Hi

we have rebuilt a lean to on the side of our Victorian semi.

we have a timber clad structure and cement fibre roof.

The timber structure is built on top of an existing 18inch deep, blue lias and lime pointed wall, and the ground is the original flagstones and steps laid on dirt.

The lean to will be used as a garden room and to house bikes etc.

we probably won’t get building regs to sign off - as we aren’t going anywhere.

so far, we have 750mm foil backed insulation taped into the voids between the rafters with the required air gap on the outside.

We are not heating this room and will have moisture through the stone wall and floor, we don’t need any more insulation - but can we apply plaster board directly on top of the rafters and insulation board?? 
Or any other ideas. We don’t have much budget left!! 

If you aren't going to heat the room, and knowingly haven't fully mitigated moisture ingress, what's the point in the insulation you've already put in and why bother plasterboarding? If it's simply for storage, ensure good ventilation to remove moisture and then just board internally with something cheap like sheets of 12mm ply or 18mm osb.

We originally thought we would have to insulate as building control were insisting, but seems we had a communication issue - now they understand the use - it’s really down to us to decide. However - we need it to be draught proof for my husbands precious plants - the plasterboarding and plaster is to make it look nice - we pass through this area to access the garden. 
 

I think it might be worth you for a minute going back and listing out your actual requirements for the performance of the space, and prioritise them in order. There are quite a few contradictions in the way you are going about this, from my perspective.

 

If your husband is keeping precious plants which are sensitive to draughts, won't they also be sensitive to temperature?

 

If you have knowingly not mitigated moisture entry, but can't tolerate draughts because of the plants, then you'll get damp & mould problems.

 

If you've got damp and mould problems, plasterboarding is probably not a good idea.