I live in one side of what is in effect a larger house divided in two, created by extending out to the side of an end of terrace. The dividing boundary is the previous external, now internal wall. There is a connecting doorway between the two sides, that is not in use at present, but cant be blocked off permanently as the owner may wish to unite the two in the future, the door is a cheap DIY store door and offers little insulation of sound or cooking smells from next door.
The dividing wall is nearly a foot thick, I plan to put an additional (non functional) door on my side the wall, so there are two parallel doors with a 6" gap between them. I was thinking about filling the space between them with some form of insulating material, Rockwool etc, foam rubber(as in furniture).
Does anyone have any suggestions, it needs to be cheap and simple, but easily removable