Imagine a home so efficient it didn't need a furnace. Sound impossible? It's a revolution taking place now in Germany, not a land particularly known for its warm climate. These so-called "passive houses" are designed to be so airtight they get all the heat and hot water they need from the amount of energy needed to run a hair dryer. Through the use of thick insulation and complex doors and windows, the airtight seal allows for the circulation of existing heating through a special ventilation system. And forget the idea of everyone wearing sweaters. The circulation of existing heat allows for all rooms to be one uniform temperature with no cold tile floors. Passive houses don't cost much more to build than conventional homes. The only downsides? Site selection is important. Also, passive houses allow for 500 square feet per person. Comfortable and cozy, but not necessarily a mansion.