September 3, 2014
Site Factors and Your Cool Roof
Much has been written of late, both positive and negative, regarding cool roofing. Some recommend a cool or reflective roof for every structure on Earth, projecting energy consumption savings in the billions. A few even say that cool roofing is overhyped, and not only won’t deliver savings, will lead to problems. To get at this topic in depth, GAF is the lead sponsor of Principia’s Energy Efficient Roofing 2013 conference.
As you might guess, the issue is complex, and you can’t make a blanket statement about cool roofing that will apply to every structure. A data center in a cold climate like Winnipeg may benefit from a cool roof, because the servers generate so much internal heat. And cooling is typically harder to do, from a thermodynamic point of view (cooling a building by one degree with conventional equipment takes approximately three times the energy that heating by one degree takes, and is usually done with more expensive energy—e.g., electric instead of natural gas). Conversely, an industrial space with very large internal heating needs in a southern climate may be a candidate for a heat-absorbing roof.
In short, the building’s intended use and site are major factors and the question is not simple. To get you started in determining if a cool roof is right for your commercial building, here are some questions to consider:
Will it lower my energy bills? There are a number of cool roof energy calculators, including GAF’s CREST (which is based on DOE’s CoolCalc and CoolCalcPeak) that will help answer this question. Projections are only as good as the data you put in, and should be used for comparison purposes only. However, generally speaking, if your building is in warmer climate zones (1-3 and usually 4); you have expensive electricity (and high demand charges); and you have older, inefficient air conditioning equipment; you’ll likely realize savings. If you don’t have much roof insulation, the savings will be greater. (Remember that all a cool roof can do is help provide savings in energy usage. If rates go up, the bills will go up – but then of course so will the savings.)
Should I just add more insulation? Additional insulation lessens the energy savings effect of membrane reflectivity and emissivity. As a rule of thumb, above R-30, a cool roof will have a much smaller contribution to savings, but membrane reflectivity is a net plus or minus on the roof no matter how much insulation there is (if a cool roof helps with no insulation, it will not hurt with a lot of insulation). System cost may be reduced by using cool roofing together with less insulation, if appropriate. However, as always, it all depends on the building and its use.
Do I have a high demand charge? Increasingly, electrical billing is about the peaks. Peaks in electrical usage force expensive electrical grid upgrades. Peak usage tends to occur on the hottest days of the year. To capture and bill for those peaks, electrical billing structures are more and more finely detailed, and capture the cost of peaks in a demand charge. Demand charges as high as $900 per kilowatt hour are not unheard of. This means that a cool roof, which can help shave off peak demand by keeping the building cooler on the hottest days of the year (or even just by moving peak load later in the day when demand charges may be lower), can make a lot more sense than you might guess in a northern city like Boston.
What is the condition of my roofing system? Insulation that’s wet has an R value of -0-. That means that a full removal (a “full rip” or “tear off” in industry lingo) will have benefits beyond what any calculator will project.
What’s my building’s use? Roofing assemblies, particularly those that have insulation below deck and no vapor retarder, can sometimes handle vapor drive and high vapor loads just by overheating. Switching to a cool roof may expose the need for additional vapor retarders and/or some above deck insulation, particularly if there is a large vapor load, such as from a commercial laundry. This is an advanced roofing system design question.
Should I care about the Urban Heat Island Effect? If you’re just trying to lower your utility bill, the Urban Heat Island Effect is not a consideration. However, if you’re pursuing a LEED®, Energy Star® or Green Globes® rating for your building, site factors such as Urban Heat Islands are a consideration. And an owner who is thinking of selling in a few years should consider the membrane reflectivity and these broader concerns when they make their roofing choice, since it could impact future sales price.
Are there other benefits to a cool roof for my structure? There is some support for the idea that reduced temperature swings will lengthen the life of a roofing system and even a building. It has also been shown that solar PV panels are more efficient when they are cooler – so if a PV project is in your future, a cool roof can be a good choice.
Are there other building considerations that trump reflectivity? I feel that highly reflective TPO is a great solution for most roofs, but it is important to remember that the roof’s primary job is to keep water out of the building. Certain types of facilities may be better served by different roofs types. It should not be overlooked that our traditional multi-ply asphaltic roofs have a history of great performance as well, and are available with high reflectivity and high emissivity.
What’s clear is that there is a right roof for every structure – and re-roofing is a phenomenal opportunity to improve your building’s energy performance.
Groman Martin (2013 April 5) Site Factors and Your Cool Roof. Retrieved on September 3, 2014 from Gaf.com