Cool Roofs

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What Is a "Cool Roof"?

Cool roof materials have two important surface properties: a high solar reflectance – or albedo – and a high thermal emittance. Solar reflectance is the percentage of solar energy that is reflected by a surface. Thermal emittance is defined as the percentage of energy a material can radiate away after it is absorbed.

Cool roofs reflect heat well across the entire solar spectrum, especially in the infrared and visible wavelengths. The less solar radiation materials absorb, the cooler they are. In addition to absorbing less heat, the coolest roofing materials radiate away any absorbed heat.

This graphic compares the reflectance of urban roof materials, which can vary significantly. Conventional surfaces- such as tar and gravel roof- have low solar reflectance (.03 to .18). Other surfaces- such as a light colored roof- have high solar reflectance (.50 to .90), which can lead to lower temperatures.

Various roof albedos

Solar reflectance and thermal emittance have noticeable effects on temperature. Conventional roof surfaces have low reflectance (0.05 to 0.25) and high thermal emittance (typically over 80%) and heat up to 150 to 190°F (66 to 88°C) at midday during the summer. Bare metal or metallic surfaced roofs have high solar reflectance (0.5 or higher) and may have low thermal emittance (20 to 60%, depending on their surface treatment) and warm to 140 to 170°F (60 to 77°C). Cool roofs with both high reflectance and high emittance warm to only 100 to 120°F (38 to 49°C) in the summer sun.

Most cool roof applications have a smooth, bright white surface to reflect solar radiation, reduce heat transfer to the interior, and save on summertime air conditioning. These properties also can extend the life span of a roof. By limiting the quantity of absorbed solar energy, damage from ultraviolet radiation and daily temperature fluctuations – which cause repeated contraction and expansion – can be reduced. A range of cool roof products are available for use in low-sloped (mainly commercial) and sloped (mainly residential) building applications.

How Much Solar Radiation Do Cool Roofs Reflect?

EPA's ENERGY STAR ® Roof Product Program has cool roofing specifications for both low-sloped and sloped roofs. Low-sloped roofs must have an average initial albedo of at least 0.65, and sloped roofs must have an average initial albedo of 0.25 or more. While thermal emittance is not a qualifying criterion for the ENERGY STAR ® label, a rating of 80% or more further reduces heat transfer to the indoors.

Do Cool Roofs Stay Reflective over Time?

The solar reflectance of cool roofs tends to decrease over time. This is because surface particles like dew, dust, and air pollutants accumulate. Another factor that affects long-term solar reflectance is slope – the greater the angle of roof slope, the more dirt and particles dislodge and fall off the roof surface. Studies done at the Florida State Energy Center show that during the first two years of a cool roof's life span, solar reflectance can deteriorate up to 11% if regular maintenance is not provided.

Research on cool roof coatings by the Department of Energy's Lawrence Berkeley National Laboratory (LBNL) quantifies the effects of weathering and dirt accumulation. The results of the study show that solar reflectance decreases most in the first year (with an average reduction of 0.15) but that the magnitude of reductions decreases in later years.

Another LBNL study showed that washing a weathered cool roof membrane resulted in a significant solar reflectance increase. This indicates that if a roof collects large amounts of particulate matter or leaf debris, washing the roof every few years can help maintain high solar reflectance. LBNL recommends considering resistance to dirt accumulation among other factors when selecting a cool roofing material.

Another way to address the issue of declining albedo values is to reapply roof coatings periodically. Some roof coating distributors recommend applying a new coat after 10 years.

What Is the Energy Savings Potential of a Cool Roof?

Monitoring of over 10 buildings in California and Florida demonstrates that cool roofs save residents and building owners 20-70% in annual cooling energy use. These products reduce heat transfer to the indoors, lowering air conditioning costs.

Bar chart demonstrating energy savings in dollars from installation of cool roofs in US cities. For every 1000 square feet of cool roofing on air conditioned buildings, savings potential ranges from $4 in Philadelphia, PA to $34 in Phoenix, AZ. An estimate of cool roof energy savings can be determined by considering the following factors:

Air conditioning: cool roofs can reduce summertime energy use in air conditioned buildings. In buildings without air conditioning, cool roofs can improve comfort by reducing top-floor temperatures.
Roof insulation: cool roofs save more energy when installed on buildings with low roof insulation.
Attic radiant barrier: these structures reduce the energy saving potential of cool roofs.
Attic ventilation: buildings with low attic ventilation see a greater benefit from a reflective roof.
Local climate: cooling energy savings are typically greatest in areas with long, sunny, hot summers.

Visit EPA's ENERGY STAR® Roofing Comparison Calculator to estimate the amount of energy and money that can be saved by installing ENERGY STAR ® labeled roof product on homes or buildings.

HIRI's Energy Savings page has more information on how using the range of heat island reduction measures – installing cool roofs, planting trees and vegetation, and using cool paving – can save energy by lowering temperatures and rdecreasing air conditioning demand.

Results of UHIPP 11-City Energy Savings Study

LBNL conducted an analysis to estimate potential energy and monetary savings resulting from the use of light-colored roofs on residential and commercial buildings in 11 U.S. Metropolitan Statistical Areas . The study estimates reductions in peak power demand and annual cooling electricity use that would result from increasing the solar reflectance of the roofs.

Annual, citywide results are shown in 1993 dollars. Energy savings are calculated by comparing decreased summertime costs (from lowered air conditioning demand) with any observed increases in wintertime heating expenditures. The results of research studies indicate that in most U.S. climates, summertime air conditioning energy savings significantly exceed this " winter penalty ."

What Are Some Examples of Action?

California's Cool Savings Program provided rebates to building owners for installing roofing materials with high solar reflectance and thermal emittance. The highest rebate went to roofs on air conditioned buildings, while buildings with rooftop ducts and other non-residential buildings were eligible for slightly less. The program aimed to reduce peak summer electricity demand and was administered by the California Energy Commission.
LBNL estimates that a large (100,000 ft²) retail store in Austin, TX will achieve annual abated energy savings of $7,400. The store replaced a black rubber roof membrane with a white thermoplastic alternative.
The City of Chicago's energy code requires that roof installations on most commercial, low-sloped air conditioned buildings have an initial solar reflectance greater than or equal to 0.25 to help reduce the urban heat island effect.
Georgia instituted the "Georgia White Roof Amendment," which requires the use of additional insulation for roofing systems whose surfaces do not have test values of 0.75 or more for both solar reflectance and emittance. The Amendment goes beyond a 1994 congressional provision, which requires compliance with the American Society of Heating, Refrigerating, and Air-Conditioning Engineers 90.1 energy efficiency standards. This regulation will serve as a model for changes in the building codes of other southern states because it addresses both energy conservation and environmental concerns.