I am probably the owner of the best insulated shutter enclosures (DIY that is) in the country, after finishing up today. It's been a long day of hard work, but sure enough it'll pay off I guess. I had to rip the current enclosure to shreds because someone deemed it necessary to use screws and nails randomly, to cover them up, before finishing off with a layer of paint. Sure enough that left me with no clues to find out how it all was built, let alone take a well thought through approach for disassembling it.
While closing off the cavity walls - which was the point entirely - I replaced the EPS insulation panels (Styrofoam) dating back from the 70's (I'm just guessing) with 40mm Jackodur XPS (extruded polystyrene foam) panels, which should give me better insulation value and a tighter fit (the old panels were sort of thrown in there, whereas I sealed everything up nicely).
So, what's left to do tomorrow? I have to get a large wooden panel, which I'll cut to the right size so I can build a new enclosure. Seems all to simple when compared to today's demolition derby, grin.
Note : using a circular saw inside is a bad idea. I'll be eating dust for weeks.