When we think of green buildings, we tend to think of new ones—the kind of high-tech, solar-paneled masterpieces that 1 the covers of architecture magazines. But the US has more than 100 million existing homes, and it would be incredibly wasteful to tear them all 2 and replace them with greener versions. An enormous amount of energy and resources went into the 3 of those dwellings. And it would take 65 years for the reduced carbon emissions from a new energy-efficient home to make up 4 the resources lost by destroying an old one. So in the broadest sense, the greenest home is the one that has already been built. But at the same time, nearly half of US carbon emissions come from heating, cooling and 5 our homes, offices and other buildings.“You can’t deal with climate change without dealing with existing buildings,” says Richard Moe, the president of the National Trust. With some 6, the oldest homes tend to be the least energy-efficient. Houses built before 1939 expand 7 time and let in more outside air. 8, there are a tremendous number of relatively simple changes that can green homes, from historic ones like Lincoln’s Cottage to your own post-war house. And efficiency 9 can save more than just the Earth; they can help 10 house owners from rising power costs. Moreover, a nationwide effort to improve existing buildings could create hundreds of thousands of green jobs.