With the ceiling done and ready to prime/paint, the floor was next. The concrete slab has never shown any signs of damp so I was pretty certain it was well laid and had a damp-proof membrane in it. I did not want to raise the floor as the head-room is tight, the roof trusses are only about 6′ 3″ from the floor. That ruled out heavy floor insulation with boards over, my end choice was a double foil-coated foam layer at 5mm thick topped with water resistant tongue and groove chipboard sheets. The total height loss was only 23mm which was ok.
23sq m of chipboard later and it looks like this. The insulation was taken up the walls a little and sealed off with foil tape.
A good tidy up and it was on to the walls. These need battening to support the 12mm ply wall lining. Behind the ply will be 30mm Celotex/Kingspan sheet – it was kept thin so as not to lose any floor space while still gaining some useful insulation value. The battens are fixed to the slab bolts with steel angle brackets, two brackets per upright seemed sturdy enough.
The far left and right battens were fixed first, then a string-line was used at the top to keep all the middle battens true, the bottom of each batten was adjusted to get them vertical. The plywood sheets will overhang the battens, this overhang will be used to fix backer-strips which will in turn be used to secure the narrow vertical sheets of ply between. The spacings are in feet and ply only comes in one size, the best-fit option seems to be one nearly full sheet per bay with a narrow sheet over the concrete support columns secured to the backer-strips.