You may observe dark areas in interior walls in some old buildings. In the old days people did not understand what caused them. So they suspected that those might be ghosts living in old buildings. Today we know they are caused by the effect of thermal bridging on steer framing (colder areas tend to have more water vapor condensation, which facilitates mold growth that makes the areas look dark, like ghost shadows). The following simulations show this effect and how to avoid it.