Our thinking has moved on from when we started work in 2009 and 2010. One advantage of repairing a house very slowly is that it gives time to learn as you go along.
This applies particularly to the question of which materials to use when repairing walls and infill panels and when plastering and rendering, and to the question of energy efficiency.
About a third of the old clay in the building has disappeared and been replaced with modern materials such as thermalite blocks. We started in 2009 and 2010 by putting back new wattle and daub where there were gaps, and much enjoyed doing this. But more recently we have been encouraged to use modern breathable insulation made from hemp fibres in the body of the wall to improve the thermal performance of the house.
After attending a course in June 2013 on improving the energy performance of old buildings, I came away with the impression that the expert pundits are also looking at a wider range of techniques than they used to. They certainly sound less purist than a few years ago, when there was a strong encouragement to use entirely traditional material for repairs, mimicking what was there before, and not to trust anything new. Now the talk is much more of using thermally efficient substitutes that will improve the energy performance, though only where it can be done reversibly, and without short or long term damage to existing materials or structure.
In 2013 we applied this new thinking when we re-rendered a gable end wall and part of a side wall which were covered 40 or 50 years ago with thick cement on wire netting with a plastic sheet underneath. After conversations with Anglia Lime, with a neighbour who has re-rendered a similar cottage and finally with lime specialists Alan Wilkins and Rory Sumerling (formerly of Anglia Lime), we have used lime plaster with chalk and hair as our external render to protect the clay daub, instead of the lime and sand mortar we used in the first stage of repairs. And instead of laths (which don’t appear to have been used on our building anyway) we are using breathable Savalit boards, which can be plastered direct. The result is hemp insulation sandwiched between Savalit boards.
Chalked lime is softer and more pliable and seems to be more resistant to movement cracks than lime and sand render. Our neighbour made a number of test panels which showed that the plaster weathers well. It is considerably stickier than the clay daub and lime plaster we were using before, and does not need quite such an effort to work it onto a clay surface, which is a major advantage. And there is evidence that in our part of Suffolk chalked lime plaster was historically the main material used, rather than lime and sand render. So we may well abandon sand and lime renders altogether.