Buildings, including residential, commercial, public, and industrial consume about one third of all energy use and about two thirds of all the electricity generated. Demand for electricity in the U.S. is increasing three times faster than new power plants and lines are being built to accommodate this rapidly increasing demand. Heating and cooling costs dominate most building’s energy usage. A building’s roof can have the greatest impact on the energy consumption by a building so any technological advances and improvements in roofing energy efficiency and performance will greatly affect both energy usage and consequent cost savings. Energy efficiency, durability, performance, sustainability, recyclability and life cycle costs are the focus of green building. Architects, builders and consumers have been changing the way they select building materials and designing buildings. They are increasingly shifting their focus to green building materials and energy performance with so called cool roofs becoming a key focus. Cooling costs significantly affect a building’s energy consumption in most parts of the country and cooling is the predominant cost in the southern states. Cool roofs can save up to 40% in cooling energy as reported by several studies including Lawrence Berkeley Laboratory Heat Island Group. Cool roofs essentially are roofs which reflect much of the sun’s radiation away from a building, employ above sheathing ventilation (ASV) cooling under the roof, readily emit any heat which they do absorb back away from the building and cool quickly at night. The less solar radiation and heat entering a building’s interior space through the building’s roof, the lower the building’s cooling costs and energy use will be.
Net Heat Flux Through Roof (courtesy coolmetalroofing.org)
Cool roofs or reflective roofs can be comprised of a variety of materials including asphalt or fiberglass shingles, clay or concrete tile, EDPM rubber or PVC roofing membrane and variety of metals as long as they can reflect away much of the sun’s radiation and readily release any heat they absorb into the atmosphere. White and the more lightly colored roofing materials are the most reflective but different material composition as well as special designed and engineered coatings or paints used on them can also greatly affect reflectivity and emissivity. Some of these new coatings or paints, especially those applied to newer more advanced metal roofing, incorporate special pigments designed to provide higher reflectance of solar infrared and ultraviolet radiation. Infrared and ultraviolet radiation can contribute even more to absorbed heat energy from the sun than visible light so any type of roofing material incorporating these special reflective coatings may outperform conventional light colored materials without them. Even darker colored roofing which incorporates these special IR and UV reflective coatings will outperform older non treated darker colored roofing saving as much as 25% in cooling costs.
Reflective Cool Metal Roofing Tile (courtesy coolmetalroofing.org)
Even with the advent of newer reflective coatings, white and light colored roofing still is the best performing cool roof providing the most energy and cost savings. Consumers, builders and architects however still like the aesthetics in having varied colors and materials and many still favor darker roofing hues. Ultra violet (UV) represents about 3% of the total solar energy striking the earth’s surface. Visible solar energy amounts to 40% while infrared (IR) energy accounts for the largest percentage 57% of the solar energy spectrum and is felt as heat. Thus roof materials which can be coated with newer UV and IR reflective coatings, even the darker shades of roof color, can now be much more energy efficient than in the past. Darker shades of roofing still won’t reflect near as much of the visible solar radiation as lightly colored and white roofing. Though with engineered reflective coatings they will now block a high percentage of the UV and IR radiation which actually constitute more of the solar energy radiation and heat gain than visible solar energy. In addition to solar reflectivity, heat emissivity of roofing materials must also be considered as a large factor affecting building energy usage and possible contribution towards heat island effects. Any solar radiation which is not reflected by the roofing surface is then absorbed, converted or transferred to heat energy and some of this heat can conduct and radiate down into the interior building space affecting cooling costs. If the roof material has good emissivity much of this heat can re-radiate back to the sky in the form of IR energy especially cooling off during the night and there will consequently be less heat build up. Any concentrated heat build up, especially in tight grouped urban areas leads to heat island effects where temperatures in the air above these heat islands can be up to 12 degrees hotter than normal, leading to higher air conditioning costs, higher use of energy and higher levels of ozone and smog.
Atlanta at Night Thermal (Blue=cool, green=lukewarm, red=warm, white=hot)
In addition to emphasizing high solar reflectance and emissivity properties, roofs can additionally be kept cooler, limiting or preventing heat gain or heat flux down into a building’ interior space below, by promoting convection cooling underneath the roof material using above sheathing ventilation (ASV). Heat emanating from the underside of the roofing can be removed using convective air flow (balanced ventilation) moving from soffit vents or eaves up to and out a ventilating roof ridge cap. Testing at Oak Ridge National Laboratory showed that by employing the natural convection of heated air below the roof covering, temperatures in an attic space are reduced. Air enters at the eave, is heated by the roof, and because hot air rises, it is naturally drawn up and vents to the outside. This natural convection controls heat gain and reduces the temperature in the
attic below. The Oak Ridge National Laboratory study found that dark-colored metal-shingle roofs using above-sheathing ventilation had similar heat energy flows compared to their lighter cooler colored counterparts. About 30 percent of measured reduction in heat gain was contributed by ASV. When a cool reflective roof surface was used with ASV, an additional 15 percent reduction in heat gain was achieved for 45% total. The improved summer performance of cooler roofs coupled with the reduced heat losses during the winter show cool roofs can both reduce energy usage in summer months while negating any supposed heating penalty in cooler months or northern heating dominated climates.
