Reflective roofs are atractive as a tool to deal with the climate change, with the increasing temperatures the world is abide to brace. They have a high level of solar reflectance, thus presenting a low level of solar energy absorption. Reflective roofs today are built by covering the roof with a highly reflective seamless coat.
The thermal properties of the reflective roofs are equivalent to those of the green roofs. As the data provided by the U.S. Department of Energy Building Technologies Program shows for the reflective roofs, there is a broad thermal equivalence between green roofs and reflective roofs.
The leaves reflect 20 to 30% of the visible radiation received from the sun. But most of the radiation they absorb is used to their metabolic work, bringing water from the soil and doing their photosynthesis, by which they absorb carbon to grow. This way they do not allow the soil temperature to increase.
The reflective roofs presents a much higher level of reflectance, absorbing the energy that it does not reflect. Both types of roof decrease the roof temperature by broadly equal level thus protecting the user of the rooms immediately under the roof from high temperatures or from high electricity bills, a benefit appropriated by users. Both protect from high themperature variations the part of the building structure that is most sensitive to temperature variations. This represent a benefit appropriated by owners. These first two benefits, the reduction of the temperature for the floor under the roof and the protection of the structure, are private benefits.
Having treated the aspects under which the reflective roofs and the green roofs are equivalente, let us go to aspects under which they work producing very divergent effects. If someone in the second floor of a two floor building take a look through the window may be exposed to be uncomfortably hit by the glare of the one level building neighbor roof solar reflection. Instead of this, a green roof would provide a nice view. The green roof would provide some sense of tranquility. The reflective roof provide some contribution to take someone more far way from his inner equilibrium. A world where a passer-by is circundated by reflective roofs is an arid world. This is a negative public good effect.
A comparison of effects should not miss another unfriendly aspect of the reflective roofs. By reflecting solar energy to the neighboring buildings fachades, it may increase their temperatures. This is another negative public good effect.
Another difference incides on the relation to the flash floods. Green roofs retard the time a strong precipitation becames a flash flood. It is a service they provid to the county community. Reflective roofs do not do so. More yet than ordinary roofs, they allow the water run promptly to the streets. This is other kind of negative public good.
By least, think about being prudent. Small airplanes do not approch landing by navigation instruments. Nowadays even for the smallest of them, the pilots are guided by GPSs. Now suppose the GPSs are out of order, because a high solar radiation episode. Or, what is more probable to happen, an airplane GPS becames out of order. The pilot needs to find the airport by focusing the green and white airport lighthouse. But how can the pilot find the green and white airport lighthouse if the roofs are reflecting the powerfull sun rays in his eyes? Who would like to be in such airplane. Who would like to be in a city under it?
One may conclude that in matters of private benefit, green roofs and reflective roofs are equivalent as a tool for adaptation to global warming. But in matters of public goods effects green roofs are very superior.
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