The thermal performance of granite walls is complex.
The thermal resistance is influenced by the thickness of the wall, the size of the stones, the type of mortar, the thickness of the mortar joints, the moisture content of the wall, the ventilation path around the wall and the internal linings.
Research by Historic Environment Scotland and others has determined the u-value to be between 1.5.... and xxxx W/mK.
If we want to insulate these walls we need to be careful that we don't cause problems with the transport of moisture through the wall, and remember sources of moisture may come from the inside or the outside.
There are various ways of insulating granite walls and different suppliers will tell you that their methods are fool-proof, but please check any claims with someone independent, such as a Retrofit Designer.
Some installers will suggest that you can fill the cavity between the stone wall and the plaster linings, but this will depend on
> the location of the building
> whether the wall faces north, south, east or west
> how exposed the wall is to driving rain
> whether air bricks are present
> whether the solum ventilates via the wall cavity
The transport of heat and moisture can be simulated using Hygrothermal Analysis.
This analysis is site and building specific and requires accurate information about the build-up of the structure being modelled and the ventilation strategy being used.
There is obviously a cost associated with conducting this analysis which can be £500-£1000 per project.
A retrofit designer needs to have a moisture and ventilation strategy for the whole building as a starting point.
The safest treatment of the cavity is to leave it alone and to insulate internally.
However, for projects where internal room space is small, or there is plasterwork and cornice that cannot be changed, filling the cavity may be the only viable solution.
This must be carefully designed and checked by an independent export for the situation at hand.
Remember: A whole house approach is essential. If energy saving measures can be made elsewhere then it may not be necessary to insulate your walls, but this depends on your ambitions and site specific circumstances