Internal Insulation needs attention
There´s no universal solution available for insulation upgrade and refurbishment.
Many home owners have been selected insulated plasterboard sheets to improve the insulation of their existing houses, and even a certain amount of new buildings get the "insulated slabs" installed to the external walls.
By doing an internal wall insulation, you should make a survey upfront, regarding the risk of interstitial condensation. The trouble of the most standard insulation products is the fact of cooling down the existing wall (the insulation layer keeps the room heat away of the blockwork). Whilst the heat is kept away, the moisture (coming from the warm indoor air) is still getting access to the wall and will cause condensation, as soon as the wall has a temperature close to the dew point temperature.
I´ve seen many houses with mould on the surface of insulated plasterboards. During my last survey, I got the chance to take off a piece of the installed insulated plasterboard. The home owner was pretty easy to convince about the dismantling, since he saw the readings of the moisture meter, measured straight on the block behind of the insulation.