The fan is connected to software package, the floor areas and wall areas of the building is entered into the software measuring device The software calculates the air flow in or out of the fan (depressurisation test or pressurisation test), the software calculates both readings, total surface area of the building and the air flow through the fan resulting in an Air Permeability reading. We divide this figure by 20 to obtain the A.C., air changes rate. The A.C. rate is the amount of times the air inside the building will change every hour. Air enters the building through air paths or gaps and cracks in the buildings' envelope which are most likely not visible to the naked eye. This happens constantly which we called uncontrolled ventilation or draughts. The more air changes, the more heat loss occurs.