Greetings Nirky.
I am no expert but will throw in a couple cents worth.
I have been toying with covering my barrel too. I am more interested in the heat getting into my (yet to be constructed) bench with water heating element. I was thinking about a variety of covering options, including insulation, thinking that would help "move the heat" along.
Come to find out it, insulating the primary bell would do exactly the opposite. The cooling of the gasses as it drops down the outside of the heat riser is one of the key functions of the RMH unit, it's part of the pumping process! That said, there are theories out there that if, like in your case it's more mass and less insulation, then it will still take heat from the gasses fresh out of the heat riser, cooling it (albeit slower than a steel drum exposed to ambient temps) and playing it's role pumping the goodies along.
A note about perlite. I too am casting a core and while shopping for ingredients was informed that using perlite is akin to adding marbles to a mix, it offers no structural integration, integrity. I was told that adding vermiculite was akin to adding oatmeal to the mix, absorbing some of the water (read slower dry) but it becomes part of the core/cast, not just an added aggregate.
That said many have used it, Matt of course showing us on his great core creation video (my inspiration to cast my own
) uses to great affect perlite. I was unable to get the stuff, and my masonry supply place had honking bags of vermiculite for 15 bucks so that's my deal!
I struggled with the fire/found clay issue for months (kudos to many on the board here for being patient with me) and recently found a source of fire clay (called a pottery place three 1/2 hours away) and perhaps even found clay, but that hasn't panned out and I will cob with that, not cast.
My thinking was to take Matt's recipe and tweak the ratios, ratcheting the fire cement up to 50% of the mix. HeatStop in a 50# bag is 45$, to me that is an investment in making a bombproof core that I wouldn't have to fiddle with. I will be casting in the next week or so and will document the progress here. Whether or not it works will be for everyone to see, and for our general edification....
My hope of course is for it to work swimmingly and to create a highly efficient moderately priced core, somewhere between going with straight castable refractory {spendy but awesome} and homebrew cob combo {less expensive but needs some tlc along the way and a cob surround.
Would love to see your build, its pretty hard to think you'll regret it, but like so much in life, we play the balance of pros and cons and learn along the way. Hopefully we can do this dance with respect for our hostess and eke out a sustainable future...