The basic standard of Passive House design/retrofit is making sure the structure is airtight. There is a standard test called the blower-door test to see how much air is escaping through parts that should be sealed. The heat exchanger pulls in fresh outside air and filters it thoroughly. It can 'passively' adjust the air temp by warming it with household heat recycled from cooking, showers, etc, or selectively pulling in cooler air. The net effect is dramatically better indoor air quality with minuscule utility bills. More decent info here: http://www.heatspaceandlight.com/can-open-windows-passivhaus...