(UPDATED) You know how walking across a blacktop parking lot on a sunny day can feel like the temperature just went up 20 degrees? It’s not your imagination – surface material has a lot to do with temperature. Move from the blacktop to the grass and you can instantly feel the difference.
It’s exactly the same with roofs. Dark roofs absorb light — up to 90% of solar energy — and convert it to heat, emitting it back into the atmosphere. White ones absorb much less light by reflecting the light without transferring it into heat.
Green roofs, on the other hand, absorb sunlight and even put it to good use.
Here’s the kind of temperatures you can expect your roof to reach this summer depending on its surface:
|Roof Surface||Degrees Fahrenheit on a sunny, 90-degree day|
|Aluminum reflective coated||Up to 68 degrees cooler than black roofs|
|Unpainted metal||138-145, but feels hotter to the touch than a black roof|
|Painted Metal (ex: BASF Cool Paint)||105-115 or up to 42% cooler than unpainted metal|
The rooftop temperature results from two different properties. The ability of the material to reflect the sun’s energy back into the atmosphere, called solar reflectance, and the amount of heat the material rejects instead of absorbs, called thermal emittance. Both are measured on a scale from 0 to 1 with higher scores translating into cooler roofs.
Differences between rooftops
Clearly, black roofs get the hottest. While most are designed to last 20 years or longer, black roofs can suffer because heat speeds up the breakdown of materials — in fact, heat wear and tear is the main issue from a longevity standpoint. This is especially pronounced in aging roofs.
Metal roofs also get hot. That’s why Sentry Roof always tries to use metal roofing that’s been coated with BASF paint as part of the manufacturing process — it helps cut down on the surface heat. We can paint the roof after it’s installed but installing pre-painted is best.
White roofs can be 50 or more degrees cooler than black roofs and almost 30 degrees cooler than gravel roofs. Coated roofs, either aluminum or elastomeric, are also cooler than black roofs — technology also makes it possible for you to choose designer Pantone colors and still reduce your roof temperatures significantly. And vegetative roofs…well, nothing beats a living, breathing cooling machine for keeping your rooftop at a pleasant temperature.
Why vegetative roofs are so cool
The temperature of a vegetative roof at its surface is actually cool to the touch on most days — it’s in total shade, with a conventional roof surface protected under a dense canopy of vegetation. The cooling qualities go beyond the individual rooftop, as vegetative roofs also reduce the heat that’s reflected back into the atmosphere, helping to cut down on a city’s heat island effect.
Effects on the temperature inside your building
Does a hotter roof make for a hotter interior? Absolutely. Heat can be absorbed by your roof surface and radiated inside, especially in buildings with older roofs that were not assembled using R20 and R30 insulation. Newer insulations do a better job than even just a few years ago. And if you also add a nice white or silver coating, you can reduce heat radiation enough to size your AC requirements differently. Which in turn reduces your energy costs and the demands on the power grid that contribute to energy pricing from the utility company.
The Green Building Alliance reports that cool roofing can result in energy savings ranging from 7%-15% of your total cooling costs.
Keep in mind that to get the greatest benefit from your painted or coated roof, you need to keep it as clean as possible and repaint every 10-15 years.
Should you change your roof to keep it cooler?
How hot your roof really gets in the summer is a variable you have some control over. It’s definitely something to consider when re-roofing and in a new construction project, and you might be able to take steps to cool things down on the roof you already have. Plus, new coating technologies are being developed all the time. A recent study in Australia showed promising results from coatings that can keep the roof even cooler than the surrounding air.
Here’s a calculator from the Department of Energy that helps you get an idea of the costs savings you could get from a cooler roof. The Cool Roof Rating Council at coolroof.org has technical information and product ratings (membership is required to access the ratings). Our team at Sentry can advise you on your best options for your particular roof that will also align with your long-range goals for your building — give us a call.
Here’s an infographic of surface temperatures — feel free to share it. And check out our other infographics here (scroll down).