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.
