Pitched roof insulation

[manchestergas]

I'm ready to start insulating the roof of my single storey pitched roof extension. The pitch is 15% and at its widest point the span is 3.55m. We have used marley wessex tiles on 38x25mm batterns on breathable felt sat on 150x50mm (c16) rafters @400mm centres as per the chartered engineers drawings.

The rafters are infact about 147x50mm so the specified 150mm Kingspan is to deep and would protrude the underside of the rafters not allowing plasterboard to be mounted directly to the rafters. The electricians have kept their cables nice a tight to the breathable felt for me.
Before I ring the engineer I would like to ask what minimum insulation is I need to use that will allow me to keep maximum head room on the ceiling. If I use 125mm Kingspan between rafters and purchase the plasterboard with king span fitted to the rear of the board what is the minimum thickness this insulation backed board will have to be. Keeping the finished ceiling height as high as possible but so it passes building control is my main objective here as at the lowest point along the external wall of the new extension the floor to bottom of the rafters height is 2.3 meters. Thanks

 

[^woody^]

150 kingspan is 100 between and 50 across the complete underside. I don't think 25 across the underside will be sufficient.

 

[Ian H]

Our extension is near identical and we did it how Woody says. We used seperate 50mm insulation and 12.5mm plasterboard rather than a combined one.

 

[manchestergas]

150mm is 150mm though no? 125 between and 25 beneath can't really be much of a difference at all in u value unless I'm missing something else?

 

[Ian H]

I've got no idea. That's just how our plans were drawn.

 

[^woody^]

It's to do with thermal bridging of the rafters, so you need more cover beneath.