What we mean by this is that when there is some moisture, it can have the opportunity to dry naturally through ventilation and heating.
The use of natural lime in mortar, plaster and renders together with water based decorative coatings is critically important. The use of cement based materials, gypsum plasters and impervious modern paints can be very harmful indeed.
Too many ‘restorations’ are done without any consideration for the long-term impact on the building. Too many remedial damp-proofing projects are done without giving any thought to alternative options.
Historic construction is not the same as modern construction. It should not have the same expectations and allowing the building fabric to naturally dry after it has become damp is the essence of what we mean by ‘breathability’.