Barnboy

I'm doing a similar thing on a solid stone barn conversion. Original walls are 5-700mm thick lime pointed granite. I've built dwarf walls inside consisting of a marmox waterproof thermal load bearing block with a standard 6" block on top to bring me up to the top of my finished screed height. I've then built 50x100 studwork on the floor with 50mm of sheeps wool inbetween counter battens on the external of the stud, wrapped the back in a tyvek membrane and then lifted each section onto the dwarf wall before packing them tight and bolting them down to the block and screwing up into the posi joists. I've left a 50mm vented cavity between the new stud and original walls . The membrane on each 4.8mtr section of studwork laps through the section joint and is joined to the next on the inner face. Inside the 100x50 stud I've filled it with more sheeps wool before fixing an "insulated " sheathing board to the inner face to keep my bco happy with any thermal bridging issues. Next I've fixed a smart vapour control layer which is taped and sealed in every way possible to create an airtight envelope inside the building. I'm currently now fixing counter battens to the front of all of this the create a service void before hopefully fitting my final layer which will be Fermacell to hopefully give me as solid as possible wall whilst still allowing the original stone wall as much of a chance as I can to breath. It's a fair amount of work but between keeping my bco happy and trying to look after the original building it's the best I could work out to do.

I've looked at skimming but it's at least 10k in labour alone, let alone the materials , which is why I've gone for Fermacell, strong but diy-able and I can hopefully route some nice rounded edges on the reveals and external corners. I'm just worried that if I have the slightest amount of out of tolerance timber work when I come to putting the boards up I'll either end up with glued edges with a slight step or have to use tapered board and fill. I'd like to use the square edge and glued boards rather than tapered and filled to give me less chance of cracks appearing in the future. Has anyone any experience of using the glued boards and what the tolerance of the stud needs to be please ? Or any 1st hand experience at all of either methods.

I've read the for and against arguments for using Fermacell. I want something strong, can't afford to plaster and don't have the time to learn whilst carrying out the job of every trade possible on my self build conversion. My main concern is the state of timber as I have internal walls and externally lined walls of timber studwork faced with sheathing board and counter battens. I've read that everything needs to be perfect for Fermacell but running a 2mtr level across my stud and counter battens, where its not all flush, I've got 2, maybe 3mm of rock over that 2 mtrs, is this going to be good enough or am I going to have to somehow get everything perfectly flush ? Thanks