Roof systems like those from Conklin that reflect the sun’s rays can save energy and money in warm climates year-round and during hot summer months in more seasonal climates, but is cool roofing beneficial in climates where over the course of the year heating energy loads dominate cooling? In other words, are cool roofs beneficial in cold climates?
The perfect roof system
The ideal roof would be 100% reflective in the summer and 100% absorbent in the winter, but unfortunately a roof can’t change its reflective and emissive properties, so the general energy efficiency of a cool roof must be assessed by comparing the energy saved during the hot summer months to the energy consumed, or the “heating penalty,” during the heating season.
What skeptics say
Skeptics of the energy benefits of cool roofs claim cold climates where heating degree-days vastly outnumber cooling degree-days, the heating penalty would be severe enough to render a cool roof counterproductive. Heating and cooling degree-days are measures of the number of degrees the average daily temperature falls below or above 65 degrees.The main purpose of heating and cooling degree-days is to serve as an index of heating and cooling energy loads over defined time periods, but they also help determine whether certain building strategies are appropriate for a particular geography.
Calculators developed separately by the U.S. Environmental Protection Agency and the Department of Energy’s Oak Ridge National Laboratory, allow users to enter the thermal properties of the proposed roof, amount of insulation, cost of energy and efficiency of the building’s HVAC systems, among other things. The calculators are not meant to be precise indicators of specific building performance - rather, they help give a general indication of how certain roofing materials with specific thermal properties will perform in various parts of the country. In many cases, even in the cold climates the calculators still yield a yearly net savings for using cool roofs.
Cool roofs counteract the winter heating penalty
There are at least five reasons why the heating penalty in the winter time isn’t nearly as severe as it could be and why the summer cooling savings, to some degree, are able to counteract the winter heating penalty in cold climates:
- During the winter, the solar angle is lower so reflectivity and absorption aren’t as important. Reflectivity and absorption are more critical during the summer when the sun is higher in the sky and solar radiation is hitting the roof directly.
- The days during winter months are shorter so less total energy is hitting the roof to be absorbed or reflected over the same period of time as during the summer.
- The ratio of cloudy to sunny days increases during the winter, so again, not as much solar energy is striking the roof.
- Snow piled up on the roof during parts of the winter reflects the sun’s energy. Therefore, it doesn’t matter how reflective or absorbent the roof is.
- In many cases, resources cheaper than electricity, such as natural gas or oil, are used to heat buildings in the winter.
Cool roofs in seasonal climates can be beneficial for other reasons
Cool roofs can have more impact on energy costs than energy use. Cool roofs cut energy use during peak demand times during summer when rates are highest and help reduce the demand charge that a facility pays all year on the basis of its greatest energy use. In fact, some northern utility companies offer rebates and incentives for cool roofs to help cut down on the peak demand load. The idea is to help ensure that there will be enough energy to go around, avoiding brownouts.
"The only building that won’t benefit from a cool roof energy-wise is one that’s not air-conditioned,” says Hashem Akbari, staff scientist and group leader for Lawrence Berkeley National Laboratory’s Heat Island Group.
Akbari’s group also researches measures cities can adopt to mitigate the urban heat island effect, another area in which cool roofs are invaluable, even in seasonal climates. The urban heat island effect is a phenomenon wherein dark building materials, dark pavement and lack of vegetation create a 6 to 8 degree difference between downtown areas of major metropolitan cities and outlying suburban areas. The elevated temperature means that cities as a whole and buildings specifically consume more cooling energy. Additionally, as the air temperature increases, air quality decreases, putting people at further risk for smog-related health problems.
Because of the lower roof surface temperature on roofs with a high emissivity and high reflectivity, urban heat islands can be reduced with widespread use of cool roofs. This is true even in cities with seasonal climates. A study performed by Akbari and his team in Toronto shows that if heat-island-reducing measures, including cool roofs, were adopted widely, the city could save more than $10 million a year on energy costs.
On a national level, widespread adoption of cool roofs - independent of other mitigation measures - could save close to $750M in energy costs in major urban areas, LBNL estimates.
“It’s a collective thing for building owners,” says Greg Crawford, executive director of the Cool Metal Roofing Coalition. “If several buildings begin installing reflective roofs, it will lower the temperature of the urban area and reduce the natural load on building systems.”
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