Walls breathe. The exchange of air and water  between porous masonry and the atmosphere is called ‘breathing’, and it’s essential that old walls should continue to breathe. Attempts to seal them, ‘to keep the damp out’, will inevitably trap moisture inside the wall and cause more serious decay. Many older houses suffer from rising damp issues and the moisture from the soil literally rises up the wall via capillary action. Internally, dampness can cause plaster to crumble, damage paint, wallpaper, and the moulds can be unhealthy, particularly for asthmatics.

Efflorescence is a crystalline deposit of salts that migrate to the surfaces of masonry leaving a coating of salt. Efflorescence clogs the pores of porous materials, resulting in the destruction of those materials by internal water pressure. This not only leads to unsightly aesthetic issues but can also indicate structural weakness of internal structures.