What is the thermal conductivity of modern houses in terms of brickwork, and how can is it tackled?

Modern houses are built from two layers: an external brick wall and an internal breezeblock wall, between the walls, is a cavity and the thermal conductivity of walls built like this is fairly high therefore a lot of thermal energy can transfer out the house. To tackle this issue, builders pack the cavity with an insulating material that has a very low thermal conductivity, which reduces the overall thermal conductivity escaping the house