residential energy use per household has declined by 37 percent, residential energy use per capita has
declined by 27 percent, and commercial energy use per square foot of commercial building space has
declined by 25 percent (Figure 2).
2
These energy intensities have decreased despite two trends toward greater building energy
services. First, the size of homes has increased significantly, which in turn increases heating and cooling
requirements. According to the vice president of research at the National Association of Home Builders,
 as family size decreased almost 25 percent over 30 years, the size of new houses increased about 50
percent, to slightly more than 2,300 square feet today, from 1,500 square feet. 
3
Second, the range of
electric equipment provided in buildings has increased significantly, especially air conditioning in the
South and electronic equipment, televisions, and other  plug loads  in buildings nationwide.
4
Central air
conditioning is now a feature of 85 percent of homes in the United States, up from 34 percent in 1970. 
Examples of technology improvements during the past 30 years help document this progress.
Compact fluorescent lamps, now in common use, are 70 percent more efficient than are incandescent
lamps; refrigerators use 75 percent less energy; and new horizontal axis clothes washers are 50 percent
more efficient than current minimum standards. Between 1978 and 1999, the typical level of insulation
in walls increased from R 11 to R 13, and typical insulation levels in ceilings and attics rose from R 19
to R 30.
5
Advances in window performance have also been notable over the same period. The market
+
penetration of high efficiency low emissivity (low E) coated windows
6
in homes grew to almost 30 percent,
and the use of insulated glass increased from nearly 68 percent to 87 percent. Finally, a research,
development, and demonstration (RD&D) partnership sponsored by the U.S. Department of Energy (DOE)
helped industry replace ozone depleting chlorofluorocarbons (CFCs) in foam insulation and in refrigerants,
consistent with the Montreal Protocol.
7
Yet despite these impressive improvements in the energy intensity of building use over the past
30 years, there is no room for complacency. The U.S. population and economy are projected to grow
+
significantly in absolute terms over the next 50 years, which will likely require a sizeable increase in the
physical U.S. building stock and corresponding energy use. Specifically, the U.S. population is expected
to grow from 295 million in 2005 to 378 million by 2035 and 420 million by 2050.
8
Over the next 
3
Towards a Climate Friendly  
Built Environment
+
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