Polystyrene as a loft insulation

Firstly you might want to improve the insulation value by removing the existing insulation and installing Knauf (250mm) loft insulation in between your joists. If you require a boarded loft space and feel you could still do with more insulation then I would use kingspan on top of the joists and then install T+G chipboard on top of the insulation. It all really depends how well the rest of your property has been insulated. Regards to your roof, felt (underlay) is used as a secondary defence against broken tiles/slates and also proves quite handy at keeping tile/slate material out of your loft space, there are plenty of reasons why you should make your roof as water tight as possible so I would suggest if your finances can stretch to installing the felt then I personally would say it’s an investment well spent. If you was to install some form of barrier as you mentioned I would advise a breathable material, and make sure the dust does not gather around the eave area, this will cause problems with air flow. I hope this helps.