maintain 50mm air gap then fill with PIR and overboard with insulated plasterboard. You are going to lose a bit of head height but at least you wont freeze taking a bath!
no issue at all. trenches often have a couple inches of water in them when conc is poured for example.
The problem is when face brickwork is not dry and it rains, struggle to ever get the staining out of the courses below.
some say a nails width gap? Our last build had a large flat (warm) parapet roof (10x6), i butted the OSB (cheaper than ply) up tight and had zero squeaks, movement.
why hemp build ? Creating a headache and cost right off the bat. Excellent insulation levels can be achieved much more cost effectively with normal traditional build types. Save the money for elsewhere!
save yourself a lot of hassle, have 400 GAPS not centres so when you rip the PIR you have zero waste. You may need an extra truss and spend 10 mins making the plasterboard work but will save hours and £££ on the hateful task of insulation ripping.