These were very famous in the 80s. Quite simply they are two walls which are built side by side with a specific gap between them. In older houses the cavity walls can be filled with insulation and today with the new homes, they are built with excellent approved insulating material. It’s very easy and fast to pump the insulation.
This is where you have to fix insulation to the inside of the wall. So where the wallpaper is currently stick is about hundred mm of insulating material on the wall and complete it with plaster board, that can be used to redecorate. This solution is not so expensive than external insulation and If you want to redecorate it, then it can be included with the project with a very small amount of hassle.
If you wish to hire home insulation services for your single room where there are no thermal bridges, this is one of the most easiest way to make that one comfy and warm.
External wall insulation
External insulation is one of the best insulation for walls. This is where you can wrap the whole house in an insulating blanket.