One third of the energy you pay for probably leaks through holes in your house. Air leaks can also cause moisture and indoor air quality problems.

Stopping air is the second most important job of a building enclosure.
Next to rain, air leaks through walls, roofs and floors can have the biggest effect on the durability of a house. Uncontrolled air flow through the shell can not only carry moisture into framing cavities, causing mold and rot, it can account for a huge portion of a home’s energy use and cause indoor air quality problems to boot.

So tight houses are good houses, right?
Tight houses are better than leaky houses — with a caveat: tight houses without a ventilation system are just as bad as leaky houses with no ventilation system; maybe worse.

Energy efficiency requires a tight shell; good indoor air quality requires fresh outdoor air. Ideally, the fresh air should come not from random leaks but from a known source; for this to happen, the house needs an adequate air barrier [see "What is an air barrier and where can I buy one?," below] and a controlled ventilation path.

In leaky homes, large volumes of air — driven by exhaust fans, the furnace fan, the stack effect, and wind — can blow through the home’s floor, walls, and ceiling. Because air usually contains water vapor, these uncontrolled air leaks can cause condensation and mold.

Read more: http://www.greenbuildingadvisor.com/air-leaks-waste-energy-and-rot-houses#ixzz4Uqh2AD7k