ADAPTATION MEASURE 035
The effect on indoor temperatures from the radiant heat of roofs heated by sunlight is often neglected, but in fact, it causes wasted energy to maintain indoor temperatures. It also contributes to the heat island effects in the cities.
Proper roof surface construction will lower the temperature inside the building and thereby cool the city. Furthermore, it will encourage energy self-sufficiency.
There are many inexpensive ways to convert an already constructed building into a climate change-adapted building with only minor modifications. Among them, conversion of roof surfaces to control indoor temperatures is an important adaptation measure. For example, the transpiration action of water and the insulating effect of soil in rooftop greenery can be expected to lower indoor temperatures. Another approach is to incorporate cool roofs, which change the roof from a darker color to a whiter color. With a cool roof, the coating reflects sunlight and reduces heat transfer to the interior. The effect is further enhanced by the use of heat-insulating paint. Solar panels installed on the rooftops of houses, factories, and other buildings not only promote energy self-sufficiency, but also reduce indoor temperatures by blocking out the sunlight.
Rooftop greening is attracting attention as a way to mitigate the heat island effect and to coexist in harmony with the city and the ecosystem. Rooftop greenery is expected to enhance the beauty of the city and the wellbeing of its residents, but there is more to be expected. The transpiration of water and the soil’s heat-insulating layer keep the interior cool in summer by preventing outside heat from entering, and in winter, it keeps the warm temperature inside from escaping. Beekeeping and planting native plants on rooftops can be expected to connect ecosystems that tend to be fragmented by urban development and restore the surrounding nature. The soil’s ability to retain water will also help create a flood-resistant community where rainwater is absorbed.
With rainwater harvesting tanks attached, water can be irrigated to plants without wasting it, which will further improve water retention capacity of the city.
Cool Roof and High Insulation Paint
Looking at aerial photographs, one can see that in today’s cities, roofs are painted in a variety of colors according to the owner’s taste and preference. This is a very regrettable situation in terms of climate change adaptation. This is because some colors and materials absorb heat, just as black clothing absorbs heat from sunlight. Conversely, a measure called “cool roofs,” which can be expected to have a high energy-saving effect simply by painting rooftops in a color with a high degree of whiteness, is beginning to draw attention around the world.
In 2013, the International Energy Agency (IEA) announced that white roofs reduce CO2 emissions and curb global warming. According to the Lawrence Berkeley National Laboratory in California, dark roofs reflect only 10 to 20 percent of sunlight, while white roofs reflect 70 to 80 percent. To make cool roofs even more effective, insulating paints are effective. In Japan, there are outdoor insulating paints such as GAINA, which are based on space engineering technology used for rockets and other applications, and offer significantly higher thermal insulation properties than conventional paints. Simply applying a highly white insulating paint to a roof can be expected to provide significant thermal insulation.
Indoor cooling effect of solar panel installation
Needless to say, solar panels help local energy production for local consumption and provide a stable energy infrastructure in the case of a disaster, but there is a major hidden benefit: heat shielding. Solar panels create shading in the summer, preventing heat from penetrating indoors via solar radiation on the roof. An experiment by the PV Solar House Association found that when the outside temperature is 36.4°C, the temperature of the ceiling surface without panels is 56.5°C, while the ceiling surface with panels is 47.1°C. The ceiling without panels is about 20 degrees higher than the outside temperature, while the ceiling with panels is suppressed by about 10 degrees. On the contrary, in winter, the panels are expected to have the ability to reduce the escape of heat from the room through radiative cooling. Thus, solar panels are hoped to be implemented throughout the city not only for the sustainable use of energy, but also for their secondary effect of blocking solar radiation. A unique solar power installation model is becoming popular in Japan. In this model, an enterprise pays the initial cost of installing solar panels on a house, and in exchange for receiving lease payments or electricity fees, the enterprise transfers the panels to the homeowner for free after a certain period of time. By proactively incorporating such a system, rooftop solar panels will become more widely adopted.
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