Hi All. We're planning a refurb on a 1920's semi-d house. It will include improving insulation, and a small extension. The house already has double glazed windows. We'll obviously need new windows for the extension, and perhaps replacement windows for some of the back windows to tie in with the new ones, for aesthetic reasons. That still leaves about 5 existing windows and I don't know if they are "good" or not, i.e. I don't want to end up with great insulation everywhere, and then 5 windows "letting all the heat out". The windows do not seem draughty, but I don't know what their UValue is etc, so how do I go about making an informed decision on replacing them or not? Any advice appreciated.