Air leaks can be simple and inexpensive to seal if you can just find them, and that is where the blower door comes in. First you use the blower door to measure the air leakage and see how the house rates on a scale of "leaky" to "tight". If it is already tight, then you can forget about air sealing and look at other ways to save energy. But if it is too leaky, then the blower door can tell you how bad it is and then it can help you to find the location of the leaks. After you seal the leaks, the blower door can tell you how well you have done.
What is Air Leakage?
In contrast to controlled ventilation, air leakage is the migration of uncontrolled air to and from a building, which does not have a design purpose such as providing fresh air. It is the flow of air through gaps and cracks in the fabric of the building (sometimes referred to as infiltration or draughts).