Damp is a key factor in almost all forms of decay, and damp fabric increases heat loss. However, identifying its cause can be complicated as moisture is so mobile. It can condense from humid air on to cold surfaces and it can spread through a structure by capillary action and gravity. Understanding these mechanisms and their consequences is vital, particularly now that we are increasing insulation levels in historic buildings and restricting the movement of air. Getting this stuff wrong can be disastrous.