Two real options here. No1 you take down the existing plasterwork and install a modern very efficient multifoil insulation, counter batton it and reboard and plaster. Advantage is you can guarantee the 50 mm air gap is there, it would now be totally sealed and insulated. Disadvantage is it's the more expensive way. Option 2 Providing you already have the air gap you could just over insulate (with a multi foil or even two layers) the existing plaster and re board. Advantage is a lot less mess and less cost too. Hope this helps

Hello
It is surprising that a relatively new loft conversion is not thermally efficient.
Often the best solution is to remove the existing plasterboard and reinsulate and then new plasterboard and skim.
There should be insultation [ celotex/kingspan etc ] between the rafters and also on the face of the rafters, a typical specification is below:

• To new sloping roof sections with non breathable felt
• 100mm Kingspan K7 insulation to be fitted between rafters with 52.5mm Kingspan K18 insulated plasterboard across face of rafters (based on 400mm centres). Note minimum 150mm deep rafters required for this application.
• Or
• 50mm mineral wool insulation to be installed between rafters. Underside of rafters to be lined with Triso Super 10 multi foil insulation, all joints to be taped with aluminium tape. Insulation to be fitted in place using 25x50mm treated softwood battens to allow fixing of plasterboard and provide necessary U-Value.
You would need to make a building regulation application for the above as you are changing an existing thermal element.
Regards
Pete Watkin
Aztech Architecture Ltd

Sounds like you have not got enough insulation in between the joists, with the walls and ceiling plastered one option would be to install some kingspan sheets to the ceiling then overboard with plasterboard and skim (you could do the same with any walls). Regards, Terry.

It may be that the insulation had deteriorated if it's of old construction or it maybe that it just had insufficient insulation to start with, worth checking in any crawl space you can get to.