Lime render

Lime render is a lime-based cementitious mix applied to the external surfaces of traditionally-built stone buildings. It allows the building to 'breathe' - as lime is porous, it allows for the collection and evaporation of moisture. Essentially it is something like Gore-Tex for stone built buildings. Cement in contrast, an often applied render for stone buildings, traps moisture behind the stonework, which may result in the erosion of the stone masonry.

External links

some explanations on a commercial site