* (see http://www.metalconstruction.org/pubs/pdf/ORNL-TM-2006-9.pdf)
A convective air space below the roofing material can be easily set up by using a batten, cross batten or metal purlin framework on top of roof sheathing or a roof deck, on which sturdy roofing panels can be attached. Less sturdy roofing material can employ a 1 1/2″ metal roof deck or else just a purlin spaced layer of OSB below the existing roof sheathing. Even darker roof materials such as weathered bare copper which does not have a great solar reflectance index (SRI), can provide good to outstanding energy efficiency results with the use of either an underneath or above sheathing ventilation approach. Additionally winter or cold weather heating performance is improved with the use of below or above sheathing ventilation as that convection air space between the roof material and lower roof deck or sheathing further acts as an insulative layer and helps to prevent heat loss and condensation. Ventilated cooled roofs work great and are energy efficient in any climate. They tend to last much longer too with less heat degradation.
Batten – Cross Batten Above Sheathing Ventilation (ASV)
ORNL Copper Roof Ventilation Test (images courtesy CDA at copper.org)
* These test results reflect using a 1½” fluted metal roof deck, ¾” OSB (plywood would work just as well), underlayment and red rosin paper underneath a standing seam copper roof. The use of a fluted metal roof deck is optional; similar results can be obtained by using other designs that provide an air space below the deck. The test roof had a 4/12 slope and faces south. The ventilated copper roof system was compared with a control roof of typical black asphalt, a typical non-ventilated copper roof assembly and two very good examples of above-the-sheathing, ventilated stone-coated steel roofs using special Cool Roof reflective coating colors. The graph above shows the ceiling heat flux crossing the attic floors of the respective attic assemblies. The light gray stone-coated cool metal roof (SR246E90) dropped the ceiling heat flux by roughly 29% of that measured for the asphalt shingle roof (SR093E89). The copper roof with the fluted metal deck further dropped peak ceiling heat flux an additional 23% of that for the coated steel shingle roof, bringing the total reduction for the copper roofing system to 50% of the asphalt control. Peak heat flux dropped an additional 1.5 Btu per hour per square foot for the copper roof as compared with the stone-coated metal roof. These results show clearly that a vented copper roof assembly will greatly reduce heat gain resulting in lower energy costs for the structure. Further, it dispels the notion that copper roofing must have high reflectance and emissivity in order to be effective in saving energy. * See Copper Roofs Are Cool
Cool roofs with their large energy saving and cost saving benefits can be realized by employing light colored roofing materials with very good solar reflective properties or roof materials using special engineered reflective coatings or most any type of roofing utilizing balanced and well functioning below or above sheathing ventilation (ASV). White or lightly colored highly solar reflective roofs employing ASV still provide the greatest energy and cost savings. It must also be stated that using higher R-values for ceiling insulation or better insulation below the roof in structures without attics will also control or limit heat gain into a building interior and consequently also lessens the heat gain differential using less reflective roofing materials or darker roof colors. Adding a radiant barrier and insulation board underneath your lower roof deck and convection space can further prevent the intrusion of heat gain from roofing into attic and living space. Additional attic vents, gable vents, dormer vents and roof vents will also reduce heat gain from roofing. More sloped roofs also perform much better and have less heat gain than flat or low sloped roofs and should be considered for any new construction. Some buildings with flat or low slope (<2:12) low-rise roofs may also be candidates for retrofitting them with a lightweight sloped roofing structure framework when re-roofing them especially with lighter weight roofing such as metal roofing. That framework could simultaneously also provide a convective roof cooling space underneath the new roofing. Resulting lower energy costs, less maintenance and no roof replacement for 50 years or longer could provide a good return on that investment. The bottom line is that when evaluating roofing energy efficiency, heating and cooling costs, looking only or primarily at roofing material properties such as SRI, reflectivity and emissivity is not sufficient to see the whole picture. You have to look at what effect the entire roofing system and structure has on heat gain, cooling load, heat loss and energy usage of the building and it’s systems. The performance of the whole system must be examined in totality when the end goals are reducing energy costs and going still further, in achieving the most eco-friendly green buildings. In a subsequent blog article, the green building aspects of roofing will be discussed in more detail. Then again there are specialized roof systems such as KME’s Tecu Solar System copper roofs below which are expressly designed as efficient solar heat collectors for heating a building’s interior space, it’s hot water and even swimming pool. Where do such technically advanced roofs fit into energy efficient roofing guidelines, requirements, ratings and green design certification?
Copper Solar System Roofs (photos courtesy KME.com – Italy)