options for adding extra loft insulation

I have a loft room which has been converted insofar as it has a carpet, and 4ft plasterboard walls and a plasterboard lined ceiling. Behind this plasterboard is about 3" of what I think is glass fibre insulation. This insulation hasn't been put in with any ventilation room whatsoever; it is snug against the plasterboard on one side and the ceiling felt on the other side. Having lived in this house for only 9 months, at the moment at least, I haven't seen any problems with damp, so the lack of ventilation isn't neccasarily an issue.

However, the insulation is not effective enough. There is a lot of heat loss out of the loft room, so I would like to add more insulation. I want to know whether it is feasable to merely add extra insultation (either isulated plasterboard, or isotherm then plasterboard...any comparative merits?) to the inside of the plasterboard that is already up. I know I will lose a few inches in every dimension of the room, but that isn't really a problem. If this isn't effective or introduces dampness, I would obviously need to do a more major job, and remove the current (unventilated) insulation and start afresh. I was keen to avoid this if possible!

Any comments greatly appreciated.

Really you should have 50mm air gap between the insulation and the felt, the gap should be vented. Ideally you would remove the plasterboard, insert 25mm Celotex or Kingspan rigid insulation between the battens and a 65mm layer bellow the rafter level, then your plasterboard fixed back to the rafters through the insulation. http://www.celotex.co.uk/downloads/PitchedRoofSarking.pdf

For the walls, if you want to avoid removing the exiting plasterboard but don't mind losing a little space fit some insulated plasterboard to the exiting walls.

agree with freddy as the insulated plasterboard route tends to be a much more expensive way of doing things try these guys they might help ive used them on last couple of jobs beat the local merchant prices by quite a bit, www.bmdinsulation.com you migh also think about using a multifoil like triso 10 if your building inspector will allow it this saves quite a bit of space an is easy to put up.