Cold roof insulation

6 May 2016
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United Kingdom
My outbuilding has a flat cold roof that is a little unusual in its design.
The joists which taper down to provide the roof pitch have no insulation between them. Instead there are 60mm foam insulation boards (60mm thick) fixed to the underside of the joists with plasterboards affixed to the underside of those.

What i would like to do is to add some insulation between the joists.
I understand that the cold roof space needs to be vented?
At the moment there are vents fitted in the fascia board at either end of the building between each of the joists to provide cross ventilation.
I have read that there must be an air gap of a minimum of 50mm to allow for this cross ventilation. Is this true? I can't just fill the entire void with insulation? Would this cause the roof to sweat and evetually rot?

Which insulation is best suited to filling the voids? Which has the best insulation properties? Sheeps wool? Foam? Rockwool? Eco-quilt?
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