Earlier posts mentioned a lantern, not a flat roof light. Lanterns would tend to have cills, but flat lights would not and instead have some sort of proprietary shroud. So what this means is that the kerb could potentially be thinner (50mm timber ) for a flat light than for a lantern.
But whatever it is, it would be up to the manufacturer to specify the kerb requirements for their product. So that should be determined first, and then the kerb and the light well and structural requirems would be translated on to the drawing.
I'm not against box sections. In fact I made these as lintels for my timber frame instead of using double timbers, for the very same reason - to allow insulation within the box. But what I am saying is think about whether its actually necessary in context of the relatively small amount of exposed area.
Thanks. Yes other posters mentioned lantern but it is a roof light shown in the section and that is what we are using.
I've looked at quite a few and as I say, most have fairly similar kerb profile requirements between 75 and 100mm before plasterboard. See for example vitral attached which is 100mm (before plaster) and suggests ply frame. But I can totally see what you are saying about diminishing returns given the glass losses above it!
Some manufacturers seem offer additional purchase of insulated kerb but most I've seen expect the builder to do it. See attached for Vittral skyvision but these seem quite pricey for a double glaze unit..
Thanks a lot for all your comments.