A characteristic cracking pattern will develop, with a triangle of brickwork detached from the rest of the wall and resting on top of the sagging window frame. This phenomenon can be observed in homes all over the UK, and it is serious bad news for the owners concerned. They have paid thousands of pounds to some chancer who has inflicted serious structural damage upon their homes, and then probably done a runner. This can knock thousands off the price of a decent Victorian or Edwardian terraced house - the price, in fact, of removing the double glazing and replacing it with the original draughty sliding sash windows. But the most widespread problem is condensation.
Replacement windows tend to make a house airtight, and when that happens condensation follows.
Black mould growth in the corners of rooms is the usual first sign, which is an indication of damp walls. Damp walls are poorer insulators than dry walls, so they allow more heat to escape through them. An article on the timber treatment trade. Is it really necessary to flood your house with all those chemicals. Textured wall coatings are supposed to keep water out of your house.