Lime - that's less than half the problem!

Sustainable and historical bodies talk about the need for lime renders and mortars in pre-1919 solid walled buildings and rightly so. Lime is a porous material and will allow moisture to pass through it. This allows the walls to 'breathe' and so can help to keep them dry.

All well and good. However, when you make up a mix of lime render what is the main constituent? Lime? No, the bulk of the material is the aggregate. I think that this can be a problem, because the main aggregate that we use is the cheapest and most common one. Sand.

Sand is not breathable / porous and so if you use lime and sand for renders then all the moisture can only travel through the lime. Given that rainwater is slightly acidic means that the lime will be weakened more quickly. However, if porous aggregates are used (or at least a portion of the mix is made from porous materials like limestone, brick dust etc) then the aggregates will take a lot of this load off of the lime, thus making it last longer. Also if external finishes are meant to be 4 times more breathable than the main structure then having a porous aggregate helps to achieve this. 

Lime with a porous aggregate is also more likely to work in an 'osmotic' manner rather than in a capillary way with sands. This means that it can 'suck' moisture out of walls more easily. So as you can see the term 'Lime render / plaster' needs a little more clarification.

I think that the choice of lime and its associated aggregates needs to be more scientific in its specification, especially since choice might also be driven by a range of other factors including:

Exposure 

Underpinning substrate 

Orientation 

Historical colours of renders

So we need to be careful. The best for renders are a mix of particle sizes and sharp angular edges and so if we allow ourselves to be driven by factors like historical use we need to take care. Many old render mixes were dictated by available local materials and these might not be best suited to the location etc, so planners might insist on a mix that is not particularly appropriate for the building!

There are other issues that surround all of this including:

Sand types (smooth or angular)

Salt contamination

Application methods

However for the sake of brevity, are there any broad recommendations that I can give?  

Well, after much thought I have ended up in the Welsh Lime Works camp where we tend to advise people to use lime putty (based on limestone rather than chalk) with a limestone aggregate, applied with a pump and then finished off with a limewash (again based on limestone). This creates a very breathable mix that allows buildings to work in harmony with nature, but it is worth noting that all projects needs to be individually approached.