Building a better roof
An old Japanese essay says, “Buildings should be built for the summer.”
By Kikuko Tagawa
This year we are having an especially hot summer in Japan. An old Japanese essay says, “Buildings should be built for the summer.” Reducing the summer heat in buildings has been a perennial issue, given Japan’s hot weather. Both individuals and organizations are taking environmental issues into consideration, trying to lower energy costs and reduce carbon dioxide (CO2) emissions in everyday life. The Japanese government has amended its energy conservation laws to strengthen energy conservation measures in residential and industrial sectors and to promote company-wide energy management systems in commercial installations.
At the annual Product Showcase held by ATA Japan in January 2010, two energy-saving thermal insulation products were introduced to visitors by representatives of the manufacturing/supplying companies. Made with net or woven screen fabric, both products are used on top of a folded metal plate-roofed building, and can significantly lower energy costs.
Material and mechanism
The Cool Roof Net was co-developed by the Itochu Corp., Toray Industries and Nakada Industrial Co. Ltd., a company specializing in net products. By installing flame-retardant polyester net horizontally on the plated roof and reducing heat conduction from sunlight, it prevents the inside temperature of the building from becoming too hot, saving cooling energy. The net has solved the challenge of combining high radiation shielding performance and high permeability/breathability. Since it is 100 percent polyester, it can also be recycled.
By reducing CO2c emissions, use of the Cool Roof Net helps prevent global warming and reduces the heat island phenomena in urban areas. The product won the Small Business Administration Secretary Award of Energy Conservation Grand Prize in 2009, and has been introduced on television and many other media.
According to Akinori Sato, Nakada Industrial Co. Ltd., “From the experiment in Kagoshima, it reduced 78.7 megajoules (MJ) per one square meter per year, 19.7 percent of the energy costs. (By AE-Sim/Heat, a thermal environment simulation program authorized by the Minister of Land, Infrastructure, Transport and Tourism.) Also it is calculated that 4.5kg/m2 of CO2 emission is reduced, the equivalent of 2 liters of oil per square meter. This means the annual CO2 emission reduction for a 2,000 square meter building is 9.0 tons, with oil savings of 4.0 kilograms.”
Roof Shade was the second product introduced, this one co-developed by the Ishikawa Tent Co. Ltd. and Nihon Widecloth Co. Ltd. Ishikawa Tent, a tent and awning manufacturer, developed the method to insulate a metal roof utilizing a tent/awning system. Roof Shade is also installed on a plated roof to reduce heat conduction from sunlight, to keep inside temperatures cooler and offer energy savings. It’s made of special metal evaporating treated (MASA treatment or sputtering treatment) woven high-strength polyethylene monofilament screen.
MASA treatment uses a high-density plasma arc to cause ionized gas to collide with the target material. With the energy bombardment, atoms are ejected from the target material and deposit a thin film on the substrate: approximately 30 nanometers stainless SUS310 (nickel/chrome, iron).
According to Hitoshi Yamamoto, Nihon Wide Cloth Co. Ltd., “This screen material is improved from the screen used in mariculture, nurturing seabream and flatfish in the sea. It has a naturally high tensile/tear strength compared to the material used in tents and awnings. By using this special MASA treatment, it enhances the heat insulation through the infrared-absorbing performance of the stainless film, as well as through its strength and durability.”
Potential energy savings
Researchers at Hoshino Shoten Co. Ltd. took the data of a facility before and after the installation of Roof Shade and compared two days of similar weather and temperature conditions. It was found that by installing Roof Shade, the roof temperature was lowered by 20.9 degrees (C), and room temperature by 5.1 degrees (C), as shown in Figure 1.
They also did an experiment with a building of 200 square meters in Ehime prefecture, and found electricity savings of approximately 23 percent.
A simulation calculation of installing Roof Shade in a building of 400 square meters floor space, 5 meters in height (one open space building in Kagoshima) showed that the Roof Shade reduced the cooling and heating load for air conditioning by 36.2 percent a year, and reduced CO2 gas emissions by about 5.7 tons, which equals roughly 4.2 kiloliters (kl) of crude oil.
Using the same simulation for the same size building in Tokyo, the Roof Shade was shown to reduce the cooling and heating load for air conditioning by 17.7 percent a year, and reduce CO2 gas emissions by about 1.8 tons, the equivalent of about 1.4 kl of crude oil.
By using the fabric screen shade on the roof, it restrained the rise in roof temperature. The thin stainless film made by the MASA treatment on the screen absorbs infrared rays and lowers heat conduction. Air between the screen and the roof reduces heat conduction, and is drawn off by the wind.
A growing need
Toshihisa Kato, executive director of Hoshino Shoten Co. Ltd., commented: “There have been needs for shielding the heat of sunlight to the roof and saving energy costs in buildings, especially by owners of those folded metal plate-roofed buildings, including office buildings, convenience stores, drug stores, food plants, medical and/or livestock industries and cold storage facilities who want to save on air-conditioning costs, or to manage the inside temperature, or to help address complaints from workers in a hot working environment—as well as those who are keen on preserving the environment and reducing CO2 emissions.”
There are numerous ways to insulate a roof from heat. One method gaining popularity is the green roof—having plants growing on the roof, often using geotextiles to contain the soil and water needed. “It is a nice method,” said Toshio Katayama, vice president of Daiichi Hanpu Co. Ltd. in Osaka, “but many clients tell us that not only the initial cost but also the maintenance costs are considerably higher. It requires a lot of watering and care not to let the plants wither under the strong summer sunlight. The Roof Shade is less expensive in itself, and needs almost no maintenance.”
Thermal shielding painting is another method of roof insulation. “There are various thermal paints, qualitywise and pricewise,” said Katayama. Comparisons show, however, that the effect of painting deteriorates through chemical reaction and from the dirt accumulating on the paint over the years—but both Cool Roof and Roof Shade sustain their cooling effects.
“One of our customers told me the sound of rain was very noisy, but it became very soft after we installed the net. That was a happy byproduct of it,” said Sato. Sato and Kato noted that Cool Roof Net and Roof Shade reduce raindrop noise and protect the roof, and are both easily installed.
“As it is made 30 cm in width, it is flexible enough to accommodate television anntenas, outdoor equipment from air conditioners and walkways for installers, and is very easy to install, ”said Kato.
Both products were wind tested; both can withstand more than 40 m/s, a wind speed equivalent to that of a strong typhoon. Takashi Okada, Hoshino Shoten Co. Ltd., added, “Typhoon No. 18 last year caused serious damage to the local area, but the Roof Shade stayed safe.”
Sato commented, “I was invited to make a presentation about Cool Roof Net at a workshop held by the Ministry of International Trade and Industry near the end of last year. They said that in the U.S. there is a plan to conduct a large-scale study on the energy savings and heat insulation of building roofs. I was impressed, and was confident that our efforts and study and our product are working in the right direction, and will contribute to the sustainability of the earth.”
It was interesting to have presented two products developed separately but intended to produce the same energy-saving effects for building roofs. Both products offer a cost-effective way to reduce air conditioning costs, and to help preserve the environment. Both have come onto the market recently and are attracting a lot of market attention … as well as a lot of interest and questions from attendees at ATA Japan’s 2010 Product Showcase.
