What's this?

[bullethead123]

Hi all,
Just peeling off some ancient, curling up at the seams, lining paper and I find some polystyrene backed plasterboard underneath. This stuff seems to be on (the inside of) all the downstairs exterior walls only. I guess it is some form of insulation but it has peeled offin some places. I'm thinking to strip it all off and skim the plasterboard. Is this a bad idea?

 

[bullethead123]

Here are some pics.

 

[RandomGrinch]

Just peeling off some ancient, curling up at the seams, lining paper and I find some polystyrene

You've already guessed what it is without knowing it!
Have a look for 'Thermal lining paper" - along the lines of:

Graphite+ Insulating Lining Paper 7.5m x 50cm | Toolstation

A 4mm layer of BASF expanded polystyrene infused with graphite particles and reinforced by a laminated backing. Delivers 20% better insulation performance than a standard polystyrene of the same thickness, equivalent to 26cm of solid concrete. Also helps to combat condensation and resultant...

www.toolstation.com

I'm thinking to strip it all off

Yes, just strip it off

 

[bullethead123]

Thanks Random. So, should I just strip it off, skim the plasterboard and paint over that or should I replace the thermally lined paper?
The house is a seni of 1880s construction with solid brick walls. The EPC says insulating outside walls would cost 4-14,000 quid so I doubt they mean 18 quid a roll lining paper.