For retrofitting as well as for new buildings a good airtightness is an important issue. In Germany, Austria and Switzerland about 1000 persons conduct blower door tests according to EN 13829 in order to characterize the air permeability of buildings. Also, preliminary measurements of the air barrier are made, often by the craftsmen themselves. Early measurements allow to repair leakages more easily than when the building is completed. In this lecture typical faults and the resulting problems as well as good solutions are introduced at planning and execution with two attic storey developments: (1) In an appartment building thick insulation layers were built in during the attic storey development in the eighties. The subject airtightness was not considered. Consequence: The rooms lying to the east didn't get warm sufficiently. (2) At the attic development in an one-family house of 1928 the airtight layer was planned in detail and checked with the blower door during the construction. In the course of further redevelopment measures the n50 value of the complete building was improved from 5 h-1 to less than 2 h-1 and a ventilation system (central exhaust ventilator) was installed. Different test methods were used: Zonal Pressure Measurement, Opening A Door, Guard Zone Measurement.