I know this subject has been covered in various ways before (quite a bit
of googling done), but I am still not sure how to interpret all the bits
of information I have found. So let me describe the problem, and
hopefully one of you will know the answer.

This is what I understand (correct me if I am wrong):

In a warm roof construction, the insulation material is above the
rafters and continuous, and ventilation is not needed. On the warm side
of the insulation a vapour barier is needed.

In a cold roof, where the insulation is for example between the rafters,
you need to keep 50mm air space between the top of the insulation and
the roof covering (felt), and this space needs to be ventilated. Again,
a vapour barrier is needed on the warm side.

In the Building regulations part F2, section 0.2 it states that roofs
where the moisture from the building cannot permeate the insulation, no
ventilation is needed.

So why is it not possible to put insulation between the rafters, and
then seal the warm side with a vapour barrier, and not have any
ventilation? The reason I ask this question is that we have a flat roof
running up against a pitch roof, and it is really awkward to ventilate
everything. Unfortunately it is a bit late in the day to use a warm roof
construction.

Any explanations greatly appreciated,

Johan