PART 1: ENERGY EFFICIENCY & ROOFING Is energy efficiency a concern for you? You’re not alone. Every year, Americans spend around $40 billion on air conditioning alone. That’s the equivalent of a sixth of all the electricity the United States consumes annually! There are many ways to address energy efficiency at home, and it starts with identifying what contributes the most to your power consumption. For decades, space conditioning (heating and cooling) has been the typical US household’s largest energy expense. While appliances, electronics and lighting are slowly catching up, heating and cooling still make up a sizeable portion of the pie: about 48%, according to Energy Information Administration surveys.
The Case for Energy-Efficient Roofs Climate is a big factor when you’re choosing a roof, but for most people, this is limited to how well the roof can protect their homes against the elements. But roofs also play a big part in making homes more energyefficient. Because the roof is responsible for about 20% of a home’s heat loss in winter and 5% of its heat gain in summer, any improvement in its energy performance will have a tremendous impact on the entire home’s energy efficiency. A roof system designed with energy efficiency in mind will be able to reduce the amount of power your HVAC system needs to stabilize indoor temperatures, which will in turn be a huge load off of your energy bill.
PART 1: ENERGY EFFICIENCY & ROOFING
Building an Energy-Efficient Roof Roof energy efficiency isn’t as easy to measure as, say, window energy efficiency. This is because so many factors contribute to a roof system’s overall energy performance, the biggest of which include:
Among these factors, roof and attic insulation provide the simplest pathway to setting energy performance benchmarks. In the case of insulation, energy efficiency is rated in R-value, the measure of how well a material is able to prevent heat transfer. As a general rule, the higher the R-value, the better a roof is at preventing heat loss. (Heat gain prevention is more dependent on sun control than on insulation.) Different climate zones have different space conditioning requirements, so ideal R-values vary widely. For instance, recommended ratings for cathedral ceilings in Zone 3 (mixed-humid, hot-humid and hot-dry) are R22 to R38, while Zone 6 (cold) calls for R49 to R60. These R-values are recommendations by the Department of Energy and the International Energy Conservation Code (IECC), the model building code for the United States. Did You Know? You can compute how much energy you can save through your roof using Oak Ridge National Laboratory’s Roof Savings Calculator at http://rsc.ornl.gov/. The online tool (which is also available as a web service) also lets users compare the performance of different roofing options, which will come in handy if you’re planning a reroof.
PART 2: RETHINKING THE IDEAL ROOFING SYSTEM Cool roofs are the biggest buzzword in roof energy efficiency today. They work by reflecting a large fraction of incident solar radiation, dramatically reducing the amount of heat absorbed by your home. They also have high emissivity, which means they quickly release whatever amount of heat they end up absorbing. But is reflectivity and emissivity all there is to an energy-efficient roof? The short answer is NO. According to former California Energy Commissioner Art Rosenfeld, the ideal energy-efficient roof is both:
The thing is, such a material doesn’t exist (yet). Modern cool roof materials are around 65% reflective (meaning they absorb around 35% of the sun’s energy). And unfortunately, the emissive and reflective properties of roofs don’t really change to suit the changing seasons. The ideal 100% reflective/100% absorbent roof is—for now—an impossibility. But Rosenfeld’s definition of an ideal roof emphasizes an interesting point: focusing on a roof’s ability to absorb heat can also make it more energy efficient. About Thermal Mass Thermal mass refers to a material’s ability to absorb and store heat (the term is also used to refer to the material itself ). A material with high thermal mass can hold a larger amount of heat and takes longer to fully absorb and release that heat compared to a material with low thermal mass.
PART 2: RETHINKING THE IDEAL ROOFING SYSTEM
You can think of a thermal mass as a high-capacity “battery” that gets charged and then releases, not electrical energy, but heat.
Roofing materials with high thermal mass can reduce the heating and cooling needs of homes in most climates. They are flexible enough to have the ability to:
a) keep a home cool when it is hot out (by slowing down daytime heat transfer from the exterior to the interior); and
b) keep a home warm when it is cold out (by slowing down heat loss from the interior to the exterior).
Roofing Materials with High Thermal Mass Two things make a material a good thermal mass: a high specific heat capacity and a high density. Roofing materials that meet these requirements include clay tile, concrete tile and slate. (Bonus: clay and concrete tile roofs can be modified with reflective coatings to make them even more energy efficient.) A Quick Note Roofing materials with high thermal mass tend to be heavy, so your roof deck and the underlying structures must be able to handle the weight. If you’re switching from a lighter material, modifications will have to be made to ensure that the structural integrity of your home won’t be compromised.
PART 3: HARNESSING THE COOL BENEFITS OF COOL ROOFS
As we’d mentioned in Part Two, cool roofs keep homes cool by reflecting most of the incident heat from the sun. This promotes energy efficiency because reducing the amount of heat absorbed by the roof and transferred to the home’s interior reduces the amount of energy consumed by HVAC systems. And that’s just half of the good news. Perhaps the best thing about cool roofs is that most roof surfaces can be made reflective without a complete roof overhaul. Making Existing Roofs Cool • Low-slope roofs – Light-colored coatings or single-ply membranes can be applied onto most existing low-slope roofs to make them cool. If you want a dark-colored roof, choose a reformulated coating that contains pigments that reflect near-infrared (non-visible) light. • Built-up roof – Substitute gray slag or reflective marble chips for dark gravel or apply a cool coating directly over the roof. • Tile roof – Most clay and concrete tiles reflect only 10% to 30% of incident solar energy, although some manufacturers produce tiles capable of up to 70% reflectivity. If your tile roof is not naturally reflective, field-applied surface treatments can still make it cool. • Metal roof – Many metal roofs have reflective factory-applied finishes, but you can also have a light-colored paint or reflective coating applied on a metal roof after installation.
PART 3: HARNESSING THE COOL BENEFITS OF COOL ROOFS
A Quick Note Modified bitumen sheet membranes can only be pre-coated with cool roof coating during the manufacturing process. Spray polyurethane foam roofs, on the other hand, are already reflective to begin with so they don’t really need further treatment. As for existing asphalt shingles, you may have them cool-coated, but you’ll be doing so at your own risk as it is not recommended by most manufacturers.
Final Thoughts If your roof is due for replacement, there is no reason for you not to get a cool roof as it will cost roughly the same as its conventional counterpart. As a takeaway, keep in mind that improving your roof’s energy performance depends on more than just what’s on the surface. For best results, consult a trusted contractor about taking a comprehensive approach to roof energy efficiency.
The Roofing and Remodeling Company 2106 Aluminum Avenue, Suite A Hampton, VA 23661 (757) 903-0050 TheRoofingCompany.com