Driven by Climate Change legislation and high rates of fuel poverty, the UK faces multiple challenges both in new build and upgrading the existing stock. How these challenges are addressed will have long term impacts on building fabric, occupant comfort and wellbeing. Building performance simulation has an important role to play in this process, yet it is widely recognised that over-simplification in the modelling of physical phenomena leads to substantial sources of error. Moisture is a major cause of damage in buildings, and the Glaser method is a widely used steady–state method used to calculate the vapour pressure difference in a building’s envelope. Although known for its limitations, it is the principal method used to assess moisture response in the UK. This paper evaluates the current situation in the UK, addressing fuel poverty targets, energy saving regulations and changing boundary conditions and their compounding implications for building envelope performance.