.

Several measures can be taken outside of the building to mitigate solar heat gain if it is unwanted. Deciduous trees provide shade in summer, but in winter they lose their leaves, allowing about 60% more sun to pass through for solar heating. Vegetation can also be provided on a trellis to block the sun from a window or porch.

Green roofs are roofs with a thin layer of planted soil to dissipate solar heat, absorb water runoff, and give the roof space a pleasing garden-like appearance.

Reflective white or aluminized coatings are also used to reflect solar heat. Water-spray systems have been demonstrated to cool the roof, but the drawback is significant water consumption.

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