Cool Roofs: A Big Deal

In the ongoing efforts to reduce our nation’s carbon output by improving the energy efficiency of our built environment, a new old idea is shaping up to be a key player: cool roofs.  Used throughout the Mediterranean and tropical climates worldwide, the solar reflectance value (albedo) of a white or light-colored roof has been long understood—the more sunlight the roof reflects, the less the building absorbs and the easier it is to keep the building cool.

A recent report by Hashem Akbari, Surabi Menon and Art Rosenfeld titled, “Global Cooling: Effect of Urban Albedo on Global Temperature”, quantifies cool roofs’ potential impact on improving energy efficiency and slowing climate change.  The report notes that painting 100 feet2 of black roof a lighter color offsets the extra heating caused by 1 metric ton of CO2 in the atmosphere.  Scaled up to the national level, converting dark-colored roofs and pavements in urban areas around the world to lighter colors would offset the extra heating caused by 44 billion metric tons of CO2in the atmosphere, effectively offsetting over 6 years of the U.S.’s CO2 equivalent greenhouse gas output and saving the country over $1 billion per year in energy costs.

Clearly, cool roofs are a big deal.  But from a building technology perspective, just painting the roof a lighter color isn’t enough, since the lighter color only solves half of the cool roof equation.  Calculating the coolness of a roof requires measuring both solar reflectance (the fraction of solar energy reflected by the roof) and thermal emittance (the measure of a roof’s ability to radiate absorbed heat as infrared light); the most useful method available for calculating roof coolness is the solar reflective index (SRI).  This index utilizes both factors to generate a 1-100 SRI rating, where 100 indicates a roof with perfect solar reflectance and thermal emittance.  The higher the SRI, the cooler a roof will be, even in full sunlight on a hot day.

Much like the HERS index for whole house energy efficiency, this rating index is essential to meeting the goal of retrofitting and constructing new buildings with cool roofs.  Without a scientifically sound method to rate the cooling properties of various roofing materials, consumers cannot make educated decisions and the maximum cooling benefits cannot be harnessed.

And while many current cool roof materials apply the latest and most advanced technologies, from spray polyurethane foam systems to brightly-colored tiles that reflect infrared energy, our historic understanding of the relationship between color and solar reflectance retains its preeminent importance.  Lighter roofing materials keep buildings cooler than darker materials, yielding more energy efficient structures that have a lower carbon footprint and are less expensive to operate.

Resources on Cool Roofs:

Hashem Akbari, Surabi Menon and Arthur Rosenfeld, “Global Cooling: Effect of Urban Albedo on Global Temperature”, 2008.  http://repositories.cdlib.org/lbnl/LBNL-63490/

Energy Information Administration, “Emissions of Greenhouse Gases Report”, December 2008.  http://www.eia.doe.gov/oiaf/1605/ggrpt/

The Lawrence Berkeley National Laboratory (LBNL) Cool Roofing Materials Database. http://eetd.lbl.gov/coolroof/

The Cool Roof Rating Council (CRRC).  http://www.coolroofs.org/

Celeste Allen Novak and Sarah Van Mantgem, “What’s So Cool About Cool Roofs”, GreenSource, March 2009.  http://continuingeducation.construction.com/article.php?L=68&C=488&P=1

The DOE Cool Roof Calculator provides an estimate of cooling and heating savings for small to medium size facilities that purchase electricity with a demand charge and an alternative version for larger facilities. http://www.ornl.gov/sci/roofs%2Bwalls/facts/CoolCalcEnergy.htm

The EPA Cool Roof Calculator allows the designer to input specific details about a building, including heating and cooling systems as well as location and the cost of energy. http://www.roofcalc.com/RoofCalcBuildingInput.aspx

A PDF version of this document is available here.

0 thoughts on “Cool Roofs: A Big Deal

  1. Thank you for a concise and informative article on the current interest in cool roofing and greenhouse gas reductions. One small correction: The Cool Roof Rating Council (CRRC) does not rate roofing products by Solar Reflectance Index (SRI). Instead, the CRRC reports the solar reflectance and thermal emittance of rated products, and includes these values on the product label. As an independent rating system the CRRC prefers to use solar reflectance and thermal emittance, which are physical properties of a material, rather than SRI which is a calculated value including certain assumptions. However, we recognize the importance of having a single value to use in comparing roofing products, and the CRRC website directs users to an online SRI calculator where they can use the solar reflectance and thermal emittance values from the CRRC to calculate the SRI.

    Molly Trombley-McCann
    Technical Coordinator
    Cool Roof Rating Council
    510.482.4420 ext239
    [email protected]

  2. Thank you for making clear the relationship between solar reflectance and thermal emissivity shown by SRI (solar reflective index). I do believe the implication you have left us with is that only very light or white roofs can acheive true “cool roof” capability in colors and make the difference in Carbon Footprint. True infrared pigment based coatings already rate LEED and Energy Star(r) status in colors that can be field applied over most steep slope roof substrates and are not limited just to bright colors or what is just available only on new cool roof tiles. I do agree that the cool roof issue certainly must revolve around both the SRI and what is realistic in terms of colors for design considerations in the case of roofs that can be seen from the street level (sloped roofs).

    Inevitably this means inclusion of cool coatings products in efficient colors that reflect infrared energy.

  3. I am not sure if I agree with your post here. See you do make the best point, I don’t think you have actually given a large amount of thought to the opposite side of the argument. Perhaps I could do a guest post or a follow-up, just tell me.

  4. Nice post!!! which has helped me decide which kind of material to choose for my roofing. When you are looking for a roofing material there are so many things that are going through your mind as you need to consider your budget, as well. However, there may be better options like using polycarbonate roof sheets. This can be one of the best options as it is really durable and reasonably prices, as well. Last night when I was on the internet looking for roofing materials for my new home I came across a website http://www.hfmfgcorp.com/index.htm that offers roofing materials. These are made up of polycarbonate that serves as an ideal roofing material.

  5. Nice post! In this blog we find about cool roof which reflects the temperature globally.these kind of cool roofs provides global cooling.when you make your new house there are various things you have to consider about roofing this information is quiet useful to us. Cool Roof Calculator provides an estimate of cooling and heating savings for small to medium size facilities that purchase electricity with a demand charge and an alternative version for larger facilities.thanks!

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