Cool Roofs in Hot Climates Deflecting the Radiation Early ZNEdesigner

A Cool Roof reflects the sunlight (solar radiation) back, significantly reducing the amount of heat that gets absorbed through the roof surface. A cool roof thus reduces the cooling loads in hot summers, reducing peak loads, urban island effect and the required HVAC system size.

Cool Roofs can be one of the most cost effective strategies towards reaching the goal of a self sustaining Zero Net Energy home. A recent study by EPA showed how using cool roofs can result in over 30% peak load reduction (and over 40% cooling energy savings in Sacramento, CA), in hot climates (cooling dominated).

The US Department of Energy recommends cool roofs for US Climate Zones 1, 2 and 3.  In colder (heating dominated) climates, cool roofs should be avoided as the roof surfaces will reflect the beneficial (in cold winters) solar radiation.  In California, cool roofs are recommended specifically for climate zones: CZ 4, CZ 9, CZ10, CZ13, CZ14, and CZ15.

MEASURING THE COOLNESS 

The roofs “Coolness” is measured by its reflectance and thermal emittance, or a combination metric of solar reflectance index (SRI):

Solar reflectance  is fraction that corresponds to the percentage of sunlight that the roof reflects back. Thus a roof surface having a reflectance of 0.75, reflects back 75% of the incident  sunlight.

Emittance  is a fraction that corresponds to the percentage of heat energy emitted by the roof surface. A surface with a higher emittance will reflect a higher percentage of heat and thus cool itself faster.

Solar Reflectance index (SRI)  is an alternative metric for comparing the “coolness” of roof surfaces1. It is  calculated i from solar reflectance and thermal emittance values. The higher the SRI, the cooler the roof will be in the sun. The Lawrence Berkeley Laboratory (LBL) has a spreadsheet  that calculates SRI.

The following chart shows the minimum cool roof requirements set by the California Energy Commission (CEC) :

US Green Building Council’s (USGBC) Leadership in Energy and Environmental Design (LEED ) program currently uses minimum SRI values of 78 and 29 for low and steep sloped cool roofs, respectively

COOL ROOF PRODUCTS

There are a number of Cool roofing product options available in the market today:

Eagle Roofing   Foorja has worked on a number of ZNE projects where the Eagle roofing products have been used. Eagle has products with varying degrees of reflectance and emittance, available in different colors.

Other companies that offer cool roofing products include Boral  and Certainteed . The Oakland based Cool Roofing Rating Council (CRRC) also has a number of cool roofing product manufacturers listed under its members page .

BEST RESEARCH RESOURCES

Reducing Urban Heat islands, Compendinum Strategies  Cool Roof  , EPA. This comprehensive study includes a discussion on how cool roofs work, different cool roof types, their benefits and costs, and various initiatives and resources.

Guidelines for Selecting Cool Roof , US Department of Energy, 2010. This study discusses different aspects of cool roofs, including their cost benefits, and the various types of cool roofs and coatings.

Cool  Roof Rating Council  (CRRC) website has a number of articles and resources on cool roofs and their advantages.


Leave a Reply