Shed roof insulation

Hello experts,

I have built a shed.
24 sand-lime brick walls.
Flat roof with 26 cm beams and a 16 mm boarding on top, covered with welding membrane and roofing felt.
Now my question.
From the inside, I would like to clad the ceiling with OSB boards.
I still have 20 cm thick insulation (compressed fleece) available, unfortunately my beams are 26 cm thick. Do I have to fill the 6 cm gap with additional insulation or can I place the 20 cm insulation on the OSB boards? I am afraid that condensation will form in the gap.
There is nothing further on or above the roof and it is meant to serve as a shed, possibly heated occasionally in winter for a party.
I am attaching a drawing.

If it is a shed similar to a garden house and you only want to have a party in it occasionally in winter, I would simply save yourself the effort of insulating. With a gas heater, the shed gets warm in no time even without insulation, and for the few hours during the party, the energy consumption is simply irrelevant. Even with insulation, hardly any noticeable improvement would occur, since you would also have to insulate the walls.€: somehow overlooked that the walls are KS. But still, I am of the opinion that insulating the roof is not worth the effort. So my tip: leave it as it is, and if you have a party, buy a gas bottle + heater or put in an old workshop stove. That is more than enough and easy.

Gas bottle + heater

In enclosed spaces, of course, one should know what one is doing. So please pay attention to exhaust air and supply air when burning.

I have the same structure, but with 24 cm aerated concrete blocks for my garage. However, I only used 16 cm or 18 cm Isover insulation wool. There shouldn't really be any problems. You can try googling a bit about ventilated flat roofs... I haven't really thought much about this so far. But I would definitely insulate... I find the insulation useful even in summer when it’s scorching hot outside.