Insulating workshop roof

[MMBHepworth]

Hi one and all , looking forward to the foreseeable future, with our new venture.
Hoping to glean some valuable expertise from the UK Workshop guys for a little help ere n there would be much appreciated, thanks in advance.
On to the first Q; On my workshop roof which has a 15 degree slope, constructed using 3 x 2 ( @ 4.8m length ) and spanning about 17 ft across, and having been covered using corrugated onduline, I'm at the stage where I wish to insulate and line the structures..

So I have 75mm timber thickness plus a 30mm void within the corrugated roofing, would a 50mm kingspan (insulation) sitting directly on say 9mm ply fixed to the rafters be sufficient? my thinking is that would give at least a 50mm air gap, I also have constructed my own opening rooflights within the roof structure for extra ventilation..

This is my first build, and has turned out to be more than just a shed, trying to build things right at a fair price and aesthetically pleasing to last for many years.. cheers for now :deer

 

[Blister]

Not sure if this will help , from a few years ago when I renovating my garage / workshop
what-s-blister-up-too-now-t34753.html?hilit=garage

The latter photos show the inside roof

 

[MMBHepworth]

Ah yes, thanks for that Blister.. by eck you have done a good job there buddy, fair bit of graft gone in there I bet
And good to know about how best to paint up the OSB (oil based paint that is) thanks again.. need to post a few times on ere b4 I can post piccies of my workshop, only just finished cladding the exterior and trying to get it watertight!