Anybody has experience trying to improve the energetic rating of

The rules are different for condos than for houses.  It depends where you are, in a centro storico you cannot do a 'wall'  capotto, and in some you cannot even change the windows without a special permit. 

However putting in double glazing is a good starting point.  

Modicasa is correct on his comments about potential restrictions on what you can do in certain situations.

IF you aren't limited in your options (ie, not centro storico, condominium etc - you need to check), I'd suggest options are:-

1.  Double glaze windows and doors

2.  Insulate the structure

          a.   "External" wall insulation, new brick skin etc, is effective but quite an outlay

          b.   "Internal" wall insulation - this is insulated plasterboard fitted odours on the external walls.  Easier and cheaper than external insulation - easier to cope with windows etc.  BUT probably not as effective as external insulation

          c.  Insulate ceilings and floors if headroom is sufficient.   Insulated plasterboard on ceiling, insulated polystyrene slabs on floor, underneath new flooring.

         d.  Fitting thick carpets helps.

Basically, it's a 'pick and mix' situation. 

Talk to a Geometra or Architect who is local to the area 

Good luck.

Found this uk website - may have some useful info

https://www.eco-home-essentials.co.uk/insulating-an-old-house.html

Otherwise change to an energy efficient boiler/water heater and aircon - that alone can raise your rating by one class.