Early on, we filled panels where the clay had disappeared with new wattle and daub. This year we have switched to a new plan: these panels, which had been filled in the last 30 years with Thermalite blocks and broken bricks, have been made into a breathable hemp sandwich.
We lined the outside of the building with Savalit woodwool boards (as described before), then plastered them with haired chalk and lime mix bought from Anglia Lime. We used battens to fix Savalit boards on the inside, recessed just enough to take the plaster, and gave them a thinner layer of chalk and lime plaster, also with hair. In fact, internally, Savalit boards can take an extremely thin, almost skim coat of haired lime plaster and still look good.
In the middle we put two 75mm layers of natural hemp batts, which have exactly the same insulation rating as wool but absorb less moisture. The hemp is also grown in Suffolk, and processed in Wales by Black Mountain (though they had a fire that stopped production in summer 2013).
We attended a course by the authors of The Old House Eco Handbook, published in collaboration with the Society for the Protection of Ancient Buildings. Marianne Suhr, co-author of the book, gave three very practical suggestions for the panels, in a private conversation: Pavaflex wood quilt, then oak laths and a coat of plaster, which she said would be thermally the best; next best would be a wood fibre board to fill the whole panel, which has the advantage that it can be plastered direct; finally hemcrete, which can be applied behind shuttering and left to set, and then directly plastered. Pavaflex sounds good, but the laths would be a fiddle.
However, when we looked in detail back home at prices and the amount of work involved, we opted for the local suggestion of Savalit boards and hemp insulation batts, which the conservation officer approved.
These pictures show the method in more detail, for those who want to try a hemp sandwich: