Thursday, September 29, 2011

Measuring Weathered Cool Roofs, Updated DOE Roof Savings Calculator

A 2010 Lawrence Berkeley National Laboratory study showed that the average annual savings when substituting a .55 weathered reflectance cool roof for a .20 weathered reflectance gray roof is 5.02 kWh per square meter of roof surface. The study also showed that the savings are greater in areas of the country with higher air conditioning loads where an upper range benefit of 7.69 kWh per square meter was possible.

Cool roofs having the greatest benefit in air conditioning-heavy climates makes logical sense, and is in line with an important change to the most recent version of the ASHRAE 90.1 energy code, making cool roofs a prescriptive requirement for Climate Zones 1 through 3. Roofs in those areas — the South, Southwest, and most of California — that adopt the code must install roofs with a minimum three-year weathered reflectance of .55, and three-year weathered emittance of .75. Infrared emittance is the measure of how much radiation a surface re-emits to the atmosphere.

Using weathered results, as opposed to new, represents a shift in the industry in how cool roof effectiveness is measured, as well. New cool roofs can exhibit reflectance numbers as high as .75 or .80. Energy Star labels roofing products with initial reflectances of .65 on low slope roofs. The industry is beginning to understand that the true measure of how well the roof will perform depends not on its initial reflectance number, but on its weathered. Every cool roof gets soiled and loses performance over time, so it's very important to base energy calculations on aged value, not new.

That is exactly what the Department of Energy Roof Savings Calculator does, and allows you to enter several variables such as area of the country, building type, heat and cooling type, efficiency, as well as characteristics of your existing roof and proposed cool roof including weathered reflectance and emittance ratings. The resulting calculation shows the potential annual energy savings (or loss) with a cool roof.

According to André Desjarlais, group leader of the Building Envelope Program at Oak Ridge National Lab, the calculator was relaunched late last year to accomplish a few goals:
  • First, it combines two calculators into one (previously, the Environmental Protection Agency (EPA) and DOE each had its own cool roof calculator). These two calculators were programmed with slightly different assumptions and therefore gave different results. The new calculator combines research and analysis from both agencies to be more comprehensive.
  • Second, it's more sophisticated than either of the previous two in terms of how it allows you to enter data about a facility (basically, there are more variables - the old calculators didn't specify a building type, they assumed the roof was floating above space, etc. and now the calculator is more sophisticated and thorough to cover more scenarios).

Part of DOE's Cool Roofs Roadmap is to expand and improve the Roof Savings Calculator and to benchmark its results against actual experimental data. Another part of the plan is to study several different types of roofing, such as vegetative roofs and roof coatings like those from Conklin, to determine energy savings potential.

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