Thermal insulation upgrades
Care needs to be taken when considering upgrading the thermal insulation of traditional solid-walled buildings. The jury is still out on whether the application of a thermally insulating layer internal causes enough of a temperature drop within the wall to cause a build of moisture within the fabric of the building (read 'Interstitial Condensation')
This is an excerpt from Historic England's website simulation-models-and-energy-efficiency-in-historic-buildings:
Traditional buildings are particularly vulnerable to unwanted effects of energy retrofit measures. Their moisture behaviour is completely different to that of a modern construction. Unlike modern buildings, in which impermeable vapour barriers are employed to keep moisture from entering, traditional constructions are composed of hygroscopic and semi-permeable materials and are naturally ventilated, allowing the transfer of moisture vapour to maintain their equilibrium. However, problems can occur when they are altered, particularly by adding modern impermeable materials which produce changes in their hygrothermal behaviour.
Building regulations give flexibility for traditionally built houses to ensure that changes don't cause deterioration of the building's fabric. The relevance here being that the timbers of historic buildings are typically seated within the wall itself. If the wall is holding moisture then the chances of timber decay is increased, not to mention the increased chance of dampness and mould.
The following is taken from Approved Document L 0.10:
The energy efficiency of historic and traditional dwellings should be improved only if doing so will
not cause long-term deterioration of the building’s fabric or fittings. In particular, this applies to
historic and traditional buildings with a vapour permeable construction that both absorbs moisture
and readily allows moisture to evaporate. Examples include those built with wattle and daub, cob
or stone and constructions using lime render or mortar.
