WALLS NEED TO BREATHE
Lime is the principal binder of traditional mortars, plasters and renders. It tends to be neglected in modern building practice, but it is central to successful maintenance and repair of traditional buildings. 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 these materials and 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.
TSB, Patrick St. Cork
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.
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.
Hot Lime Rendering
The use of hot lime renders in construction these days are virtually non-existent. It is a lost trade and its use plays a vital role in the conservation and restoration of Ireland historic buildings.
These days 50% of my work is using hydrated and 50% is using hydraulic. I use hot lime renders when matching into existing render and where historic fabric is decayed and frail, hydrated lime in this case is more sympathetic to the building and more authentic.