Roof Coatings, White
White roof coatings contain transparent
polymeric materials, such as acrylic, and a white pigment, such as
titantium dioxide (rutile), to make them opaque and reflective.
Other white pigments sometimes used are the anatase form of titanium
dioxide, and zinc oxide. These coatings typically reflect 70 to 80 %
of the sun's energy. Despite the white appearance, these pigments
strongly absorb the 5 % or so of the sun's energy which falls in the
ultraviolet. Thus, the pigments help protect the polymer material
and the substrate underneath from uv damage.
These coatings are applied in thicknesses
considerably greater than typical white paints, ranging up to about
1 mm. Some of the Oak Ridge data in the table show how the
reflectance increases with thickness. The substrate must be clean
and compatible with the coating system. The achievement of the very
highest reflectance values requires sufficient pigment and a smooth
substrate. If the substrate is already light in color, it can be
made highly reflective with less pigment (fewer coats).
To help maintain the high reflectance of a
freshly applied white coating, several issues are important. A
completely horizonal roof, with ponding water after rain, is likely
to become quickly soiled, with a corresponding loss in reflectance.
Of course it is also very likely to fail by leaking! A mildewcide
additive can retard biological growth with its resulting stains.
There is some variation in how tightly dirt adheres to
Roof Coatings, Aluminum
Aluminum roof coatings generally employ an
asphalt-type resin containing "leafing" aluminum flakes.
The term leafing refers to the tendancy of the aluminum flakes to
accumulate at the exposed upper portion of the coating, which is
accomplished with specialized coatings on the flakes. Thus the upper
surface is a nearly continuous aluminum layer, which protects the
asphalt material from the sun's ultraviolet rays. The aluminum
flakes greatly enhance the solar reflectance over the 4 % value for
bare asphalt, to above 50 % for the most reflective coatings. The
industry regards a visible reflectance above 50 % as a bright
coating. (For aluminum coatings, it happens that the visible
reflectance is roughly equal to the solar reflectance. This is not
true in general due to the fact that about 1/2 of the solar energy
content is in the invisible near-infrared region.)
While the 50 % solar reflectance of a
bright aluminum roof coating is a great improvement over the
performance of a black material, the aluminum content has the
offsetting effect of lower infrared emittance.