The concept of "Perfect Wall" is well-known in the industry for its premium performance in cold climates. It consists of a vapour retarding membrane outboard of the wall structure, complete with all the thermal insulation outboard of that membrane. The vapour retarding membrane serves as vapour retarder, air barrier, and the last line of defense against rain entry. This wall configuration results in superior condensation resistance, uniform temperature of the wall structure, reduced thermal bridging, several lines of defense against rain penetration, and a service cavity inboard of the environmental control layers. It is acknowledged that the "Perfect Wall", using a vapour retarding membrane, offers robust performance. That said, would there be room for improvement? For example, is the vapour retarding characteristic of the membrane still necessary in assemblies that incorporate concrete or mass wood, both of which already offer reasonable resistance to vapour diffusion? Is it possible to improve the performance of these high moisture storage capacity walls by switching to a vapour permeable membrane? This paper provides answers to the above questions using a holistic perspective. Supported by physics and hygrothermal analysis, the paper compares the performance of exterior insulated concrete and mass wood walls using vapour permeable vs vapour impermeable membranes. Various conditions will be explored such as normal environmental loads, incidental rain, and construction moisture. The study suggests that when it comes to exterior insulated concrete and mass wood walls, switching to a vapour permeable membrane would take the "Perfect Wall" concept to the next level, by improving its year-round performance and drying potential, with the latter being an important consideration for areas with high precipitation and low sun exposure.