Care needs to be taken when considering upgrading the thermal insulation of traditional solid-walled buildings. There is a risk that the application of a thermally insulating layer internally causes enough of a temperature drop within the wall to cause a build of moisture within the fabric of the building (read 'Interstitial Condensation')

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Latest guidance can be found here.

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Traditional buildings are particularly vulnerable to unwanted effects of energy retrofit measures. Their moisture behaviour is completely different to that of a modern construction. 

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Building regulations give flexibility for traditionally built houses to ensure that changes don't cause deterioration of the building's fabric. 

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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.

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