Insulating Small Pitched Roof From Below

[carson321]

Hi, 

 

I currently have a single storey wrap around section to my house containing utility and toilet that is part of the original early 80's construction, as shown. It has a double pitched roof over the utility, becoming mono pitched over the WC area.

 

Currently doing some renovation work and have all the ceilings down, this area has no insulation in the roof, which goes some way to explaining why it has always been freezing cold! 

What is the most practical way to insulate this roof from below, before I put new plasterboard up? I know I can just stuff some mineral wool up there, but how do I maintain ventilation? There were indicators of condensation (damp patches) on the back of the ceiling when I took it down, does that mean I already have poor ventilation and need to look at doing something to the roof construction? If I just stuff 300mm batts up there, I will have to avoid a large area around the edges to ensure I am not just contacting the felt under the tiles. 

 

Any advice or pointers greatly appreciated. 

[Russell griffiths]

Need some pictures and a diagram really. 
do you have a flat ceiling or is it all vaulted.