Ceiling Insulation - from underneath

Hi,
I need to improve the insulation of all my upstairs ceilings, but I can't access the roofspace far enough to add insulation there - the roof pitch is very shallow and the space towards the edges is full of trusses that are too small to get through. The only insulation at the moment is a couple of inches of now patchy and scruffy glass fibre that was installed at build in 1971.

So I want to find a means of adding insulation below the existing plasterboard ceiling. I can easily sacrifice 100mm of room height. I would prefer not to use a tile-type suspended ceiling as I want a smooth finish - these are living rooms, kitchen, bathroom, bedrooms. But the only insulated plasterboard that I can find seems to be for new-build, and not for old ceilings.

1. Can anyone please point me at suitable materials?

2. Is there a condensation / humidity issue that I need to address?

Any ideas will be gratefully received.

GLW

how about kingspan then plasterboard and skim?

You can buy plasterboard with insulation (Celotex type) already bonded to it in varying thicknesses.

Many thanks to RSSteve and GaryinAshby - I'll get some more info from those two makers.

How about the condensation / ventilation issue? Can I fix insulation directly under the existing ceiling or do I need a vapour barrier, or allow an air space? Do I need to ventilate the air space into the loft (which would negate the value of the existing insulation, I suppose)?

Ideas welcome.

GLW