In parts 1 and 2 of this occasional series on lime we looked at the background to lime and the use of lime mortar in re-pointing walls, today in the last parts we will look at the use of lime plaster. Warning- I have found this the most challenging of the lime materials to work with. Unlike re-pointing walls I have had to use tools such as floats (mostly a steel float).
The reason why I wanted to use this is to insulate an “Edinburgh Press”. This is a shallow cupboard built into the wall, often with a door so it looks like it leads to a room. We have four of these. Two of these have caused no problem being on the boundary wall with our neighbours and therefore internal. The other two, one upstairs in our lounge and the other downstairs in one bedroom have had huge damp problems. The upstairs one had a door, we removed this and gave it to a homeless charity (this only gave modest damp improvements). The problem is they cut into external wall and act as a “cold bridge”, that is an area of heat loss which leads to a build up of condensation. My attempts at insulating both presses with non-breathable insulation failed. They were still damp and had Aspergillus niger growing (which is an increasing health hazard) effectively making them unusable as shelves.
I needed a solution and my first thought was to brick the ground floor press up. As a last resort in 2011 I rang Historic Scotland who in those days had a free advice line. They recommended calcitherm and told me how to use it. I rarely mention products by name and I’m certainly not being paid for it, but its hard to avoid and this is one I would highly recommend. Essentially calcitherm is a breathable board formed of calcium silicate through a chemical reaction, it is then dried and compressed. Its highly insulating keeping heat in but letting water vapour out. Basically its use is as follows. Skim a layer of flat lime plaster onto the surface you are going to mount the board on (in my case stone), stick the calcitherm on with special breathable glue, then skim another layer of lime plaster on top and paint this either with breathable paint (or better still lime wash).
All this will be described later on this blog site.