What makes a cool roof “cool”?
A whole range of design elements fall under the umbrella term of “cool roofs”. Essentially, a cool roof is one that reflects the suns heat and emits absorbed radiation back into the atmosphere at a higher rate than standard materials, literally staying cooler than a standard roof. This reduces the amount of heat transferred into the building below, keeping it at a more constant temperature.
Cool roofs are one element of passive design which can help keep a building and its occupants comfortable by lessening temperature fluctuations. Incorporating a cool roof into a design in the appropriate climate can make a building “greener” by reducing cooling energy costs, minimise the use or need for installation of air conditioning systems and subsequently reduce utility bills. Cool roofs can also mitigate a community’s Urban Heat Island Effect.
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UNDERSTANDING COOL ROOFSA cool roof can be installed on any building. Some elements are built into materials before construction, and some are applied after, allowing a standard roof to be post-fitted with cool roof technology. Without a cool roof, on a 35°C day, the roof space can become superheated up to 90°C and the temperature of the rooms below can become unbearable, even with insulation. These temperature extremes can have an impact on the durability and the lifespan of the whole roof structure.
There are a few terms which come into play when determining the “coolness” of a roof. The ability of a material to reflect the solar radiation of light, infrared and UV (Solar Reflectance) and the measure of the material’s ability to absorb and re-radiate heat into the atmosphere (Thermal Emittance) is combined into what is known as the Solar Reflectance Index (SRI). SRI is a measure of the constructed surface’s ability to reflect solar heat. The higher the SRI value, the lower its surface temperature and consequently, the heat gain into the building.