The moisture performance of timber-framed external wall assemblies with and without a vapour barrier has been the subject of continuous discussion in Finland as well as other countries for several decades. It still remains one of the most hotly contested issues in building physics research. Despite numerous studies, agreement has not been reached on the minimum water vapour resistance that should be required of the interior wall lining of a wall assembly under different conditions. Consequently, different guidelines and regulations for water vapour resistance values of vapor retarders exist also in countries located in highly similar climatic conditions. This paper presents the new analysis method for determining sufficient water vapour retarder in timber-framed walls. It contains ten years research results examined by laboratory tests, field tests and calculation analyses in Tampere University of Technology, Finland. The basis of this method is that all factors affect to the water vapor diffusion in wall assemblies have been taken into account. Comprehensive assessment of the moisture performance of wall assemblies requires establishing the performance criteria and limit values that an assembly must meet, the outdoor and indoor conditions to be used in designing moisture performance, and the assembly solution and used materials as well as their building physical properties. The acceptability of the moisture performance of an external wall assembly depends essentially on all these factors during the service life of a building. The research results indicate that a plastic vapour barrier behind the interior board is safe in all typical indoor and outdoor air conditions occurring in Finnish climatic conditions. However, a wall may be implemented also without a plastic vapour barrier in the interior wall lining. Then, the required water vapour resistance ratio between the interior and exterior wall linings depends on the sheathing and thermal insulation used. In winter conditions, the minimum water vapour resistance values between the interior and exterior wall linings are typically in the 0 to 80:1 range. The key properties of the layers which improve the moisture performance of a wall assembly are the high thermal resistance and low water vapour resistance of the sheathing.