In reality, air leaks mean you’ve lost control of air movement. Air and moisture can be forced into wall and ceiling cavities where water vapor condenses and fosters the growth of mold. Warm air exiting the top of the house can draw in cold air to replace it, wasting heat and energy. In many ways, uncontrolled air movement wastes energy and increases the risk of long-term damage to building components.
Effective air and moisture barriers reduce those problems, but they come with a few caveats: Tight houses need mechanical ventilation to ensure a supply of fresh air to keep people healthy; and existing houses should not be tightened without assessing whether the existing combustion appliances have an adequate source of combustion makeup air.
As far as insulation goes, there may be a theoretical point of “too much, ” but in most cases buildings have too little. At the very minimum, insulation should meet recommendations of the Department of Energy, but adding more is always a good thing. Properly insulated buildings are cheaper to heat and cool.
Where insulation is added can be as important to how much is added. Walls and roofs with an extra layer of rigid insulation outside the framing help cut energy losses due to thermal bridging. What’s more, some types of insulation are inherently more effective than others. But using too much should be the least of our worries.