Cool Roofs: How They Work

What makes a roof cool?

The "coolness" of a roof is determined by two properties of roofing materials and their combined effects on temperature:

  • Solar reflectance : ability to reflect sunlight
  • Thermal emittance : ability to cool down by emitting heat that is absorbed

These properties explain how cool roofs absorb less heat and stay up to 28-33°C (50–60°F) cooler than conventional rooftops on the hottest summer days.[1] Both of these properties are measured on a scale from 0 to 1 (0-100%), with higher values indicating a cooler roof. Another measure often cited is the solar reflectance index (SRI). Solar reflectance and thermal emittance can be combined to calculate a material’s SRI, which is a measure of a roof’s coolness, with higher values again indicating a cooler roof.


science of solar reflectance image

Figure 3. Cool roofs stay cool because they have high solar reflectance and thermal emittance (source: Cool Roof Rating Council)

Cool roof options are available for a wide range of materials, but the values for solar reflectance and thermal emittance vary with the material. See the below table for the approximate ranges of these radiative properties for various cool roof options.

Cool Roof Material Options:

Example Images

Product

Application

Solar Reflectance (initial)

Thermal Emittance (initial)

Solar Relectance Index (initial)

asphalt shingle

Asphalt shingle

Pitched roofs

0.20 – 0.30

0.85 – 0.90

16 – 32

clay tile

Clay tile

Pitched roofs

0.40 – 0.50

0.80 – 0.90

41 – 58

concrete tile

Concrete tile

Pitched roofs

0.20 – 0.60

0.80 – 0.90

14 – 72

coated metal tile

Coated metal

Pitched roofs

0.25 – 0.70

0.80 – 0.90

21 - 86

liquid applied coating

Liquid-applied coating

Low-slope roofs

0.60 – 0.90

0.85 – 0.90

71 - 114

modified bitumen

Modified bitumen

Low-slope roofs

0.60 – 0.75

0.85 – 0.95

71 – 94

single-ply membrane

Single-ply membrane

Low-slope roofs

0.70 – 0.85

0.85 – 0.95

85 – 108

Cool roofs in practice

All cool roofs reflect more sunlight than conventional roofs, but that doesn’t mean all cool roofs look alike! One popular cool option for flat, low-slope, roofs are white roofs, which are available in a variety of materials. For pitched roofs (like on many residential buildings), cool-colored roofing materials can reflect more sunlight without sacrificing traditional aesthetics.

White roofs

White buildings with white roofs have been common architectural practice in Mediterranean and Middle Eastern societies for thousands of years because they keep the interiors of buildings cooler. It has taken some time, but white roofs are now spreading throughout other regions of the world as a cost-effective way to reduce indoor temperature and energy use.


Cool versus standard shingle - thermal emitttance

Figure 4. Cool asphalt shingles look just like conventional shingles to the naked eye, but thermal imaging reveals a key difference - they stay much cooler in the hot sun. (source: LBNL-Heat Island Group)

White materials are a popular cool option for building surfaces that cannot be seen from the street, such as low-sloped “flat” (that is, nearly horizontal) roofs. In some parts of the world, pitched roofs may also be white. White roofing products are available as membranes, coatings, coated metal, tile, and ballast roofs, among other materials.

Cool-colored roofs

There are also cool alternatives that mimic the appearance of traditional roofing products. “Cool-colored” roofs increase reflection of sunshine while maintaining the color and aesthetic of nonwhite roofing products. They reflect more invisible infrared radiation while reflecting the same amount of visible radiation as the conventional products. Cool-colored products are a good choice for pitched roofs that are visible from the street, so that cool-colored asphalt shingles, metal, and tiles products can preserve the aesthetics of the building.


Footnote:

[1] Calculated under the standard summer afternoon weather specified in ASTM Standard E1980-11, using the medium wind speed condition.

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