Cut Costs with Cool Roofs

01/29/2016 | By Thomas M. Gernetzke

How to evaluate reflective roofing materials

Tan cool roof installation Industrial roof Cool roof membrane Roofing material Metal roof Low-slope roof

The commonsense benefits of cool roofing are numerous. A roof that reflects extra UV and heat instead of absorbing it keeps the building underneath cooler, leading to a reduction in cooling costs and the opportunity to downsize HVAC equipment. Those of us who have visited facilities of questionable construction over the years can vouch for the comfort difference – in buildings where the roof absorbs too much heat, you can practically feel the heat radiating down from the ceiling.

There are more benefits than just using less energy, however. UV and heat exposure together are the primary cause of breakdown and deterioration of roofing material, so in many cases keeping the roof cooler allows the material to last longer. And the same reflectivity that keeps buildings from overheating and roof membranes from prematurely aging also battles the urban heat island effect.

But how can you determine which of the hundreds of cool roof products fits your facility best – or whether you need a cool roof at all? This guide to cool roof specification will help you decipher performance data and understand where to use a cool roof.

What Performance Metrics Mean
There are three key metrics that can help you evaluate the performance of different materials and slopes when it comes to keeping your roof cool.

Solar Reflectance Index (SRI) rating: A measure of the roof’s ability to reject solar heat. Ratings are assigned based on testing by the Cool Roof Rating Council – high-performance roofs can reach over 100. To put this in context, the Lawrence Berkeley National Laboratory defines a “perfect” SRI as roughly 122, but that score would require your roof to have very low emissivity and be a perfect mirror that absorbs zero sunlight. Some products also provide an aged SRI value to show how the product will perform after a few years of weathering.

Solar reflectance (albedo): The fraction of the incident solar energy reflected by the surface in question. A higher number means more energy is reflected rather than absorbed.

Thermal emittance: The relative ability of a surface to radiate absorbed heat. The higher the number, the faster the surface sheds the heat it has absorbed. Thermal emittance and solar reflectance are both measured in values of 0-1.

Of the three, comparing the SRI value is the easiest way to see how one roofing material stacks up against another. For example, white TPO single-ply membranes can have SRI values as high as 101, while the SRI of granulated SBS or modified bitumen cap sheets that aren’t cool roof rated averages around 26. One black EPDM roof I found actually has an SRI of -1. However, SRI may not tell the whole story, especially when you’re trying to compare like materials. If you need additional performance information, bring reflectance and emittance ratings into the mix. For example, the white TPO specimen with a 101 SRI carries a reflectance value of 0.77 and an emittance value of 0.87. Many materials have a similar emittance value, so reflectance is usually a better indicator of which roofs qualify as cool.

The SBS cap sheet in this example has the same emittance value, but its reflectance is 0.26 – it sheds absorbed heat at the same rate as the TPO, but doesn’t reflect nearly as much solar energy. The black EPDM product has a reflectance of 0.06 and an emittance of 0.88, showing that it barely reflects any of the solar energy hitting it.

What should you look for when you compare these values? The roofing industry generally defines cool roofing materials as having reflectance and emittance ratings that are at least 0.65 to 0.70. Areas where cool roofs are mandatory may set a certain minimum standard for roof ratings – for example, California’s Title 24 requires at least 0.70 reflectance and 0.75 emittance. Chicago takes a slightly different approach and allows low-slope roofs to have either an initial reflectance of 0.72 or a three-year aged reflectance of 0.5.

How to Compare Materials
Armed with roof rating information, you can now start evaluating products to see which ones best fit your specific application. Thermoplastic membranes like TPO and PVC are often lumped in together because both are plastic and heat-weldable, and both are likely to have cool roof rated products available in white, gray and tan.

White EPDM now features new formulations that promise better performance than past iterations, making it a valid option for a cool roofing system.

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