Energy Myth: It is dangerous to tighten up a house too much.
Fact: It is almost impossible to make a house too tight. Reducing air leakage in an existing home is easy and cost effective. If the air in the house is too "stuffy" then increase ventilation (something that you can control)-don't depend on uncontrollable air leakage.
Home Series Booklet: Home Tightening (PDF)