A House is not a Home without a “Cool Roof”
As we are looking to gain every advantage we can in building our retirement home, naturally we would look to make our roof as energy efficient as possible.
Also known as albedo, solar reflectance is expressed either as a decimal fraction or a percentage. A value of 0 indicates that the surface absorbs all solar radiation, and a value of 1 represents total reflectivity. Thermal emittance is also expressed either as a decimal fraction between 0 and 1, or a percentage. Another method of evaluating coolness is the solar reflectance index (SRI), which incorporates both solar reflectance and emittance in a single value. SRI quantifies how hot a surface would get relative to standard black and standard white surfaces. It is defined such that a standard black (reflectance 0.05, emittance 0.90) is 0 and a standard white (reflectance 0.80, emittance 0.90) is 100. The use of SRI as a combined measurement of reflectance has been disputed, since it has been shown that two different products with identical SRI numbers can yield significantly different energy savings results depending on what geographic region they are applied in, and the climatic conditions present in this region.
Benefits of cool roofs
Most of the roofs in the world (including over 90% of the roofs in the United States) are dark-colored. In the heat of the full sun, the surface of a black roof can increase in temperature as much as 50 °C (90 °F), reaching temperatures of 70 to 90 °C (150-190 °F). This heat increase can contribute to:
* Increased cooling energy use and higher utility bills;
* Higher peak electricity demand (the maximum energy load, in megawatts, an electric utility experiences to supply customers instantaneously, generally experienced in summer late afternoons as businesses and residences turn up their air conditioners), raised electricity production costs, and a potentially overburdened power grid;
* Reduced indoor comfort;
* Increased air pollution due to the intensification of the “heat island effect”; and
* Accelerated deterioration of roofing materials, increased roof maintenance costs, and high levels of roofing waste sent to landfills.
Any building with a dark colored roof, but particularly large buildings, will consume more energy for air conditioning than a “cooler” building – a strain on both operating costs and the electric power grid. Cool roofs offer both immediate and long-term savings in building energy costs. White reflective membranes, metal roofing with “cool roof” pigments, coated roofs and planted or green roofs can:
* Reduce building heat-gain, as a white or reflective roof typically increases only 5–14 °C (10–25 °F) above ambient temperature during the day.
* Create 15–30% savings on summertime air conditioning expenditures.
* Enhance the life expectancy of both the roof membrane and the building’s cooling equipment.
* Improve thermal efficiency of the roof insulation; this is because as temperature increases, the thermal conductivity of the roof’s insulation also increases.
* Reduce the demand for electric power by as much as 10 percent on hot days.
* Reduce resulting air pollution and greenhouse gas emissions.
* Provide energy savings, even in northern climates on sunny (not necessarily “hot”) days.
Note that today’s “cool roof” pigments allow metal roofing products to be EnergyStar rated in dark colors, even black. They aren’t as reflective as whites or light colors, but can still save energy over other paints.
Bob over at goILOILO.com has a excellent write up / pictures on the methods he used on his house to keep the roof cool have a read: http://goiloilo.com/cool-roof-philippines-metal-roofs-and-cool-buildings-in-the-tropics/. He also has a wide variety of topics regarding the building of his home, a full read of his blog would be helpful to anyone looking to build their home in the Philippines.
*Photo of home on front page is attributed to Bob at goILOILO.com