Hi there , I don't think you actually referenced the type of insulation you were using but my ears twitched when I saw the mention of sound proofing and fire proofing in the same sentence. I built a 55 foot alloy cruiser and used rock=wool or navy
board for insulation. It is made by Roxul and is totally organic, made from spun volcanic rock or slag. It is stable to 2000 F +/= and is also quite a soundproofer. It is pretty much the product of choice on commercial
vessels for soundproofing/fire so I have 7 inches on my engine
room bulkhead for instance, 3-4 inches everywhere. I bought it from C&I insulation in Vancouver , Canada
. Heavy, for sure, but my boat is warm and fireproof and...quiet.
cheerio and stay warm and shhhh .ps, my boat is built to Dutch ice-class standards so maybe we have different cruising grounds.