THE NEED FOR WALLS TO BREATHE

Traditionally-constructed masonry buildings used permeable materials such as stone, brick or cob (dried earth) in conjunction with permeable mortars and sometimes renders to keep out the weather. Moisture is both freely absorbed by thees materials and readily released by evaporation from their surface. Damp is kept under control and prevented from reaching the interior by the dept of the wall and by the rate of evaporation.          
     Traditional materials are notoriously difficult to fully weatherproof, even by the application of modern materials such as silicons. Any impermeable coating such as a paint, cement render or sealant inhibits the ability of the surface to breathe and prevents evaporation, causing damp in the interior, and in time it is inevitable that it will crack, flake off or break down, allowing further moisture penetration to compound the problems. Any attempts to weatherproof a traditional wall is therefore pointless. A permeable render provides far more effective protection.
     Lime mortars used as the bedding material for masonry and for pointing the joists provide a permeable route for the passage of moisture from the core of the wall to the surface where it evaporates. Where the stone is very hard and impervious,this may be the only route available. Where the stone is relatively soft, a more porous mortar will always be preferred to reduce the amount of evaporation which takes place through the stone.
 
THE NEED TO ACCOMMODATE MOVEMENT

All materials expand and contract with changes in moisture content and temperature. Modern walls, built with Portland cement, are relatively rigid and rely on their strength to resist movement although, in long walls, expansion joints need to be introduced to accommodate the proportionally greater expansion.

     Traditionally-built masonry walls work in an entirely different way: they are much more flexible and will accommodate minor structural and seasonal movements by minute adjustments over many joints. Hairline cracks in the mortar may subsequently be resealed by the precipitation of lime.
     The use of  cement in modern construction works well if the brick or stone is stronger than the mortar, but problems arise when a hard cement mortar is used to re point a traditional wall. This is because the shallow dept of hard cement mortar at the surface is rigid, whilst the body of the wall behind is flexible. Any movement which occurs introduces stress in the narrow band of rigid materials at the face, which is only relieved by the failure of either the stone, the brick or the mortar. As the stone or brick is usually less hard than the cement,it is this that normally fails.