Inovative insulation ?

We live in a converted barn which in winter is difficult to keep warm, 2 main reasons being :
1) It's of exposed King post truss construction so no ceiling as such but lots of volume to heat - 20' interior height max & big rooms.
2) When converting, I used the tri-iso roll insulation which was easy & all the rage at the time - don't rate it myself now though.
The construction from the inside is ceiling boards attached to 8"rafters, tri-iso roll over the top, batten vertically,membrane then battens horizontally then pantiles. What I would like to do is get some more insulation in the gap (8") between the foil tri-iso and the ceiling boards. The correct way would be removing all tiles & battens & shoving kingspan in cavity but that's a big job.
Just thinking (don't laugh) could I hire one of the machines that pump the foam in for cav wall insulation & pump in the foam from various points (ie just lifting up a tile & pushing injector through membrane along the ridgeline of the roof ? It's steep at 43 degrees so should find its way down ! anyone done something similar ?

Or do that from the inside.

I know people have insulated between a solid stone wall and lathe and plaster by pumping from the inside; I think that was polystyrene beads.

Question is, is there a requirement to keep some of that gap free for ventilation?

Yes the ventilation would be cut-off by filling with foam or beads - not sure of any long term issue's here though - damp ingress from below is none - the tri-iso foil prevents it - from above there are tiles & roof membrane - to my mind, ventilation is only reqd where damp ingress is probable for example in a cavity wall where moisture on the outer wall can permeate in etc.
TBH ventilation here is something of a disadvantage as it draws heat away & presents cooler air to the tri-iso foil and so affects what I have negatively, Would be interested to here from members who have filled this space & wished they hadn't ! (rotting rafter's would be the worst case in my situation I think)

Is it a breathable membrane you have, or normal felt?

If breathable, and the tri-iso is acting as a vapour barrier to the room below, the concept to fill the void is viable.

But the issue would be that any air large gaps left under the membrane could potentially allow a bit of local condensation. It might be that any such gap can be dealt with by the membrane, but it has recently been found that older style membranes are not as breathable as they should be.

You also need to consider settlement of the insulation, and whether it will over time, settle to create a continuous gap below three membrane - which would be a high condensation risk unventilated void.

Warmcel?