US homes have gotten huge — offsetting the gains from energy efficiency

Source: Vox
  The average size of American homes has ballooned since 1970. While American homes today are about 31 percent more energy-efficient (as measured in energy use per square foot) than they were in 1970, their growing size has negated these gains in efficiency. In fact, since square footage has risen about 28 percent, on average, there's basically been no change in overall energy intensity. That being said, the shift in US population to the South and Southwest has saved energy for heating and cooling homes.
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