The two primary thermal properties that characterize roofs are solar reflectance and thermal emittance. Surfaces with low solar reflectance, absorb a high fraction of the incoming solar energy. A fraction of this absorbed energy is conducted into ground and buildings, a fraction is convected to the ambient air, and a fraction (termed emissivity) is radiated back to the sky. For equivalent conditions, the lower the emissivity of a surface, the higher will be its steady-state temperature. Surfaces with low emissivity cannot effectively radiate to the sky and, therefore, get hot. (Cool roofs for cool Delhi: Design Manual – BEE).

Thermal Emittance or Emissivity of a material is the ratio of energy radiated by a particular material to energy radiated by a black body at the same temperature. It is a measure of a material’s ability to radiate the absorbed energy. A true black body would have an e = 1 while any real object would have e <1. Emissivity is a dimensionless quantity (does not have units). In general, the duller and blacker a material is, the closer it’s emissivity is to 1. The more reflective a material is, the lower its emissivity. The emissivity of building material, unlike reflectance, is usually measured in the far infrared part of the spectrum. (ECBC Envelope for Warm & Humid Climate)

Solar reflectance is the fraction of solar radiation reflected by a roof. The complement of reflectance is absorptance; whatever radiant energy incident on a surface that is not reflected is absorbed in the roof. The reflectance and absorptance of building materials are usually measured across the solar spectrum, since these are exposed to that range of wavelength. Reflectance is measured on a scale of 0 to 1, with 0 being a perfect absorber and 1 being a perfect reflector. Absorptance is also rated from 0 to 1, and can be calculated from the relation: Reflectance + Absorptance = 1. (ECBC Envelope for Warm & Humid Climate)

The Solar Reflectance Index (SRI) is a measure of the solar reflectance and emissivity of materials that can be used as an indicator of how hot they are likely to become when solar radiation is incident on their surface. The lower the SRI, the hotter a material is likely to become in the sunshine. (www.designingbuildings.co.uk)

Cool roofs can be selected from a wide variety of materials and colors, and can be advantageously applied to almost any building or roof type, and in most locations. Conventional materials for standard roofing are now available with their cool roof counterparts. The table below lists the SRI for common roofing materials. White roofs are characterized by a high SRI. (Cool roofs for cool Delhi: Design Manual – BEE).

Climate is an important consideration when deciding a cool roof installation. Cool roofs achieve the greatest cooling savings in hot climates, but can increase energy costs in colder climates due to reduced beneficial wintertime heat gains. Roof coatings should include special chemicals that prevent mold or algae growth for a few years. In warm, moist locations, cool roof surfaces can be more susceptible to algae or mold growth than hot roofs. Proper design techniques should be used to avoid condensation, especially in cold climates. In cold climates, roofs can accumulate moisture through condensation, and it is possible that cool roofs might be more susceptible to accumulating moisture than dark roofs of the same design. Decision on cool roofs should be taken by keeping in mind both installation costs (material and labour) and ongoing maintenance costs (repair, recoating, and cleaning). In most cases, however, they are considered an inexpensive energy efficiency measure in buildings. (www.nzeb.in)

• Recommendations on cool roof materials includes use of well-graded broken pieces of glossy glazed tiles (broken china mosaic), modified bitumen with plastic and a layer of reinforced material, RCC roof topped with elastomeric cool roof coating, or simply finished with broken white glazed tiles. (www.nzeb.in)

• Specialized white elastomeric coatings are available for low-sloped products and cool color polymer coatings are available for tiles – these can be sprayed on existing roofs. The white pigment in coatings is typically titanium dioxide. Once applied, the coating is what determines the reflective properties of the roofing product. (Cool roofs for cool Delhi: Design Manual – BEE).

• Slate and tile products are available with solar-reflective surfaces that offer a wide range of cool colors. Additionally, the dense, earthen composition of slate and tile products provides increased thermal mass, yielding additional energy savings not realized through solar reflectance and thermal emittance measures alone. Concrete and clay tiles may be obtained in white, increasing the solar reflectance to about 70 percent (compared to the 20-30 percent range for red tile). (Cool roofs for cool Delhi: Design Manual – BEE)

• Broken China Mosaic Terracing. Well-graded broken pieces of glossy glazed tiles provide an inexpensive and conducive cool roofing option. Broken pieces of glazed tiles (preferably white) are embedded in wet mortar to provide a smooth surface that does not undulate. The joints are then grouted using cement mortar with waterproofing material. (Cool roofs for cool Delhi: Design Manual – BEE)

• The metal roof systems are available in white, which raises their solar reflectance to about 65 percent. Unpainted metal should be covered with a white coating to increase its emittance. (Cool roofs for cool Delhi: Design Manual – BEE)

