Lime is a interesting subject and one that I have covered on a number of occasions in this blog. The reason why it is a controversial topic that it is a really important one where there are lots of vested interests and little in-situ research. Lime is material that is a vital element to the construction industry, especially in the traditional building element of it. Lime, though has become a generic term. We use it all the time to express the need for a breathable mortar, plaster and render. You will commonly hear people use the term lime render, but do we actually know what it means?
Some people use cement and just add powdered hydrated lime into it and then refer to it as a lime mix. Others will use the other common powder form of lime (Natural Hydraulic Lime) and use it with a sand aggregate. The purists will stick to Lime Putty. However, even the purists then disagree with the aggregates that should be used and also on how to apply it.
All this is a bit more confusing and less clear cut than the term ‘lime’ conveys. This is why I worry sometimes about the conservation industry and its use of the generic term.
I have always been clear on my thoughts about lime render and the aggregates that support it. These are the reasons why. I believe that:
- The walls of houses, where rendered, should have the most breathable (porous) element on the outside. So one should create a gradient of porosity from the main structural wall to the outer finish. This means that the outer surface will always help to draw moisture away from the inner structure, thus keeping it as dry as possible
- The pressure to take the movement of water through a render should be shared as evenly as possible between the lime and the aggregate. This means that the lime is not taking all the responsibility for the movement of the water, thus allowing the water to move more easily through the structure
- Lime renders need a consistency of mix through the structure in order to minimise the risk of de-lamination. Using different mixes and limes can create this difference in the structure and so should be avoided.
- Application of the render is really important as one needs to apply the mix in such a way as to encourage the different coats to chemically bond together. This again minimises the risk of de-lamination in the structure
Having sorted a specialist order for a limestone mix from a local supplier, the combination of lime putty and limestone aggregate has proved too much of a leap of faith for my current lime specialists on site. They have backed out of doing the job (unless I forgo by principles and stick to the common knowledge of the ‘lime industry’). This means that I have been left with a house that has been hacked back to the stone / brick / concrete blocks etc and no contractor to put a proper lime render back on. I was a tad ‘hacked off’ as you can imagine!
Knowing that I am right (see picture above) is proving to be a problem!
However, there is a knight in shining armour. Mark from Welsh Lime Works has offered to step into the breach and do the rendering. This means more of a wait, of course, due to his other commitments, but at least I will be able to get what I want and have it done by people who know what they are doing. A huge thank you to Welsh Lime Works for this, true saviours.