In the context of building materials, and with particular reference to old and historic buildings, the generic term ‘breathable’ is used to describe an important property of building fabric: the extent to which building materials are able to transmit moisture. Breathable materials should be sufficiently permeable to enable absorbed and surface moisture to evaporate during ambient drying conditions. Loss of moisture from the material needs to occur rapidly enough to ensure that there is no accumulation of moisture which can lead, over time, to significant deterioration of building fabric, development of mould growth, infestation, or other form of decay. A breathable material, by definition, cannot be impermeable and should not trap moisture.