Old Buildings Need to Breathe

(Extracts from Information Sheet 2.3 issued by the NSW Heritage Office)

Under normal circumstances, older buildings will function well if they are allowed to work as they were intended. Mortars, plasters, renders and finishes should all be of relatively permeable materials, allowing moisture to pass through them and evaporate from the surface. Traditionally, mortars, plasters and renders were usually lime-based and decoration was with limewash.

The concept behind the construction of old buildings is that moisture entering the wall should be allowed to evaporate. Whereas modern buildings rely on keeping water out by a system of barriers.

Old buildings will become damp if an impervious layer is applied to them, because this prevents water within the structure from evaporating. As the moisture content of the wall increases, the likelihood of decay also increases.

Care needs to be taken with putting any “layer” on older buildings such as external strong cement render, external cement-rich pointing, dense internal plasters and external impervious paint.