随着2019年创下的新高温记录，冷屋顶已成为降低建筑物温度和空调成本的一种经济有效的解决方案。其原理简单明了：日光充足时，白色屋顶涂料可以反射太阳光。许多建筑屋顶都可以改装为冷屋顶。
一说到被粉刷成白色房屋，就会让人联想到希腊基克拉迪群岛的惊艳之美。选择白石灰不是没有道理的，它不仅可以反射太阳光紫外线，而且还可以起到给房屋内部降温的效果。法国冷屋顶公司（Cool Roof France）的首席执行官安托万·霍雷洛（Antoine Horellou）说：“众所周知，浅颜色比深颜色吸热少”。他还补充道：“这其实不是什么新发明！冷屋顶的原理是：其涂层具有反射性，可以降低建筑屋顶的热量，从而变得比周围空气更凉爽。由此得名‘冷屋顶’。采用冷屋顶可减少最高达30％的空调能源费用。而对于那些原本无需空调的区域，冷屋顶也会使其内部变得更加舒适。”这一解决方案非常有效，以至于政府间气候变化专门委员会（IPCC）认为，它是“减缓全球变暖最快且最便宜的方法之一”。根据IPCC的计算，“将屋顶涂成白色每年可减少1 Gt的温室气体排放，相当于全球2.5亿辆汽车的排放量！”。随着2019年创下的新高温记录，是时候采取行动了。今年夏天，巴黎经历了前所未有的高温天气，7月25日当天创下了42.6°C（108.7°F）的高温记录。
美国：避免城市热岛（UHI）效应的白色屋顶先行者
在美国，这种冷屋顶的概念已经得到广泛应用。实际上，早在1998年，美国便成立了一个“冷屋顶评级委员会”。这个非营利组织的宗旨是，实施和传播一种用于评估屋顶表面辐射性能并提供研究支持的精确体系。纽约的白色屋顶总面积已经超过85万平方米（9,149,324平方英尺）。这得益于前市长迈克尔·布隆伯格（Michael Bloomberg）于2010年发起的一项运动，他曾投入巨资用于抗击气候变化。值得一提的是，纽约经常会因空调的过度使用而遭遇停电。因此，在使用白色屋顶后，会将建筑物的温度降低20°C（68°F），在夏季最热的时候，在标准屋顶表面上测得的峰值温度仅为80°C（176°F）——降低了一半！
五年多来，以法国为首的欧洲各地，纷纷利用公共建筑、购物中心、仓库等不同日照水平的建筑，在多个地区开展冷屋顶工程。巴黎市新的《空气质量、能源和气候计划》（“Plan Climat Air Énergie”），甚至试图“重新改造建筑物和屋顶以适应连续高温天气。”有关当局发现，巴黎的屋顶通常使用的是常规深色密封材料（锌、沥青），这些材料会积聚建筑物内部及周围的热量；因此，他们建议使用反射性涂料来粉刷屋顶，以减弱UHI（城市热岛）效应。
卓有成效的冷屋顶项目
有三位法国创业者，在仔细研究过纽约的例子后，对冷屋顶的有效性深信不疑，于是在2015年创立了法国冷屋顶公司（Cool Roof）。同年，这家位于布列塔尼（Brittany）的初创公司，在坎佩尔（Quimper）达成了第一笔交易：占地7,000平方米（75,347平方英尺）的Leclerc购物中心项目。“我们的客户想减少能耗，尤其是减少空调能耗，同时限制超市内的热度。所收集到的数据表明，这家购物中心的能耗降低了15％，相当于每年减少近4公吨的二氧化碳当量，每年可节省20,000欧元的能源费用。除此之外，冷屋顶还具有降低维护成本,以及提高屋顶寿命等优势。”从那以后，已有50多个客户登门拜访。其中包括亚眠的Boulanger购物中心、巴黎戴高乐机场的2G航站楼、圣乔治新城（Villeneuve-Saint-Georges）的巴黎消防局、Berlemont的体育馆以及同样位于巴黎的Adamant流动医院。
省去空调的反光屋顶
Cool roofing can also be used to avoid investing in costly air conditioning. Aximum, another company providing cool roof paints and a subsidiary of the Colas group, completed a project in Oise on an R&D site of Bostik, which manufactures glues and adhesives. As Jean-Baptiste Izart from Aximum explains, “this reflective coating applied to 1,000 m² (10,764 ft²) limits the impact of solar rays on the facilities and improves thermal comfort in the summer, since we are in a building without air conditioning. The first measurements indicated temperatures lower than 25°C (77°F) on average in August, with peaks at greater than 30°C (86°F). The following summer, after the application of the white paint, we had almost no days when the inside temperature exceeded 25°C!” the expert from Aximum shared with us.
