In the present work, the air tightness of flat angled roof structures is analysed under varying factors. Air tightness is closely related to thermal insulation and humidity protection as well as the fire protection and sound insulation of buildings. The air tightness has a significant impact on indoor climate, structural damage and loss of thermal energy. Only by ensuring the air tightness of the building shell, structural damage caused by water vapor entrained by airflow can be avoided. In the first part of the study, effects of various leaks are investigated as a function of their size and type on the air thightness of a low pitched roof construction. The leaks are implemented on the inside or on the outside of the roof construction. The roof constructions are analysed by performing different experiments with and without air tight level. The measurements of CO2 concentration are divided into three zones: in the laboratory, the timberframes and the room. In the second part of the thesis effects of mineral wool insulation in timber frames are analysed with variation of partial and full insulation. In this study a 10 mm thick leakage on the inner side and different leaks on the outside of the roof structure have been arranged.