Leaderboard

Have you considered fitting air to air heat pumps (A2AHP) to warm up your parents? Probably the cheapest and most cost effective thing you can do. Insulating the floor, if you have the room, is effective as well. If no stairs to worry about it is a lot easier. I would not worry too much about the windows, and bless they are really dreadful i.e. rotted through timber. May be possible to just replace the glass.