Rock wool is made from slag of steel production. It catches fire at several thousand degrees (very safe)
Mineral wool is similar but can be made from molten spun rock, slag from steel production, or any other minerals
both virtually cannot catch fire.. If that's any worry of yours. You should use one of those. They're available all over. You can put a blow torch on them and nothing will happen to confirm.
Cellulose also has very heavy soaking process of flame retardant. It's the second most flame resistant but also probably terrible to work with to do around batteries.
Pink foam board is the best foam board and also is "fire retardant" it'll still melt though, as will every foam except thermal spray/treated foam but that's the treatment spray protecting it. Also resistant to most weather and molds etc. It's used in pool/hot tub more than others
Fiberglass just melts
If you want spray foam to just "fill everything" like you say, you can get
FROTH-PAK Spray Foam Sealant is a 2-component, quick-cure polyurethane foam that fills penetrations and cracks in rim joists, roof wall junctures and around pipes and ducts greater than 2 in. and less
www.homedepot.com
or
FROTH-PAK Spray Foam Insulation is a Class A Fire Rated 2-component, quick-cure polyurethane foam that fills cavities, penetrations and cracks in rim joists, roof wall junctures and around pipes and ducts
www.homedepot.com
red one is more but it has some more ratings on it, including fire retardant etc
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If you want a giant pack of something you can get
https://www.lowes.com/pd/ROCKWOOL-C...th-Sound-Barrier-15-25-in-W-x-47-in-L/3610406 which will give you half your R value you're after.
Or just use a few pink foam boards (only home depot carries them)
https://www.homedepot.com/p/Owens-C...Foam-Board-Insulation-Sheathing-45W/100320352
cost adds up but there you can easily see it'd be 10 inches thick to get your R50 (since the boards are 2 inches R10)
This product is extremely easy to cut and you can use a variety of tools but the easiest is a very small toothed saw like a japanese style pull saw or a hacksaw just with the blade. Sawzaw makes a huge mess and hot knife stinks and also shrinks the product
Personally for your R50 I'd do a concrete board structure for the internal space to get the size right. Wrap it with rock wool and then make the outside look nice with whatever.. wood etc.
I've already done this and you only need like 20 watts of power and <R10 so if you're actually wanting to do R50 you probably don't need heating at all
There has been a few discussions on self heating LFP or using external heaters and it occurred to me I used to have an iguana and used a small pad heater in the tank that actually got very warm and used low watts. So I looked for them on Amazon and they have lots of models big and small and...
diysolarforum.com
If it's windy against your battery you want a wind barrier on the outside of the insulation or it'll pierce through it and cool faster... literally anything. Plastic, wood, concrete board etc. Sounds like your batteries might be outside idk
Aerogel also should be non-flammable until thousands of degrees.. but it probably has an insane cost lol
but yea I think that'd win, I don't think anything is more insulative than aerogels are (that we've discovered yet)