Cold Roof air gap

[mrkjd]

I have an existing 2 car long garage with square section metal roofing fixed directly to 5x2 joists with the profile running across them. This means there is an (approx) 25mm "gap" venting either side of a joist where the highpoint of the square section roofing crosses the joist. Roof is sound in terms of weatherproofing.

I am thinking of converting the bottom end of the garage in to a workshop / office by building a stud wall and my question is re insulating the roof of this new space.

The roof is vented at the garage back wall eaves (just resting on the wall plate with the high ridges of the roofing left open) and I can leave open the ridges at the new dividing wall in to the garage space. The high ridges also vent across the joists (4 of them). I intend to fit flashing with vents at the outside rear of the roof to tidy it up and stop insects etc.

I am thinking of creating a cold roof for the workshop / office by 75mm insulating between joists leaving the 50mm gap between roofing underside and PIR topside then vapour barrier / 25mm insulation and 9mm ply/osb ceiling across the underside of the joists.

Is the gap between joists created by the roof panel high ridge sufficient to cross vent and compliant with BC?

 

[blup]

You seem to be describing a hybrid roof (two lots of insulation), so maybe BC input would be wise. And shouldn't the vcl be on the underside of the insulation?

Blup