On hotter days, operable windows along the roof line and vents in the floors allow hot air to escape out the top and draw the same cooler air up from the lawn. By relying on this energy-efficient system during all but the hottest months (when they run a small air-conditioning unit only in the evenings when they’re home), the couple has chopped their monthly power bills in half—no small feat in a climate where summertime temperatures can top 115 degrees.