of energy consumption. Therefore if the potential for CO
2
reduction is judged by the amount of energy
used, then the greatest potential among residential users lies with single family residences.
Commercial buildings range widely in size, energy intensity, function, and ownership. Table 2
characterizes them according to primary function. Offices dominate, both in terms of square footage 
(22 percent) and energy consumption (21 percent). 
In sum, these statistics suggest that the most obvious opportunities to reduce GHG emissions
through improvements in end use efficiency are space heating (especially in the residential sector), air
conditioning, lighting (especially in the commercial sector), and water heating (especially in the residen 
tial sector). In the residential sector, the biggest opportunity lies with single family residences; in the
commercial sector, office buildings are the most important single target.
B. New Construction versus Renovation
For policy purposes, it is important to distinguish between new and
existing buildings and communities.
New construction can more easily incorporate novel,
low GHG technologies and is therefore often a harbinger of future trends. In addition, new building tech 
nologies are often introduced in the new construction market but then spill over into the building retrofit
and renovation trades. While new buildings amount to only 2 to 3 percent of the existing building stock
+
in any given year, new construction practices will have an increasing impact over time. 
The value of U.S. construction in 2000 is estimated to have been $1.3 trillion (2000$) including
new construction, renovation, heavy construction, and public works. This represents 13.2 percent of U.S.
GDP. New buildings construction represents almost half of this total ($562 billion), and building renovation
was valued at $265 billion.
22
Given the longevity of buildings and the amount spent annually on renova 
tion, the existing building market represents a key, yet often harder, opportunity for GHG reduction.
The vast majority of the buildings that exist today will still exist in 2015, and at least half of the
+
current stock will still be standing by mid century. As a result, retrofitting structures and upgrading the
efficiency and operation of their heating, ventilation, and air conditioning systems offer an important
near term opportunity to significantly reduce GHG emissions. Existing communities also can be made
more efficient by adding new structures in passed over parcels of land, allowing mixed uses that reduce
transportation requirements, and building new pedestrian and bicycle paths to encourage non motorized
11
Towards a Climate Friendly  
Built Environment
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