Best practice among builders of modern high-performance homes in most North American climates is to build as tight as possible, and then ventilate with a well-designed mechanical system. Even though this uses a little energy, you are likely to save more heating and cooling energy from having a tight house than you will use to ventilate. Mechanical ventilation is achieved in one of three ways: exhaust-only, supply-only, and balanced. To varying degrees, all of these strategies are more economically installed in new houses and more difficult to do as retrofits, because access in existing houses may be poor.
Exhaust fans (kitchen, bathroom, and/or whole-house fans) tend to “depressurize” the building, causing infiltration of outside air through any cracks or openings it can find. In the North, where winter is more intense than summer, exhaust-only ventilation may be adequate without inviting damage from moisture. Because summers tend to be short and moderate in cold climates, except for a few days, the building is unlikely to be damaged by occasionally drawing in very hot and humid air through the structure. Conversely, exhaust-only ventilation strategies should not be used in the South. If hot and humid air is drawn into the building for months on end, condensation, mold, and damage are likely to develop.