Cool Roofs

Policy

The International Institute of Building Enclosure Consultants (IIBEC) affirms that the designer of record should consider the effect that a cool roof has on a building enclosure when making design decisions.

Issue

Some codes, rating systems, and owners specify cool-roof properties such as solar reflectance, emittance, or solar reflectance index (SRI) values. SRI, calculated per ASTM E1980, is a measure of the constructed surface’s ability to stay cool in the sun by reflecting solar radiation and emitting/releasing thermal radiation back into the environment.

Cool roofs are typically defined by a combination of their aged solar reflectance (0.63 or greater) and aged thermal emittance (0.75 or greater), or by their aged solar reflectance index (SRI) (75 or greater).

While the primary advantage of a cool roof is a reduction in the urban heat island effect, there are also disadvantages associated with its use.

Advantages of a Cool Roof

• Reduce energy use of a subject building during warm weather by reflecting and emitting the energy of the sun back to the sky in lieu of absorption by the roof and transfer to the building.
• Will lower the surface temperature of the roof assembly during a sunlit day, which will slow the degradation of many roofing materials and may increase the performance of thermal insulation when compared to use under a non-reflective membrane.
• Scientific studies suggest that the collective use of cool roofs in urban environments may reduce the urban heat island effect by lowering ambient air temperatures.

Disadvantages of a Cool Roof

• A high SRI may increase glare from reflective surfaces.
• May increase energy use in cool climates during heating months by lowering the surface temperature of the roof.
• Increased risk of condensation forming under the membrane in cool-roof assemblies without vapor retarders in cool and temperate climates.
• As a result of their lower surface temperature, do not absorb as much heat as non- cool membranes or coatings and will have less downward drying during summer months.
• Due to lower surface temperatures moisture can remain on the surface for longer periods, which could result in biological growth on the surface.

Rationale

There are many project-specific requirements that require consideration when designing a roof assembly to ensure the roof system performs as intended. The use of cool roofs has increased significantly in recent years to reduce the urban heat island effect, which reduces energy consumption, and to comply with local and state requirements. Designers of record are best suited to evaluate the effects a cool roof has on the performance of a roof assembly and choose the appropriate system for that project.

References

1. CoolRoofRatingCouncil(CRRC)ProductRatingProgramCRRC-1.(http://coolroofs.org).
2. ASTM E1980, Standard Practice for Calculating Solar Reflectance Index of Horizontal and Low-Sloped Opaque Surfaces.(https://www.astm.org/Standards/E1980.htm).
3. S. Environmental Protection Agency. 2008. “Cool Roofs.” in: Reducing Urban Heat Islands: Compendium of Strategies.(https://www.epa.gov/heat-islands/heat-island-compendium).