THE PASSIVE HOUSE: Sealed For Freshness. “The basic idea is that these houses are so airtight that warm air won’t leak out in the winter, and cool air won’t leak out in the summer. Windows are three panes thick, and there is far more insulation than you would find in a standard American home. Stale indoor air is exchanged for fresh outdoor air without altering the internal temperature by mechanical systems you would not find in a conventional house: things like heat-recovery ventilators, which draw the heat from outgoing air and mix it with incoming air from outside in the winter, and do the opposite in the summer. (In high-humidity climates, an energy-recovery ventilator is used instead to strip moisture from incoming air.) Vents that look like small, round audio speakers are placed throughout the house to exchange fresh air. These devices have prevented the formation of mold, which plagued the passive-solar movement of the 1970s and 1980s.”