We have a 1920s house with a ground floor bathroom. The bathroom is entered through what was once the old back door to the house and there is a step down of approx 130mm into it. The concrete slab floor in there is currently tiled, but I'm fairly sure not insulated as it's freezing cold most of the time.
As part of the bathroom renovations, I'd like to insulate the floor, ( perhaps adding electric underfloor heating) and raising the floor level at the same time to match that of the adjacent room. If it makes any difference, I'd also like to insulate the walls and ceiling at the same time. The existing ceilings are at the same level in both rooms so height is not an issue.
A few Google searches say that the best method for this (typically when converting a garage into a usable room) is to lay rigid form insulation (e.g. Celotex etc.) directly onto the slab, then a damp proof course, then an 18mm ply/chipbord floating floor over the top.
So far so good, but would a floor like this take the weight of bathroom fittings and could I tile onto it? Surely it would be too flexible/delicate? Should I lay joists and insulate between them instead? Any suggestions?