Re insulating 1970s timber frame dormer

[Jonnykerb]

Hi
I am intending on re insulating the timber frame dormer from the inside.
Current insulation was done in 1970s and not up to standard causing damp issues and significant loss to heat.
Question is which side should the vapour barrier be on and should the void be between the plaster board and celotex or between the exterior board and celotex.
Thanks in advance

 

[MeldrewsMate]

The vapour barrier is to keep moisture from outside getting inside. Moisture generated inside should be moderated with ventilation, therefore the vapour barrier that you fit should go directly onto the board. I would then use thin battens to affix the Celotex (which has its own vapour barrier), followed by the plasterboard. You can buy Celotex already bonded to plasterboard. I have used the 25mm + 12.5mm PB =37.5mm overall size and was very impressed with its performance and with the finish. Go as thick as you can unless it starts to block out the light!