For the Mindemoya Old School we chose a light grey steel from local building product manufacturer TerraStar in Manitowaning. It has a Solar Reflectance Index (SRI) of 54. So what exactly makes it "cool"? Read on!
First, the roof is a light colour. The cooling and warming effects of colour have been understood for millennia. We wear white clothing on hot days because white is effective at reflecting the energy of the sun, keeping us cooler. Dark clothing, on the other hand, tends to absorb the heat of the sun. The heat absorbed by the fabric transmits to our skin, and we get hotter. This works for any kind of surface. What's less known is where the sun's energy goes. Fresh white snow can reflect up to 90% of the sun's energy back into space. Dark oceans, on the other hand, absorb the sun's energy. Colour, materials and texture make a difference in whether our roofs make houses hotter in summer or help to keep them cool. The rooftops of every building are exposed to sunlight, and some roofing materials will reflect much more of the energy back into space than others.
Roofing materials are rated according to a Solar Reflectance Index (SRI), so that they can be compared. The SRI is actually made up of two measurements -- how much sunlight a surface reflects, and how much energy the surface absorbs and emits into the air or objects around it. The SRI number ranges from 0 to 100, where 100 is "high reflectance, high emissivity". In general, white and pale colours are high on the scale -- they are "cool". Blacks and deep colours are generally in the low SRI range.
The Cool Roof Rating Council rates thousands of roofing and siding products. As you might expect, bright whites are the highest rated metal roofing products. Asphalt roofing tiles generally do not reflect much sunlight and feel hotter to touch on a sunny day. Cool roofs make financial sense -- they save on air conditioning costs. The U.S. government began promoting cool roofs more than a decade ago as a way to save energy and money for air conditioning, and to lower peak demand on the electricity grid, especially in the southern states. Reducing summer demand for air conditioning also reduces pollution from electricity generation, crucially greenhouse gas emissions like carbon dioxide.
Just a few decades ago, summers on were cool enough that very few buildings had air conditioning, and on average there were only 1.8 days of 30 degrees C. or greater. With global warming, more people are turning to air conditioning to keep their homes and businesses comfortable. Global warming affects northern climates more than latitudes closer to the equator, and temperature records are being broken across Canada year after year. Climate projections based on "business as usual" predict 25 days per year of 30 degrees C in the years 2051 to 2080 on Manitoulin.
Manitoulin Island is in Climate Zone 6. At 45+ degrees latitude, Manitoulin is considered a northern climate, but cool roofing can be beneficial in northern climates like ours, according to the International Institute for Building Enclosure Consultants. What is helpful now will be more helpful in the future.
Cool roofs are not just good for reducing air conditioning, they can also help combat climate breakdown. As the Earth warms, glaciers and sea ice melt, so less sunlight is reflected and more is absorbed in oceans and land, and the world temperature rises. It's a feedback loop that at some point can become unstoppable -- to the point where glaciers and permanent ice disappear. Most homes have dark, asphalt roofs that absorb heat from sunlight, but if they were all replaced with cool roofs, the effect could be remarkable. According to the Cool Roof Rating Council, studies have estimated the cooling effect of installing solar-reflective buildings world-wide at as much as 45 - 57 billion tonnes of greenhouse gas emissions per year. Just 1000 sq ft -- the roof of a small home -- could offset as much as 10 tonnes.
And that's cool!
Jan McQuay
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