Posted By: Victorianecorequires a re-roof anyway

Ideal - this is your chance to do the best kind of roof insulation, which is EWI-alike on the roof as well as the walls. Tiles and battens stripped, 11mm OSB3 sheathing laid across the rafter tops (if you gapfilling glue and screw it, all edges supported, it'll be airtight too). Glue 150 EPS or woodfibre on top of that. Then breather felt, long-screwed downslope battens, slating battens, slates, Glidevale 250 or equiv continuous ventilation of the downslope batten space, in one eave, uninterrupted over the ridge, out the other eave. 100 insulation fitted between the rafters, from below. 250mm total, optimum before diminishing returns set in, crazy to do less, while you're at it. Roof 'EWI' joins continuous with wall EWI, none of the usual eaves thermal bridge. None of the usual fiddly interruption by partitions, timber members etc that's usual with doing all the insulation from below. The roofspace is fully insulated, part of the internal environment.