A White Roof Isn't Always the Right Roof
By: Robert Reale | May 15, 2009 | 10:25 PM (PDT) | 
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A "winter penalty" may erode the value of a reflective roof's albedo effect in the reduction of greenhouse gas emissions.
Even partisans of using reflective roofs note that a 'winter penalty' in cooler climes may mean their albedo benefits could pale compared to heating requirements.
Recently I have come across numerous articles promoting cool roofs, and although I work for a company that does indeed manufacture cool, white TPO (for thermoplastic olefin) roofs as well as dark-colored materials, I would like to offer some statistics and information that will inform your readers of the benefits of other roof systems.
By having information on the different roof choices, building owners can choose the roof that is best for their location and climate, and that best solution isn't always white.
The Cool Roof Rating Council admits to a "winter penalty" when cool roofs are installed in northern climates. An article in the April edition of Maintenance Solutions magazine that was penned by the CRRC states, "Heating costs may increase slightly for buildings located in cooler regions." CRRC's justification for increasing heating costs and greenhouse gasses was that days (sunlight hours) are longer in the summer than they are in the winter, thus more energy is consumed to cool a building than to heat it.
The article failed to mention spring and autumn. However, information gathered from the U.S. Department of Energy clearly demonstrates that heating is a much more significant factor in energy usage than cooling. For instance, Grand Rapids, Mich., has 7,153 heating degree days versus 508 cooling degree days per year, which means heating equipment runs approximately 14.3 hours for every one hour that cooling equipment runs.
Furthermore, the 2008 Buildings Energy Data Book, published by the U.S. Department of Energy, concluded that space heating accounts for 29 percent of all commercial buildings' energy end-use intensities, while space cooling only accounts for 6 percent. These numbers, combined with the increased heating consumption in northern locales, clearly depict that there is no acceptable justification for increased heating costs in cooler climates.
To illustrate this point, I've chosen three major U.S. cities and provided analyses of energy and carbon footprint models, all generated using the DOE's cool-roof calculator, to help lend some clarification and hopefully dispel some growing cool-roof myths.
• Boston, Mass.: 5,841 HDD versus 646 CDD
• Grand Rapids, Mich.: 7,153 HDD versus 508 CDD
• Albuquerque, N.M.: 4,361 HDD versus 1,211 CDD
Due to the overwhelming amount of heating degree days and positive winter heat gain in these locations, in all instances, energy consumption and carbon emissions are fewer with a dark roof surface than with a reflective surface. This is true even as far south as Albuquerque. It is also important to note that increased insulation dramatically reduces energy consumption in every model.
Reflective, cool roofs are environmentally responsible when properly insulated and installed in warmer climates. Roofing, like most industries, has many solutions to accommodate many sets of circumstances. The media must be responsible when looking for ways to conserve energy and stop global warming. It can be difficult to navigate, especially when the same agency that provides an Energy Star rating for reflective roofing materials also provides a tool that demonstrates this product to be less energy efficient in approximately half of the country.
Many roofing manufacturers, most of which only offer reflective materials, are promoting reflectivity as energy efficient, regardless of climate zone. My company, Carlisle, supports the reflective roofing requirements of the American Society of Heating, Refrigerating and Air-Conditioning Engineers climate zones 1-3, as they are clearly the more energy-efficient roofing materials for predominantly cooled spaces.
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Comments
POSTED BY: Green Earl, May 17, 2009, 12:56 PM
This is coming from a roofers, son, and grandson, now a 30 year pioneer in conservation and renewable energy, solar generation. I agree that heat loss, in generally colder climates of the US, is certainly a consideration. But I would consider a white roof for any climate where temps reach above 100 degrees, say for periods of 3 to 6 months or more. Sorry no factual data this time, just common sense. Heat loss, through attics, or even flat-top roof construction can easily be addressed today with new space age foils and other cutting edge insulation. What I did not see mentioned in the article is the fact that the life-span of a black roof, generally will be less, and require the structure be re-roofed sooner. Measure the cost, carbon footprints on multiple roofing jobs over the life of the structure and I think I can still make a pretty good case for a light or white roof, with added coating of same every 5 to 10 years to increase the roof life-span even longer. Either way, no longer is "That's what the customer wanted" cutting it. In climates like ours, here in the far reaches of N. CA. it's May 17, 2009 and it was 103 degrees here yesterday, today is forcasted 106 and we will see 115 this summer, we always do. No respectable roofer should install a dark or charcoal colored roof in this area. Green Earl, Founder American Energy Conservation Group Producing Negawatts...Since 1981 http://yeswecansolveit dot blogspot dot com
POSTED BY: Baka Greg, May 19, 2009, 12:26 PM
Please expand on the mechanism of how, and how much of, the heat from a dark roof in the Fall/Winter/Spring actually transfers into the building? My understanding was that the heat captured in a dark roof coating has ony two choices of where to go: either downward through the roof insulation to the relatively warm space below OR upward into cooler sky. Seems that because of the higher delta T, and the opportunity for convection, that heat gathered on the dark roof is going to head for the sky in cooler weather...
POSTED BY: Matt Wootton, May 21, 2009, 01:36 AM
Have you factored-in that in colder climates, a well insulated house will have a "white roof" for most of the winter season no matter what colour you choose for your building material? Well-insulated houses maintain their snow-cover for longer periods of time so you might as well take the gain from having a white roof in the summer. Billion's of people living in the tropics can't be wrong and having personally lived/travelled in the Caribbean, India, SE asia and East Africa and Southern Europe one thing that stands out is the fact that 90% of the roofs are white or light-coloured. There is also further evidence to supportwhite roofs in higher latitudes. The heatwaves hitting Northern Europe are greatly magnified by the dark-coloureed slate they use on the older buildings. The elderly leaving in the top floor apartments in Paris literally baked to death. Type of roof is also a factor (flat being much hotter in summer and colder in winter). If we were smart, roof colour wouldn't matter so much. More importantly, roof angle, overhang, placement of windows should all be considered when designing a home. You can get much larger gains in efficiency by simply understanding the min/max angle of the sun at your specific latitude. Build a house for all seasons and paint it whatever colour you want!
POSTED BY: Anonymous User, May 28, 2009, 08:18 PM
I would like to make a clarification that the article referenced from Maintenance Solutions was not written by the Cool Roof Rating Council. Some information must have been referenced form the CRRC website as the original article references: "The Cool Roof Rating Council — www.coolroofs.org — provided information for this article." Information about cooler climates is on the CRRC website, located at: http://www.coolroofs.org/coolroofing.html#heating
POSTED BY: Anonymous User, August 18, 2009, 02:27 PM
Everybody is aware with the fact that darker materials absorb more heat from sun than white/light colors, but an interesting data I want to share that black surface in the sun can be 40°C (70°F) hotter than the reflective white surface. This phenomenon occurs in the case of roofs also and heated roof then transfer their heat to surrounded air and contribute to heat island effect, while reflective metallic/ ceramic roof can reflect 65-75 %solar light. According to California energy commissioner “White roofs can cut a building’s energy use by 20% and save consumers money,” and “The potential energy savings in the U.S. is in excess of $1 billion annually.” http://buildinggreenlife.wordpress.com/2009/08/17/magic-of-white-roofs/