Another example is the Louis Blanc school in the 10th arrondissement of Paris, where Cool Roof France installed a white roof. The measurements taken measured the average variation in inside temperature, comparing a “cool roof classroom” to its conventional neighbor. The result showed less than 6°C (42.8°F) of difference and the elimination of periods of discomfort in the summer (above 28°C [82.4°F]).
A white paint that reflects 95% of the sun’s rays
The quality of the white coating remains crucial. Cool Roof France thus relied on Arkema’s expertise in improving their application process: a water-based white paint, highly reflective, protected by a resin-based anti-UV and anti-mildew top layer using Kynar Aquatec® PVDF. This highly resistant fluoropolymer developed by Arkema in the United States has been widely used, in particular on buildings in hot states such as California, Arizona, and Nevada. The impermeable top coat made of Kynar Aquatec® considerably improves the effectiveness of the white roof and extends time between renovations. The result: 95% solar reflectance—whereas traditional paint reaches only 70%—for many years.
The sun’s infrared and ultraviolet rays end up gradually disintegrating a simple white paint. You then have to repaint the roof often, which quickly becomes costly. The Kynar Aquatec® top coat avoids this phenomenon; you get a coating with an effect similar to an oilcloth. Thus, the rain naturally cleans any deposits of pollution and soiling that ‘blacken’ the roof and diminish its reflectance".
Kynar® expert at Arkema for the building market
Cool Roof France confirms: “This formulation ensures a durability of at least 20 years. The recommendation is one cleaning per year, but for the Leclerc roof in Quimper, the first cleaning was not required until after three years!”
The cool roofing market
Cool roofs are becoming popular all over the world. “In France alone, flat commercial roofing makes up about 5 million m² (53,819,552 ft²) of roof surfaces,” estimates Cool Roof. “Most of them are more than 10 years old, poorly insulated, and full of micro-cracks. Their renovation with the use of cool roofs is increasingly recommended by project owners, architects, and engineering firms. It’s clearly a sign. We are also calling for the implementation of a large-scale cool roofing plan, to create a system for registration/certification of this technique in metropolitan France and the overseas territories.” Changes in regulations are paving the way. Just take the example of the national decree for the tertiary sector of July 23, 2019, requiring all businesses with surface areas greater than or equal to 1,000 m² (10,764 ft²) to reduce energy consumption by 40% by 2030 with respect to 2010.
Developing energy intelligence with cool roof technology
The Brest-based company is making use of its clients’ empirical data to educate prospective clients: “To prove that it works, we have instrumented cool roofs using heat sensors and air conditioning consumption control.” There is still more educating to be done. “We often go about doing things the wrong way. You’re at the beach and you’re hot. You’re not going to start by putting an air conditioner next to yourself. You’re going to start by moving into the shade! The same goes for a building. You have to start by addressing the source of heat, in this case solar radiation. The solution of air conditioning is nearly systematically selected since people think it has a bigger direct impact. It isn’t always appropriate. We help our clients perform energy assessments in their buildings and hope to show them that cool roof technology contributes significantly to improving them. We’re developing energy intelligence!”