A blower door is the internationally recognized tool for measuring the air tightness of enclosures. It consists of a powerful calibrated fan mounted in a frame within an exterior doorframe, gauges to measure the air going through the fan and at what pressure relative to the outside pressure, and a software program to convert the data into standardized measurements.
Typically one would run the blower door to depressurize, or deflate, the building. Using the volume of the building and the measurements taken from the gauges of the blower door, one can determine the number of air changes the building would undergo in an hour, the total amount of air leakage often expressed in square feet or inches, the relative size of the holes, and, by allowing the blower door to run and inspecting the house, where the leaks are coming from.
All this information gives evidence of the state of the house as well as the success of the remediation effort. Remember that even if your house is built of insulation, if you leave a window open, it'll still be cold!
The picture above shows the complete blower door in position. The rings within the fan casing are removed depending on the relative tightness of the building envelope.
The picture above shows the two gauges. The top gauge measures the pressure of the building (the red tube goes outside as a baseline), and the bottom gauge measures the pressure of the air going through the fan.