White Roofs
for
Cool Homes
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If a building is too warm in the summer or uses
too much air conditioning energy, then the occupants are probably considering
options to reduce air conditioning energy use and improve comfort. Reroofing
is a good time to add more ceiling insulation, radiant barriers in the
attic, and attic venting. Another option is to install reflective roofing
materials.
In the summer, the roof receives strong radiant
energy from the sun. If this energy is absorbed into the attic, insulation
and ventilation can reduce its penetration into the living space. But a
dark roof can reach temperatures of 180°F on a sunny, windless day,
and some of the heat from the roof inevitably makes it into the building.
A reflective roofing material can reject this heat before it has a chance
to penetrate indoors.
Look for a material with a high solar reflectance
(sometimes called albedo). High solar reflectance is usually accompanied
by a bright or white appearance. The best commercially available materials
are usually smooth and white. There are, for example, white roof coatings
that can be applied over asphalt shingles and most other roofing substrates
(see Table 1). When first applied, these can provide
a solar reflectance of up to 80%, which means that only 20% of the sun's
energy is being absorbed as heat. White roof coatings do weather and get
dirty, however. After several years they may reflect only about 50% of
the incoming solar flux. This is still a significant benefit, but the degradation
is worth keeping in mind if the payback is long.
Some coatings, termed ceramic by manufacturers,
contain microscopic glass spheres. Reflectance tests show that these materials
perform similarly to other white coatings, with reflectances above 80%.
Other coatings are described as elastomeric, referring to their ability
to stretch rather than break.
Conventional "white" asphalt shingles, in contrast,
typically reflect only about 25% of sunlight, because of their low brightness--they
are actually gray--along with their rough texture and black substrate.
Premium white asphalt shingles use a whiter white granule, providing a
reflectance of up to 35% (see
Table 2).
Solar reflectance isn't the only property to
look for in a roofing material. It should also have a high infrared emittance
to help the roof shed heat by reradiation. Most materials do--with the
notable exception of aluminum roof coatings. Aluminum will stay warmer
at night, while a white roof coating will radiate more of its stored heat
back to the sky. For this reason aluminum will not perform quite as well
as a white material with similar solar reflectance.
Regardless of reflectance, the material from
which the roof is constructed also affects how well it sheds heat. For
instance, curved tile roofs and wood roofs usually allow air to circulate
through them, helping to remove solar heat.
Paul Berdahl is a physicist with Lawrence
Berkeley National Laboratory.
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