• The roof surface should be a light color with the following characteristics: Solar Reflectance (SR) of > 0.7; Thermal Emittance (TE) of > 0.75; and Solar Reflectance Index (SRI) of 50% or more. (www.2030palette.org)

All roofs that are not covered by solar photovoltaics, or solar hot water, or any other renewable energy system, or utilities and services that render it unsuitable for the purpose, shall be either cool roofs or vegetated roofs. For qualifying as a cool roof, roofs with slopes less than 20° shall have an initial solar reflectance of no less than 0.60 and an initial emittance no less than 0.90. Solar reflectance shall be determined in accordance with ASTM E903-96 and emittance shall be determined in accordance with ASTM E408-71 (RA 1996).

Cool roofs are very effective in reducing the summer electricity use with minimal impact on winter heating. In general, savings in annual net utility costs can be expected for most buildings. Research has shown that cool roofs also help increase the life expectancy of roofing systems because extreme cycles of heating and cooling tend to wear out materials as they expand and contract with the temperature. Cool roofs on the other hand, keep the roof a more constant temperature and therefore, tend to last longer. While reducing ambient air temperatures, cool roofs also improve air quality by curtailing or eliminating smog formation. Smog is created by photochemical reactions of air pollutants – higher temperatures provide impetus to these reactions. Improved air quality also results in a reduction in heat-related and smog-related health issues, including heat stroke and asthma. (Cool roofs for cool Delhi: Design Manual – BEE)

Studies show that in warm and hot climates cool roofs perform well in meeting energy efficiency, cost-effectiveness, and sustainability. Cool Roofs reduce annual air conditioning energy use of a single-story building by up to 15%. They cool interior spaces in buildings that do not have air conditioning, making occupants more comfortable. Cool Roofs reduce carbon emissions by lowering the need for fossil-fuel generated electricity to run air conditioners and they potentially slow global warming by cooling the atmosphere. (www.nzeb.in)

Results of the study “Calibrated simulation for estimating energy savings by the use of cool roof in five Indian climatic zones” show that the maximum reduction in energy consumption is observed in Mumbai (warm and humid climate), followed by Ahmedabad (hot and dry climate), Bangalore (temperate climate), and Delhi (composite climate). Least energy saving is observed in Shillong (cold climate). In all the climates except cold, the financial payback period is acceptable, being less than three years, whereas for cold climate, the payback period is over six years. (www.coolrooftoolkit.org).

Office Buildings in Hyderabad

The complex houses two near-identical buildings – this facilitated the study through ensuring identical parametric values for floor area, number of floors, roofing material and system, occupancy and schedules, and cooling systems. These two buildings are is a two-stories with a roof area of 700m. The roof of one building was painted black, and the roof of the other was painted with a white reflective cool roof coating. Weather towers, temperature sensors, current transducers, and data-loggers continuously monitored the weather, energy-use, and temperature data for the two buildings. The data points monitored weather conditions (outdoor temperature, relative humidity), building temperatures (surface temperature, heat flux through the roof, roof underside temperature, indoor air temperature), and energy use (whole building electricity use, cooling energy use). The average summertime daily roof surface temperature was reduced by 20 degrees celsius. Cooling energy savings due to cool roofing (from gray concrete to white roof coating) can vary largely, for example, ranging from approximately 15% to 20% during hot summer days. (Cool roofs for cool Delhi: Design Manual – BEE)

Al-Obaidi, K.M., et al., Passive cooling techniques through reflective and radiative roofs in tropical houses in Southeast Asia: A literature review. Frontiers of Architectural Research 2014, http://dx.doi.org/10.1016/j.foar.2014.06.002

BEE, “Cool roofs for cool Delhi”: Design Manual, 2011. http://shaktifoundation.in/wp-content/uploads/2014/02/cool-roofs%20manual.pdf

ECBC, 2017

ECBC Envelope for Warm & Humid Climate http://high-performancebuildings.org/pdf/ECM2/ECM2_Technical_Information_Warm-Humid.pdf

Levinson R, Akbari H. 2010. Potential benefits of cool roofs on commercial buildings: conserving energy, saving money, and reducing emission of greenhouse gases and air pollutants. Energy Efficiency. 3:53-109

http://coolroofs.org/

http://www.coolrooftoolkit.org

https://heatisland.lbl.gov/coolscience/cool-roofs

Accessible Ventilated Roof – Cold Roof by Comfortable Low Energy Architecture (CLEAR)

Indian Cool Roof Calculator: http://coolroof.cbs.iiit.ac.in

US Cool Roof Calculator: http://web.ornl.gov/sci/buildings/tools/cool-roof