Seems to be a lot of hammer-in fasteners on the market.
They are attractive because of drill - insert - whack, rather than
messing about with wooden battens. That said a wooden batten at the
bottom seems a good idea to act as a shear-brace.
Good point about fire, Rawlplug do polypropylene (Screwfix) and
Fischer do nylon & metal, something metal is required for fire because
if the plaster falls off PIR foam is pretty nasty. That said I find it
an odd argument for domestic because you are dead if not out in a few
minutes anyway, long before plastic plasterboard fixings have failed.
Re dot-n-dab, was that onto the foil and onto solid wall? Interesting
and quick "splodge" solution.
I think I will go with the hammer-in fixings, 40mm celotex should
solve that icy box-room solid wall.