It's a good idea in principle but immensely complicated because of the detailing required at every point where new add-on meets old, mainly in terms of keeping water out, as well as cladding the insulation itself. Effectively building a new house around the old.
Much cheaper and easier to line the inside and add about 100mm to all the external walls, deal with floors and roof.
Yes but that doesn’t solve the aesthetics and causes me more issues….ie ”much easier“ - for starters you haven’t met my missus
…..also, and back to the condensation issue, further INTERNAL insulation will only pull the dew point backwards into the cavity increasing the likelihood of IC……I need to get out my graph paper, get hold of the u-values and do some calcs……..my old lecturer would be very proud