Lime Rendering / Lime Wash
Lime rendering is a traditional method of smoothing over imperfections on external walls of a structure or building. Render can be applied to follow the undulations of an underlying substrate, or applied in layers to provide a smooth final coat. The finish can be left as is, or further enhanced with a pigmented Lime Wash to add some colour.
The advantage of using lime over modern cement based materials is that it remains flexible enough to allow slight movement in a buildings structural core, and is breathable allowing the ingress and egress of moisture with the seasonal variations of precipitation and temperature. Cement based renders on the other hand set hard providing a rigid and waterproof coat, which is fine until there is structural movement and the finish cracks allowing the ingress of moisture via capillary action through these fissures. Movement and thermal cracks can be minimised by applying a layer of expanded metal or plastic lathe to the substrate prior to rendering, but this is often omitted on cost grounds.
The characteristic of cement renders to maintain a waterproof layer tends to work against it when it cracks as moisture ingress becomes trapped behind it keeping the structural elements damp and making them prone to degradation either through being continually damp or the freeze, thaw action of frost. From an environmental perspective a damp wall also has a greater ability to transmit heat through it thus reducing its insulating properties.
Ultimately a cement based render will become 'blown' and drop off a structure and if constructed in a soft material taking a chunk of substrate with it. Lime render will deteriate with age and erode away from a building but leave the basic substrate intact. As lime is a relatively soft material patch repairs can easily be carried out.