Breathability in old buildings
Old buildings must be allowed to breathe. Whereas modern buildings rely on keeping water out with a system of barriers, buildings that pre-date the mid-19th century are usually constructed of absorbent materials that allow any moisture that enters to evaporate back out.
Because most old buildings were constructed with solid walls without DPCs and originally had no roofing felt, rain or below ground moisture could both enter. This did not, however, mean dampness was inevitable.
Before central heating was commonplace, heat from open fires drew in large quantities of air through loosely fitting windows and doors. This high rate of ventilation would have quickly evaporated moisture from permeable internal surfaces while the wind dried out any damp roof timbers or permeable external wall surfaces. An equilibrium was therefore established whereby the moisture being absorbed was equal to that evaporating.
When upgrading an old building, you must maintain this equilibrium for the building to work as intended and remain dry.