TJL

Hello, I have decided to convert part of my garage into an office and though this would be a good place to come for advice. In it's current state it's a timber framed timber cladded garage with plywood sheathing on the inside rather than the outside. The frame is larch cladding, breather membrane (not sure if it has battens behind the cladding and over the breather membrane), 60mm batten then 12mm plywood sheets. A lot of stuff seems to have the sheathing on the outside unlike this one but I would rather not have to take the outside cladding and breather off if I don't have to though I don't know how hard or not that would be. I was wondering what insulation would be best to fill in between the studs with if they are straight behind the breather membrane. I am guessing I want something breathable but I don't know if rockwool slabs or wood fibre would be ok directly behind a breather membrane. Finding info on this doesn't seem to be very easy. Also if I wanted thicker insulation in the walls would I be ok adding internal battens to the frame and then placing the internal sheathing on that (going straight through the battens and into the frame) or would that nullify the ability for the sheathing to prevent the frame twisting etc. The floor is a solid concrete floor with a single course of bricks under neath the sole plate with a dpc under the sole plates. I am also looking to install a stud wall inside the garage to keep some of the building as an uninsulated garage space. Should I place another course of bricks where I want the stud wall or could I put a sole plate directly on the floor with the dpm either going under it and up the cold side (still inside the garage) or a dpc under the sole plate and lap it up on the inside and keep the dpm on the inside of the sole plate and lapped up against the turned up dpc? Thanks in advance for taking the time to read this a lot of questions I know. Cheers, TJL