White or Light Colored Cool Roofing Materials

This chapter is a state of the art report focusing on the aspects that concern cool materials for buildings that are white or light colored. It is structured in four sections; in the first section cool white or light colored materials i.e. materials characterized by high solar reflectance and high infrared emittance are defined. Typical values of the two properties are given according to independent studies, the U.S. Cool Roof Rating Council‘s database and the IEE Cool Roofs project database. The impact of solar reflectance and infrared emittance on the surface temperature is explained and performance examples according to experimental results are compiled. The second section focuses on the types of cool white colored materials that are commercially available per roof type. These include build up roofs, single ply membranes, modified bitumen, tiles, coatings, metal roofs etc. A short description of each technology is given and their main characteristics are reported. The benefits of using cool white colored materials on buildings and the urban environment are analyzed in the third section. The benefits include increased thermal comfort conditions in buildings, reduced cooling energy consumption and peak electricity loads, increased life span of the roof system, reduction of the air pollution and greenhouse gas emissions and mitigation of the heat island effect. Each one of these benefits will be supported by experimental and simulation data available in the bibliography. Finally, the problem of ageing is analyzed. Exposure of cool materials to outdoor conditions has the effect of changing their main properties: solar reflectance and infrared emittance, compromising their performance. The main mechanisms (photo-degradation, thermal stress, deposition of pollutants etc.) that are responsible for causing the ageing effect of the materials are reported. Experimental results that estimate the aging effect of various types of materials and for several outdoor conditions are presented. The case of artificial and natural ageing is discussed as well as any effort to model the ageing effect of cool materials.

Keywords: White cool materials, cool roofs, optical properties.