Insulating a plasterboard tent

[Triassic]

I’m going to insulate a plaster board tent for a good friend of mine, it’s a bedroom in a roof space and is as cold as hell on winter. 

 

Ive got a load of second hand fibreglass insulation that was removed from a similar property just prior to demolition. I was going to place this between the studs in the walls and use garden twine to hold it in place.

 

I was wondering if there is any benefit to applying a breathable membrane over the insulation once I have it all in place? I need to keep costs to a minimum as she is as poor as a church mouse, so suggestions for low or no cost options welcome.

 

The room in the room cross section is as follows...

 

roof tiles

roofing felt

ceiling void space

The insulation I’m adding

plasterboard.

 

The flat portions of the ceiling are insulated.

[Onoff]

A breathable membrane imo would  see a reduction in draughts. 

 

Any dry lining boxes with open knockouts I'd tape up too. 

 

Gaps in floorboards and around the edge.

 

Seal it up too well and you'll have ventilation issues though!