Many vacation photos from the Mediterranean or the Middle East show a townscape of light-colored buildings with white roofs. These have been traditional architectural characteristics for thousands of years – and these roofs are in fact cool roofs. Although cool roofs are one of the most cost-effective ways to reduce indoor temperatures in summer, they have not yet been widely adopted in western architecture. In this article you will learn how cool roofs work and how their energy efficiency is measured with solar reflectance index.
How do Cool Roofs Work?
Cool roofs have the ability to reflect sunlight and repel heat because the roofs are prepared, covered or coated with materials that have special characteristics. These are typically white roofs and they reduce the heat island phenomenon, minimizing thermal impact on the microclimate and on the local environment. Modern cool roofs include highly reflective thermoplastic and liquid-applied membranes and coatings that provide a full range of benefits over a long service life.
The following illustration depicts the energy flow of sunlight hitting a conventional flat roof (left) and a cool roof covered with a white membrane or coating (right).
The Efficiency of Cool Roofs is Measured with 3 Main Indicators:
SOLAR REFLECTANCE (SR)
The ability of the surface of a material to reflect visible and non-visible solar radiation (infrared and ultraviolet) is known as solar reflectance, or albedo. Solar reflectance ranges from 0 for black surfaces to 1 for white surfaces. White surfaces have high solar reflectance and low absorption, whereas dark ones have low reflectance and high absorption.
THERMAL EMITTANCE (IE)
The ability of a surface to emit thermal radiation in the infrared (heat) range is known as thermal emittance. Thermal emittance ranges from 0 to 1, depending on the type of material. The higher the emittance, the lower the surface temperature will be. Coatings on metal have lower emittance than polymeric synthetic surfaces.
SOLAR REFLECTANCE INDEX (SRI)
The solar reflectance index (SRI) expresses the ability of a roofing material to reflect solar energy. It is defined such that a standard black color (solar reflectance of 0.05, emittance of 0.90) has a value of 0, whereas a standard white (reflectance of 0.80, emittance of 0.90) has a value of 100. The higher the SRI value, the more suitable the material for use on a cool roof. SRI values can even exceed 100. SRI values are calculated using the SR and IE values defined by the ASTM E 1980 standard “Calculating Solar Reflectance Index of Horizontal and Low-Sloped Opaque Surfaces.” A similar European standard was issued in 2017: “EN 17190 Flexible sheets for waterproofing – Solar Reflectance Index.”
Some Typical Cool Roof Colors and Initial SRI Values
Cool Roofs and Green Building Certification Systems
The benefits of cool roofs and their value to communities, the environment and building owners is recognized by green building certification systems such as Leadership in Energy and Environmental Design (LEED).
LEED is the world’s best known and largest green building certification system. It provides third party verification that a building has been designed and built using strategies to improve its sustainability performance.
LEED v4 is the latest version. It recognizes several options for roofing solutions in new buildings or renovation projects that can earn LEED credits: energy-efficient roofing, water run-off management and renewable energy are all important points to consider.
The use of a cool roof system can earn Credit 5, Option 1 “Heat island effect – Roofing” in the Site Sustainability category (SS) of the LEED v4 protocol.
International Associations on Cool Roofs with Sika's Participation
Cool Roof Rating Council (CRRC) is a nonprofit organization established in the USA in 1998. It has developed a product rating program under which companies can label roofing surface products with radiation property values. CRRC lists the measured radiative property values in its Rated Products Directory. Find out more: www.coolroofs.org
European Cool Roofs (ECRC) is the European counterpart to CRRC. It is active since 2011. Find out more: www.coolroofcouncil.eu. It maintains a Rated Product Directory as well.
Sika products suitable for cool roof applications can be found in the publicly accessible Product Rating Databases of the Cool Roof Rating Council and European Cool Roofs.