It all depends on what you are trying to achieve?
If you want to heat the room with a candle in a flower pot and, still feel too warm?
Then the best bet is to stick five inches of polystyrene to the walls and ceiling and to plaster over.
On the other hand one inch of polystyrene (taking up less space) also stuck to walls and ceiling will reduce the heat loss by about 90%
The thing with heat, is that as you block one exit, it promptly finds the next best exit, probably in your case the windows and doors.
To get the best overall result you need quadruple glazing on all windows and probably two insulated doors, making an air lock.
Then there is the question of ventilation - too much ventilation, throws all that effort out the window, not enough makes the room unpleasant.