CC 2.0 // photo credit: Jeremy Levine Design. "The house encourages outdoor living to allow the owners to take advantage of Southern California’s mild climate," says designer Jeremy Levine.

Just how much can energy efficiency lower your utility bills? Well, for the uber-green, a “passive house” can literally reduce your monthly bill to nothing.

The Independent Weekly recently ran an interesting story on the home of architecture professor Corey Saft, the first certified passive home in the southern United States. Despite the hot, humid climate of Lafayette, La., Saft’s first energy bill was for -$0.62. That’s right. A negative amount.

The Passive House Institute explains why such a home is labeled “passive”:

A “passive” house achieves overall energy savings of 60-70% and 90% of space heating without applying expensive “active” technologies like photovoltaics or solar thermal hot water systems. Energy losses are minimized, and gains are maximized.

The New York Times offers a cool infographic on how passive houses work: it’s heavily insulated and sealed super-tight. This way, the homeowner knows exactly where air is coming in and going out. An energy recovery ventilation system employs a heat exchanger, which uses inside air to either warm or cool (depending on the season) fresh air from outside.

For those fortunate enough to live in moderate climates, there are other ways to control temperature inside a passive home. In the house pictured above, “temperature control is maintained by utilizing mobile shade screens, [an] interior courtyard which functions as a thermal chimney, deep set opening on the second floor which shade the glazing, ventilation breezeways carved through the house, and exterior living spaces which use decks to extend the living space into the outdoors,” says residential architect Jeremy Levine.

Of course, heating and cooling is just one component, albeit a major one, of a passive home. As the NYT graphic illustrates, solar panels, house orientation, and eco-friendly flooring and windows all come into play too.

For somewhat less expensive ways to greenify your home, see the DexKnows guide on How to Make Your House More Eco-Friendly.