How does solar reflectance of metal roofing compares to other roofing material?

How does solar reflectance of metal roofing compares to other roofing material?

How does solar reflectance of metal roofing compares to other roofing material?

Solar reflectance (SR) is the ability of a material to reflect solar energy from its surface back into the atmosphere. The SR value is a number form 0 to 1.0. A value of 0 indicates that the material absorbs all solar energy and a value of 1.0 indicates total solar reflectance

The solar reflectance of metal roofing is a function of the surface of the product. Metal roofing can be offered with pre-painted surfaces, natural or mill finishes, and even granular-coated surfaces. A mill finish such as unpainted Aluminum-Zinc coated steel sheet has a solar reflectance of 0.68-0.78 depending on the surface treatment used. Other natural metals such as zinc, copper and aluminum have somewhat lower solar reflectance values. When a paint system is applied to metal roofing, the solar reflectance depends on the color and type of pigment in the paint film. Reflectance values can range from 0.10 up to greater than 0.75. Granular-coated metal roofing can have solar reflectance values of 0.05 to 0.30 depending on pigment and glazing used on granules.

Solar Reflectance

Metal Unpainted 0.50 – 0.80
Metal Painted 0.10 – 0.75*
Comp Asphalt Shingles 0.05 – 0.28
Black EPDM 0.05 – 0.10
Single Ply Membrane 0.70 – 0.80
Smooth Modified Bitumen 0.05 – 0.25
White Granular Bitumen 0.20 – 0.30
Concrete/Clay Tile 0.20 – 0.75*
*Depends on colour & pigment    Source: ORNL and LBNL database

In comparison to these values, asphalt shingle has a relatively low solar reflectance of 0.05-0.28 even with cool granules. Single-ply membranes in light colors have high initial solar reflectance values but some are prone to dirt pick-up which can reduce the aged reflectance values. Darker roofing products such as BUR have low solar reflectance values due to their color. The above
table, which is a compilation of data from ORNL and LBNL summarizes the differences.

Let us look at concrete/clay tile and painted metal roof. The upper limits for solar reflectance may look to be equal but there is a difference when we compare attic temperatures and energy usage.

Monitoring the attic temperatures over the course of time also shows how the roof products heat up and cool down during a day. This curve shows that the white painted metal roof reaches a slightly higher peak temperature compared to a white tile roof (due to the difference in the solar reflectance), however in the afternoon and early evening hours, the metal roof, with lower thermal mass, dissipates its heat better than the tile. What this means is that the homeowner will realize lower air conditioning usage in the late afternoon and early evening hours when compared to a tile roof system.

Results from experiments (i.e. experiments on various white coloured types of roofing material) showed that white painted cool metal roof can save 23% in cooling energy, which was the highest of the roof systems tested.

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