Why does paint cause the deterioration of historic buildings?

Buildings constructed in Fremantle prior to the Second World War were generally built with traditional construction materials and methods. These stone and brick buildings have solid external walls without a cavity, and sometimes without a damp-proof course. This is fundamentally different to modern buildings, which are generally constructed with some form of cavity wall which allows moisture to penetrate the outer leaf of the building but protects the inner leaf from damp. 

When traditionally constructed masonry buildings were built, it was accepted that a certain amount of dampness would penetrate the walls and that this moisture would evaporate naturally, mainly on the external face of the walls.  To allow this to occur, traditional mortars, plasters, renders and decorative finishes were permeable to water vapour. The exchange of water vapour and air between porous masonry materials and the atmosphere is often described as ‘breathing’

However, because modern construction practice aims to keep all moisture out of masonry walls, relatively impermeable surface coatings such as plastic paints and sealers are often applied and this substantially inhibits the ability of the traditionally built solid wall to breathe.

Moisture and dissolved salts still enter the solid wall when they are drawn through hairline cracks in the impermeable finish by capillary action but then, because they cannot escape by natural evaporation, they build up and cause the masonry and mortar to deteriorate. Also as the moisture builds up it migrates to the inner face of the wall where it damages finishes and embedded timbers and steel.

Modern building materials such as acrylic paints, or sealers, damp proof coatings and strong cement render and mortars are examples of the types of low permeability materials that can prevent the natural evaporation of moisture from solid masonry walls. 

Due to the damage caused by entrapped moisture and salt, a fundamental principle in the care and conservation of buildings with solid masonry walls is to carefully remove impervious materials to allow the solid wall to breathe.