As solar and heat pump prices fall, these highly energy-efficient homes are paying for themselves faster. Here’s how they work and why they’re spreading northward. LAMBERTVILLE, Mich.—On a drive down a country road, builder Bill Decker gives an off-the-cuff seminar about energy efficient homes. He shifts from carpentry to electrical engineering, and then to theology—his belief that his faith compels him to take care of the earth. Every few minutes, he pauses and points out a house his family-owned company has built.