An easier solution is plasterboard already bonded to PIR or XPS. The XPS is cheaper and I don't think it will make a massive difference depending on all the oth

An easier solution is plasterboard already bonded to PIR or XPS. The XPS is cheaper and I don't think it will make a massive difference depending on all the other challenges like windows and doors. You can simply stick this to existing walls with adhesive foam and then dry-line or skim the boards afterwards. To do it well, though, you will often need to cut out/remove any laminate flooring so the boards can meet the floor and they won't do anything about leakage between floors, which can be significant.