When we think of green buildings, we tend to think of new ones — the kind of modern, solar-paneled masterpieces. But the US     1     (have) more than 100 million existing homes, and it would be incredibly wasteful to tear them all down.

An enormous amount of energy and     2     (resource) went into the construction of those houses. And it would take     3     average of 65 years for the reduced carbon emissions from a new energy-efficient home to make up for the resources       4     (lose) by destroying an old one. So in the broadest sense, the     5     (green) home is the one that has already been built. But at the same time, nearly half of US carbon emissions come from heating, cooling     6     powering our buildings. They can’t deal with climate change without dealing     7     existing buildings.

The oldest homes tend to be the least energy-efficient, mainly due to the tiny cracks and gaps that expand over time and let in more outside air.

    8     (fortune), there are a vast number of relatively simple changes     9     can green older homes. And efficiency upgrades can save more than just the earth; they can help protect property owners from     10     (rise) power costs.