|What is a cool roof ?|
|Technical aspects of cool roofs|
|Thursday, 20 November 2008 10:46|
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Passive cooling relies on the use of techniques for solar and heat control, heat amortisation and heat dissipation. Solar and heat protection techniques may involve: Thermal improvement by the use of outdoor and semi-outdoor spaces, layout and external finishing, solar control and shading of building surfaces, thermal insulation, control of internal gains, etc.
Cool Roofs have high solar reflectance (high ability to reflect sunlight) and high thermal emittance (high ability to radiate heat) and stay cool in the sun.
The daytime surface temperature of a roof is raised by absorption of solar radiation and lowered by thermal radiation to the sky. Solar heating is proportional to solar absorptance (absorptance = 1 – reflectance), while radiative cooling is proportional to thermal emittance. The term solar reflectance (SR) designates the total reflectance of a surface, considering the hemispherical reflectance of radiation, integrated over the solar spectrum, including specular and diffuse reflection.
The infrared emittance (e) specifies how well a surface radiates energy away from itself as compared with a black body operating at the same temperature. Hence, other factors (e.g., incident solar radiation, convective cooling, and conductive cooling) being equal, a roof with high solar reflectance and high thermal emittance can stay cooler than a roof with a low solar reflectance and/or low thermal emittance.
Increasing the reflectance and/or emittance lowers a surface’s temperature, which in turn decreases the heat penetrating into the building, if it is a surface of the building envelope, or contributes to decrease the temperature of the ambient air as the heat convection intensity from a cooler surface is lower. During the summer, this results in lower cooling loads if it is an air-conditioned building, or in more comfortable thermal conditions if the building is not air-conditioned.
The large-scale use of cool materials in an urban area leads also to indirect energy savings due to the increased solar reflectance that contributes to the reduction of the air temperature because of surface heat balance at the urban level. The indirect benefits arise from this ambient cooling of a city or neighborhood that will in turn decrease the need for air-conditioning
Cool roofs typically yielded measured summertime daily air-conditioning savings and peak demand reductions of 10 to 30%.
|Last Updated ( Tuesday, 02 February 2010 12:00 )|