Measuring the energy efficiency of cool roofs

Photographs of Mediterranean or Middle Eastern cityscapes often depict light-colored buildings with white roofs. Representing the traditional architecture of these regions, the buildings are designed to respond to the local climate and the roofs are actually cool roofs. Although cool roofs are one of the most cost effective ways to reduce interior 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 the solar reflectance index.

How do cold roofs work?

Cool roofs have the ability to reflect sunlight and repel heat because roofs are prepared, covered or clad with materials that have special characteristics. They are generally white roofs and they reduce the heat island phenomenon, thus minimizing the thermal impact on the microclimate and the local environment. Modern cool roofs include highly reflective, liquid-applied thermoplastic membranes and coatings that provide a full range of benefits over a long life.

The following illustration shows the energy flow of sunlight striking a conventional flat roof (left) and a cool roof covered with a white membrane or coating (right).

The effectiveness of cool roofs is measured with 3 main indicators:

Solar reflectance (SR)

The ability of a material’s surface to reflect visible and non-visible (infrared and ultraviolet) solar radiation 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, while 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 varies 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 synthetic polymer surfaces.

Solar Reflectance Index (SRI)

The Solar Reflectance Index (SRI) expresses the ability of a roofing material to reflect solar energy. It is defined so that a standard black color (0.05 solar reflectance, 0.90 emittance) has a value of 0, while a standard white (0.80 reflectance, 0.90 emittance ) has a value of 100. The higher the SRI value, the more suitable the material will be for use on a cool roof. SRI values ​​can even exceed 100. SRI values ​​are calculated from the SR and IE values ​​defined by the ASTM E 1980 standard “Calculation of the solar reflectance index of opaque horizontal and low slope surfaces”. A similar European standard was published in 2017: “EN 17190 Flexible sheets for waterproofing – Solar reflectance index”.

Some typical cool roof colors and initial SRI values

SRI value

Darker colors do not meet the SRI > 82 (initial value) requirement to qualify for LEED credits.

Cool roofs and green building certification systems

The benefits of cooling roofs and their value to communities, the environment and building owners are recognized by green building certification systems such as Leadership in Energy and Environmental Design (LEED).

LEED is the most well-known and largest green building certification system in the world. 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 roofs, stormwater management, and renewable energy are all important points to consider.

The use of a cool roof system may earn Credit 5, Option 1 “Heat Island Effect – Roof” in the Site Sustainability (SS) category of the LEED v4 protocol.

International associations on cool roofs with the participation of Sika

The Cool Roof Rating Council (CRRC) is a non-profit organization established in the United States in 1998. It has developed a product rating program where companies can label roofing surface products with ratings. of radiation property. The CRRC lists the values ​​of the radiative properties measured in its directory of classified products. Learn more at www.coolroofs.org.

European Cool Roofs (ECRC) is the European counterpart of CRRC. Active since 2011, it also manages a directory of classified products. Learn more at www.coolroofcouncil.eu.

Sika products suitable for cool roof applications can be found in the publicly available product rating databases of the Cool Roof Rating Council and European Cool Roofs.

Image: Application of Sikalastic-560 diluted with 10% water as a primer