While T37Chef will probably be okay with some sort of off-the-shelf insulation board, I'm going to disagree a bit about whether the fire/fume hazard from certain insulations is a big deal or not.
Over the years, I've read several accounts describing smoldering insulation fires that forced abandonment of the boat because the toxicity of the fumes was overwhelming, making it impossible to combat what would otherwise have been a manageable fire.
We tend to read about these incidents in connection with insulated metal boats, but some insulated fiberglass boats are also vulnerable. Builders of metal boats always face a dilemma as they decide on how/whether to insulate their hulls (besides the thermal and acoustic qualities, proper insulation can minimize the corrosive effects of condensation on the interior of steel hulls.) Most opt for insulation, since the benefits generally outweighing the risks, but as BillyR mentioned they live with the nagging worry about fire (especially insidious electrical fires from wire runs through the insulation).
The U.S. Navy recently (within the past few years) developed and tested a new foam insulation that is supposed to be non-flammable/non-toxic. I think it was spray-on, but I'm not positive of that. I don't know whether it has trickled into the market yet.
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Pacific Seacraft Crealock 31 #62
NEVER CALLS CRUISINGDAD BACK....CAN"T TAKE THE ACCENT