Full-scale laboratory measurements were conducted to determine moisture convection performance on the joint of external wall and attic floor. This joint is one of the most typical air leakage paths, where also the highest air pressure difference forms in winter. Two commonly used external walls (timber-frame and autoclaved aerated concrete walls) and two different sheathing materials (wood fiberboard, mineral wool) were studied. The attic floor was in both cases a timber-frame structure. A two-dimensional heat, air, and moisture transport computer model was used to study the hygrothermal behavior of the joint of external wall and attic floor. Results from the laboratory measurement series and computer simulation showed that in leaky joint the moisture convection due to positive air pressure remarkable raised moisture accumulation rate on the inner surface of sheathing. At same time the sheathing material played important role on the hygrothermal behavior of the joint of external wall and attic floor. The results of this study are used to determine air tightness requirements of structures for several positive pressure and air tightness levels.