Review and Critical Analysis of International UHI Studies

4.5.1 Review for Cool Roofs:
4.5.1.1 Cool Roof Principles
The lower the albedo of the surface roofing material of the more sunlight that falls on it is
converted into heat rather than reflected back into space. The higher the emissivity of the roof
surface the more that the heat stored within it is radiated away. So called “cool roofs” exploit
these two phenomena by using combinations of high‐albedo and high emissivity materials to
minimize the amount of sunlight that is converted to heat in the roof material and maximize the
amount of heat that is radiated away from the roof. This results in a cooler roof than one built of
materials that trap more of the sun’s heat within them. In principle the use of cool roofs will tend
to lower cooling loads and cooling energy demand because they reduce the amount of solar
energy entering the conditioned space from the roof and thus reduce the cooling loads. For the
same reasons cool roofs tend to increase heating loads during the heating season in non‐
equatorial climates because less of the sun’s energy is converted to heat in the roof material and
conducted into the conditioned space. Nonetheless, as there is less solar energy in the winter than
the summer the quantity of solar energy converted to heat within the roof is greater in the
summer than the winter, thus raising the roof surface’s albedo and emissivity displaces more
summer heat gain than winter heat gain.
As roofs are either horizontal, or angled off horizontal, a significant proportion of the solar
radiation reflected from the roof will be directed back to the sky vault. This means that increasing
the albedo of roofs not only has the potential to lower air conditioning loads, and thereby lower
energy use and associated CO2 emissions, but also has the potential to reduce the amount of solar
energy trapped in the troposphere by increasing the reflection of solar radiation back into space.
Thus in theory there are two mechanisms by which cool roofs can mitigate global warming:
a) by lowering space conditioning loads and associated power‐related emissions
b) by reducing the amount of solar radiation converted into tropospheric heat.
Cool roofs will only directly lower total annual space conditioning loads if they cause a greater
reduction in cooling energy than they cause an increase in heating energy over the year. This is
dependent on:
� The degree to which there is a change in the roof surface material temperature from the use
of the cool roof technology.
� The thermal coupling of the roof space to the conditioned space i.e. the extent to which
internal space conditioning loads are influenced by the roof surface material temperature.
� Whether or not the building stock has both active cooling and heating systems installed
(otherwise the impact is only felt via changes in thermal comfort, which is the trigger event
for the installation of active space conditioning when thermal discomfort outweighs the
inconvenience and cost of installing and operating space conditioning).
� The respective energy efficiency of the active‐cooling and heating systems.
� The control regime of the active‐cooling and heating systems.
Review and Critical Analysis of International UHI Studies
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