Insulating a wall in the loft

Hi, looking for some advice please on insulating a plasterboard finished wall which backs into the loft from an extension 
I’m adding loft roll over existing insulation which is fine. But I’ve noticed that the extension we had built which has vaulted ceiling, the top of the wall backs into the loft space. It’s a mix of cavity wall in the centre holding up the roof truss, but the builder has not extended the wall into the eves, used a mix of timber and PIR board. Bit of a mess really. I’m looking to improve the insulation and was planning to add additional 50mmPIR boards over the top at 90 degrees and seal in with expanding foam. I don’t really want to touch the plasterboard so all works done from loft side. Appreciate guidance on proposed, as reading I may need vapour barrier on plasterboard which I can’t really get to? Hopefully pics help explain situation