Blower Door Testing
Blower doors test the amount of natural air leakage in a house. Air leakage is also the same as "letting a house breathe" and a house can have too much or too little. Too much and its drafty, too little and you have to install mechanical ventilation, which can be expensive. There are also a ton of negative things with a house being too tight, the main one being water in the form of condensation which also means mildew and mold. So in existing houses we want to tighten up the house to the point where we don't need mechanical ventilation and the only way to measure tightness is with a blower door test. Sealing up a house without having a blower door test is playing with fire.
So why does the energy code mandate that we build houses so tight that we have to have mechanical ventilation? Because it saves energy. Whether it makes financial sense or not...meaning the cost to seal up the house then add mechanical ventilation versus the energy savings is another question. There are also other benefits to using mechanical ventilation, like guaranteed fresh air. The code states that all houses built in Michigan must have a blower test to prove the house has an infiltration rate of 4 ACH (air changes per hour) at 50 pascals.