Review and Critical Analysis of International UHI Studies

the global emitted CO2 offset potential for cool roofs was about 24Gt of CO2 [035]. This estimate
however, is a result of aggregation of previous results/estimates/phenomena observed at the
micro scale, aggregated and translated to the macro scale – such estimates should be approached
with caution.
Fourteen studies examined the temperature reduction ode to increasing the albedo of the roofs. It
was first noted that for low albedo roofs the temperature difference between the surface and the
surrounding air can be as high as 50K and secondly that for high albedo surfaces this can be
lowered to 10K [007, 025, 026, 027, 028, 059, 067, 070, 072, 074, 079, 087, 112, 205, 236]. One recent
study, that conducted extensive testing on cool colored roofing materials found that on average
cool dark roofs were 14K cooler than the traditional dark roofs (with a 0.25 difference in albedo
between them) [025].
Two studies examined the relationship between the roof temperature and the internal building.
They found that for cool dark roofs conduct about 20‐40% less heat into a home’s conditioned
space and that a building with cool colored roofing can have building air temperature inside the
building reduced by 7K over the traditional colored roofs [025, 079].
Two studies tested new paints and coating for roofing and shingles:
� The first study tested eleven thermochromic coatings for building surfaces and compared
them to color‐matched cool surfaces and a traditional roof surface. Thermochromic coatings
have the ability to change color depending on their temperature, which in this case was
30°C, from colored (absorptive) to colorless (reflective). The study showed that mean daily
surface temperatures were lowest for the thermochromic coated surfaces, up to 38.4°C, up to
44.6°C for color matched surfaces and up to 48.5°C for a traditional surface. Thermochromic
coatings had higher solar reflectance values at colored and colorless stages compared to the
other surfaces. Further research is currently being conducted to improve the material’s
photodegradation [205].
� In the second paper Levinson et al at LBNL produced a recent paper on a new technique to
produce cool colored roofing products. Two layers of quick‐dry paints were applied to 24
tiles and shingles. The first layer was a white basecoat of titanium dioxide rutile which
increased solar reflectance, the second layer was a cool color topcoat (red, brown, green and
blue) with weak near‐infrared absorption and/or strong near‐infrared backscattering. The
results of the experiment were that the dark brown tiles and shingles had the lowest solar
reflectance value and the light green had the highest in both tiles and shingles. The paints
increased the solar reflectance in over 50% of the samples by 0.25 which means that they can
be successfully used on darker products making them more solar reflective than
conventionally colored products. Further increases could be achieved by increasing the
thickness of the paint and making further alterations to the materials in the top and base
coats [236].
In general, most studies recognized the savings are clearly smaller for buildings with higher roof
insulation and concluded that albedo modification are more effective as an energy reduction
measure for buildings with little or no insulation [015, 025, 079, 176]. One study indicated that
A/C energy savings were approximately inversely related to roof insulation ‐ from 11% savings in
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