One of the oldest, simplest, and least expensive ways to reduce the heat absorbed by a sun-exposed roof or other exterior surface is, as Department of Energy Secretary Steven Chu recently put it, to “make it white.”

As noted in关于白色屋顶的故事in Thursday’s纽约时报, Chu made that observation during a July 21 appearance on Comedy Central’s每日表演.

True, most of Chu’s chat with the show’s host, Jon Stewart, focused on the merits of the cap-and-trade provisions of the美国清洁能源和安全行为. But near the end of the conversation, Chu briefly discussed the energy-saving potential of white roofs (and white roads). And the topic seemed to strike a chord with folks who heard Chu’s remarks: almost all of the viewers who posted comments on每日表演Web page featuring the Chu segment focused on the benefits of white roofs and roads.

Chu’s comments were timely. A draft presentation on the subject was released on Wednesday by the Lawrence Berkeley National Laboratory, in a document titled“酷屋顶Q＆A。”

A primer on roof color

Q＆A提供了屋顶材料的消费者信息和技术描述，以及它们的颜色影响其“辐射”属性 - 它们反射太阳辐射的能力（以0至1的等级）并发出热辐射（按比例0到1） - 暴露在阳光下。The coolness rating of a material, known as its solar reflective index, is often compared to that of a reference black roof, which is assigned an SRI of 0, and that of a reference white roof, whose SRI is 100. That doesn’t rule out the possibility, however, that some materials may earn index values above or below the references.

The Q&A cites as an example a roof with a clean, smooth bright white surface that reflects about 85% of incident sunlight (giving it a reflective, or “R,” value of 0.85) and emits thermal radiation with 90% efficiency (giving it an emittance, or “E,” value of 0.90). This surface, the document explains, has an SRI of 107 and will be only 9°F warmer than the outside air on a typical summer afternoon. Meanwhile, the surface of a standard gray roof that has the same emittance value but reflects only about 20% of incident sunlight (R = 0.20, E = 0.90) would have an SRI of just 19 and a surface temperature elevation (surface temperature minus outside air temperature) of 69°F.

Rosenfeld’s big picture

该文件确实注意到，在三年期间暴露于天气通常不会改变热量发射率，但确实将低倾斜的明亮白色屋顶的反射率降低至约65％，将其SRI降至79。在类似的暴露后，the reflectance and thermal emittance of a standard gray roof remain unchanged – but so does the roof’s relatively poor SRI.

California Energy Commission member Arthur Rosenfeld, who, the时代笔记，是朱议员的英雄之一，以及酷屋顶的长期倡导者，最近写了一个意见片the赫芬顿邮报这引用了他最喜欢的凉爽屋顶效率的插图之一。

“From a global perspective,” Rosenfeld wrote, “replacing dark roofs with cool ones would be equivalent to taking half the world’s cars – 300 million vehicles – off the road for 20 years, or reducing 24 billion tons of CO2 emissions for the same period.”