Replacing Polystyrene Insulation

[The Lady Mary]

I'm in the process of gutting a Calypso 28 cruiser and rebuilding the interior. It has taken on a lot of water from all angles, so I'm keen to do a slow, thorough restoration, fixing the core issues before I rebuild anything. I pulled away most of the interior plywood from the walls, most of which was wet and rotten. In parts, especially around the window frames, there are slips of polystyrene (pictured). What is the best solution to replace these with? Please excuse my lack of expertise, this is the first restoration I have undertaken, so I'm keen to learn from you guys on here. Thanks in advance!

 

[LadyInBed]

Best wishes for your restoration.
I've been thinking about alternative insulation and reckon that laminate floor insulation would do a job.
5mm SuperGold Wood & Laminate Flooring Underlay (15m²)
Screwfix, B&Q, Wicks etc do cheaper fiber material.

 

[waynes world]

That looks a good thickness. of around 25mm, if so then i would use PR foam board type Kingspan etc. same for same but better. If its just for more condensation reduction heat retaining then try the bubble wrap packed in tight. or as above. But unless you insulate the whole cabin for heat insulation there is no point in much less than PIR foam board IMO. been there and done it a many a time.