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COOL ROOF RESOURCES & GUIDE
 

GUIDE FOR SELECTING COOL ROOFS
Prepared by the Fraunhofer Center for Sustainable Energy Systems for the U.S. Department od Energy Building Technologies Program and Oak Ridge National Laboratory under contract DE-AC05-00OR22725.

Authors: Bryan Urban and Kurt Roth, Ph.D.

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INTRODUCTION

Cool roofs can help many building owners save money while protecting the environment. This guidebook has been created to help you understand how cool roofs work, what kinds of cool roof options are available, and how to determine if cool roofing is appropriate for your building. If you are planning a new building or replacing or restoring an existing roof, cool roofs should be considered as an energy efficiency option. Cool roof products exist for virtually every kind of roof.

Just as wearing light-colored clothing can keep a person cool on a sunny day, cool roofs use solar-reflective surfaces to maintain lower roof temperatures. Traditional dark roofs can reach temperatures of 150°F (66°C) or more in the summer sun. A cool roof under the same conditions could stay more than 50°F (28°C) cooler.

WHAT IS COOL ROOF?

Cool roofs are roofs that are designed to maintain a lower roof temperature than traditional roofs while the sun is shining. Sunlight is the primary factor that causes roofs to become very hot.

WHY USE COOL ROOF

A cool roof can be desirable to a building owner for several reasons. Cool roofs can

• reduce energy bill by decreasing air conditioning
   needs,

• improve indoor thermal comfort for spaces that
   are not air conditioned, and

• decrease roof operating temperature, which may
  extend roof service life

In many cases, cool roofs cost about the same as non-cool alternatives. The energy cost savings you can realize from a cool roof depends on many factors, including local climate; the amount of insulation inn your roof; how your building is used; energy prices; and the type and efficiency of your heating and cooling systems.

Cool roofs can also benefit the environment, and policymakers may issue cool roof regulations to provide these benefits to society. Cool roofs can

• reduce local air temperatures, which improves air
   quality and slows smog formation;

• reduce peak electric power demand, which can
  help prevent power outages;

reduce power plant emissions, including carbon
   dioxide, sulfur dioxide, nitrous oxides, and
  mercury by reducing cooling energy in building;
  and

reduce heat trapped in the atmosphere by
   reflecting  more sunlight back into space, which
   can slow climatechange.

WHAT QUALIFIES AS COOL ROOF

Typical minimum cool roof requirements are shown in Table 1, and this is what we mean by "cool roof" through this document. A roof can qualify as cool in one of two ways. The first way is by meeting or exceeding both the minimum solar reflectance and thermal emittance values. The alternative way is to meet or exceed the minimum SRI requirement. This allows some roofs that have a low thermal emittance and a high solar reflectance (or visa versa) to still qualify as a cool roof.

Cool roof requirements depend on the roof's slope. Low sloped roofs have a pitch of 9.5° or less (2:12 rise over run), while steep sloped roofs have a higher pitch greater than this. Requirements are usually less stringent for steep sloped roofs. Some heavier roofs – such as those with concrete pavers, ballast, or vegetation – also have less stringent cool roof standards. The weight of these roofs causes them to heat up more slowly, and during the night some of that stored heat is returned to the outdoor environment.

Others use different cool roof definitions. For example, the US Green Building Council's (USGBC) Leadership in Energy and Environment Design (LEED) program currently uses minimum SRI values of 78 and 29 for low and steep sloped cool roofs, respectively. The U.S. Department of Energy (DOE) has decided to implement cool roofs on all buildings whenever practicable. The DOE uses the low sloped cool roof definition from table 1 and defines steep sloped cool, roofs as those with a 3-year aged SRI of 29 or greater. The ENERGY STAR® program specifies minimum solar reflectance (low slope: 0.65 initial, 0.50 aged; steep slope: 0.25 initial, 0.15 aged) and does not consider thermal emittance. To satisfy local building codes or to meet rebate program requirements, be sure to find and use the appropriate cool roof definition.

HOW COOL ROOF WORKS

Cool roofs have surfaces that reflect sunlight and emit heat more efficiently than hot or dark roofs, keeping them cooler in the sun. In contrast, hot roofs absorb much more solar energy than cool roofs, making them hotter. Solar reflectance and thermal emittance are the two key material surface properties that determine a roof's temperature, and they each range on a scale from 0 to 1. The larger these two values are, the cooler the roof will remain in the sun.

Since most dark roofs absorb 90% or more of the incoming solar energy, the roof can reach temperatures higher than 150°F (66°C) when it's warm and sunny. Higher roof temperatures increase the heat flow into the building, causing the air conditioning system to work harder and use more energy in summertime. In contrast, light-colored roofs absorb less than 50% of the solar energy, reducing the roof temperature and decreasing air conditioning energy use.

Reducing the roof's temperature with a cool roof can also increase the need for heating during heating seasons. Later sections of this report show you how to evaluate the resulting cost savings for your building.

Solar Reflectance is the fraction of the sunlight that a surface reflects. Sunlight that is not reflected is absorbed as heat. Solar reflectance is measured on a scale of 0 to 1. For example, a surface that reflects 55% of sunlight has a solar reflectance of 0.55. Most dark roof materials reflect 5 to 20% of incoming sunlight, while light-colored roof materials typically reflect 55 to 90%. Solar reflectance has the biggest effect on keeping your roof cool in the sun.

Thermal Reflectance describes how efficiently a surface cools itself by emitting thermal radiation. Thermal emittance is measured on a scale of 0 to 1, where a value of 1 indicates a perfectly efficient emitter. Nearly all nonmetallic surfaces have high thermal emittance, usually between 0.80 and 0.95, that helps them cool down. Bare, shiny metal surfaces, like aluminum foil, have low thermal emittance, which helps them stay warm. A bare metal surface that reflects as much sunlight as a white surface will stay warmer in the sun because it emits less thermal radiation.

Solar Reflectance Index (SRI) is another metric for comparing the "coolness" of roof surfaces. It is calculated from solar reflectance and thermal emittance values. The higher the SRI, the cooler the roof will be in the sun. For example, a clean black roof could have an SRI of 0, while a clean white roof could have an SRI of 100. Dark roofs usually have an SRI less than 20.

Table 1: Typical Minimum Cool Roof Requirements, California Energy Commission

Roof Type Low Sloped Steep sloped
Solar Reflective[3-year aged] 0.55 0.2
AND
Thermal Emittance [new or aged] 0.75 0.75

OR

 

 

Solar Reflectance Index (SRI) [3-year aged] 64 16
 
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