The surface of a dark roof can reach 180° on a sunny day. A roof that hot does a lot to the temperature inside the structure, a situation that would be welcome in Minnesota in January but is more likely to occur in Texas in August. The heat is absorbed into the structure where, even with its tendency to rise, will still elevate the inside temperature increasing the need for and cost of air conditioning.
That superheated roof will also send some of the heat back out into the surrounding air, creating what are called “heat islands” in densely populated areas. While absorbing the suns heat, a dark surface also absorbs its rays, lowering the earth’s overall solar reflectance (called albedo) and possibly promoting climate change.
For those reasons there is a growing movement toward white roofs. As mentioned earlier, Energy Secretary Steven Chou, who is also a winner of the Nobel Prize in Physics, has lately brought a lot of attention to white roofs with his frequent speeches on the topic.
For reflective purposes the best surface is white but also smooth, something that traditional roofing materials are not. According to tests run by Lawrence Berkeley National Laboratory, Oak Ridge National Laboratory, and the Florida Solar Energy Center, black or dark shingles or roofing materials reflect between 5 and 9 percent of the sun’s rays and have roofs that reach temperatures 76° to 90° above the air temperature. A generic white shingle roof (asphalt) will have solar reflectance of 25 percent with a 70° temperature difference and high grade white tiles or shingles (white clay, concrete, or fiber cement)-- score at 35 percent and 60°.
White shingles score so low because the generic types, while they appear white, are actually grey over a black substrate. The premium roofs use a whiter white granule.
From an aesthetic standpoint, most persons who have pitched roofs will want shingles for a reroofing project and white ones are not yet easily available. For a flat room or one with a low pitch there are other alternatives. A bright white ceramic and elastomeric coating on a smooth surface will provide 80 percent reflectance and a temperature rise of only 15°. On a rough surface (such as an existing asphalt tile or shingle roof) the reflectance drops to 60 percent with a temperature differential of 36°. If a rubber roof is an option, white membrane is 70 to 80 percent reflective with 15° to 25° temperature increases.
The ceramic or elastomeric coating, which may be retrofitted over an existing roof, will degrade over time, dropping to as little as 50 percent in reflectiveness as they wear and get dirty.
Other options are white metal roofs (metal is a common roofing material in the South, even on pitched roofs) with 60 to 70 percent reflectance and bright aluminum. The latter is in the middle range of reflectance and temperature difference but has the disadvantage of retaining heat through the night.
Modern roofing materials are designed to last for 25 to 35 years so it is impossible to imagine that white roofs will become a significant factor in energy conservation or climate change any time soon. But should homeowners become accustomed to the idea, there is no real downside. Converting to white when replacing a roof will cost about 15 percent more than using a dark roof. This is a differential which will be made up in lower energy costs and the greater durability of a light colored roof which does not expand and contract as much as a dark roof and thus lasts longer.
If the idea of a dazzling white roof is off-putting to some people, there is a scale of economies to using other light colors. Scientists at Lawrence Berkley have tested the effectiveness of alternative shades and at least one manufacturer is using this data to create reflexive roofing materials in traditional colors like sienna or cream.