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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.
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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.
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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.
Fluid-applied and retrofit coatings are a catchall category for coatings in white or other light colors that can be applied to a variety of roof systems. Many are intended for existing roofs that don’t yet meet cool roof requirements and can help add a layer of protection to the roof underneath. For example, you could apply a coating to an older built-up or modified roof to both capitalize on the benefits of cool roofing and lengthen the life expectancy of the roof system.
Modified bituminous systems can achieve cool roof ratings in a number of ways. Some vendors offer factory-coated systems where a light-colored coating is added to the top ply during manufacturing. Factory-ply systems involve a separate ply of white material that is bonded to the sheet in the factory. Other products feature reflective granule surfacing bonded to the top ply.
Ballast and aggregate surfacing generally has a reflectivity of around 0.30, but this can vary depending on the surfacing. Make sure to take a look at emittance ratings – studies by Chicago roofing contractors have shown that it may take longer for the layers of surfacing to release accumulated heat, which is a bigger issue in cooling climate states. If the roof is still emitting heat hours after the sun goes down, the air conditioning has to continue working into the night to maintain moderate temperatures inside the building. Aggregate surfacing qualifies as cool surfacing in some parts of the country, but not others.
Cool roofing technologies for steep slope roofs have come a long way. The most common cool roof materials for these buildings are architectural metal systems where the metal is coated with reflective finishes or light colors. A quick review of current product shows a wide SRI range of 24-81 and reflectance values of 0.26 to 0.67, with white or light-colored materials achieving higher reflectance and SRI values.
Note, however, that even some of the dark-colored materials are considered cool despite their lower SRI and reflectance ratings. Steep slope projects may also benefit from architectural shingles, with one sample product boasting an SRI of 29, reflectance of 0.27 and emittance of 0.92. Shingles aren’t necessarily as cool as other materials, but SRI and reflectance values like these represent a significant improvement over traditional shingles that aren’t cool roof rated.
The best resource to compare materials that I’ve found is the Cool Roof Rating Council product directory at coolroofs.org – it includes all products that have received the organization’s cool roof designation and provides extra detail in addition to ratings.
Where Cool Roofs Don’t Work
Despite the benefits, cool roofs don’t make sense for every facility. Building owners can find themselves thinking that they need a cool roof on every property, but it’s not always appropriate to do that. Some roof assemblies need extra heat to dissipate accumulated moisture, and a cool roof that maintains a lower roof temperature won’t help with that.
In one project I worked on, a wooden building with a white membrane had moisture draining from the fire alarms and light fixtures. We reviewed the assembly – which included a vapor retarder, rafters with fiberglass insulation and roof sheathing – and were stumped as to why this was happening until the owner mentioned that other buildings in the area had the exact same construction but with black membranes on top. We performed a WUFI hygrothermal analysis on both membrane colors and it became clear that the black membrane generated enough heat to dissipate that moisture but the white did not.
In some areas of the country, you may be confronted with a heating season penalty where you spend more heating a “cool” building in the winter than its light-colored roof saves you on air conditioning in the summer. Most buildings’ weather-related energy costs are spent on air conditioning, however, so unless you already know heating is the bigger problem for you, this probably doesn’t apply.
Also note that even if your climate is perfect for cool roofs, your specific location might not be. A pet peeve of mine is owners who are adamant about specifying a cool roof but know it won’t remain clean and have no interest in keeping it white. If it won’t have a chance to remain reflective or cool, why bother?
Understand that scrubbing the roof will lower the life expectancy, so if you know your building will attract a lot of grime and you let it build up instead of just addressing minor dirt deposits during regular maintenance, a cool roof may not be for you. If you do decide to clean the roof, take into consideration what the cleaning process will do to the membrane and make sure you’re using safety precautions for people who are cleaning a slippery roof.
Finally, it’s crucial to take the buildings around you into consideration when you’re weighing cool roof implementation. Reflective roofs can cause serious glare for adjacent buildings, but you may be able to mitigate the issues by opting for a lower reflectance rating or using a gray or tan product instead of a white one.
Thomas M. Gernetzke is a Project Manager for Facility Engineering, Inc., in Madison, WI. He specializes in roofing and waterproofing systems. Gernetzke is a Past President and Fellow of RCI.