vapour check membrane

[swge1]

Hi

just wondered about the purpose of the vapour check membrane.

am i right in thinking it is to prevent moisture from migrating from the room to the underside of the felt??

i am creating a room in the loft. already have the insulation fitted between the rafters and 50mm ventilation gap behind that.

on the underside i am going to place foil backed plasterboard.

but do i need to use / or would it be better to put a vapour check membrane in as well.

just wondering as to create a complete air tight seal, do the joints of the plasterboard not need to be taped and sealed. this will obviously be quite difficult to do on the underside of the plasterboard

thanks

 

[SurreyRoofingEstimator]

you need the vapour control layer on the room (warm) side of the roof so put 1000 gauge polythene between the plasterboard skeiling and the rafter.