The cavity brick thickness relates to each leaf of brick, not the total wall system. If 2 x 90mm bricks were used with a 50mm cavity - the total wall system would be 230mm thick. This means 90mm needs to be modelled to represent a 230mm brick cavity wall.
If a 110mm face brick was being used with a 90mm internal brick with a 50mm cavity - the total system would be 250mm thick. This means the difference would need to be split by modelling 100mm to represent this scenario.
