As consciousness grows regarding the negative impacts most buildings have on the Earth's environment, techniques to mitigate this impact must emerge in mainstream design practices. A calibrated hygrothermal simulation was conducted using WUFI® Pro to assess predicted hygrothermal performance of a variety of wall assemblies that are likely to enter into mainstream design practices. The results of these simulations reveal the importance of designing assemblies that are resilient to field conditions that introduce more severe hygrothermal loads than standard vapour diffusion. It is clear that in order for a wall assembly to perform adequately under moisture ingress conditions, it must be able to dry freely to at least one side of the building enclosure. High-RSI assemblies with exterior XPS exhibited far diminished resiliency to driving rain penetration as compared to those without exterior insulation and those with exterior mineral wool.