Review and Critical Analysis of International UHI Studies

� residential sector – not yet cost effective except for premium roofs (metal, tile), could be
very cost effective especially when ducts are in attic
In other words cool roofs become a cost effective option in many cases where other aspects of the
building and HVAC design are inefficient.
5.4.2 Cool Pavements (Roadways)
The existing economic calculations regarding cool pavements suggest they struggle to break even
if just the life cycle value of energy and local pollution abatement are considered. If the value of
the negative radiative forcing abatement impact is monetized they become far more cost effective.
Reflectivity of pavements is also a safety factor in visibility at night and in wet weather, affecting
the demand for electric street lighting. Street lighting is more effective if pavements are more
reflective, which can lead to greater safety; or, alternatively, less lighting could be used to obtain
the same visibility. These benefits have not yet been monetized.
5.4.3 Economics of Shade Trees
UHI reduction from tree planting has been found to lower peak urban diurnal temperatures by
0.3 to up to 3K and to give air conditioning savings of $5‐$10 per 100m 2 of roof area for residential
and commercial buildings. For example a study of the impact of planting 100m trees in the Los
Angeles basin estimated $270 million from direct and indirect energy savings and smog
reduction benefit of which the value of smog reduction accounted for about 2/3 rds of the total. The
present value of planting trees was found to be $68 per tree for the direct energy savings and $24
per tree for indirect energy savings. The present value of smog savings was about $120/tree.
Estimates of the cost of planting, pruning and tree removal vary and have been found to vary
from $45 to $500 per tree. On this basis tree planting is often not cost‐effective in pure energy
savings terms and even if the value of local air quality improvement is incorporated the cost‐
benefit calculations do not always favor tree planting, however, the aesthetic and quality of life
benefits of greener spaces are not included in this assessment and they may well tip the balance
firmly in favor of greater urban greenery.
5.4.4 The Value of Mitigation Benefits from the Negative Radiative Forcing Effect
The value of the negative radiative forcing effect is purely in terms of the public good value of
reduced global warming. According to the only global estimate so far conducted this would be
worth a massive US$1,100 in carbon offsets were CO2 valued at �$25/tonne of CO2 and were all
viable urban roofs and pavements to have their albedo increased by 0.1. This is equated to a 44 Gt
CO2 emission offset.
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