Kaowool by B&W is for very high temps like 1400 degrees. It is a waste of money on this.
The ceramic fibers that I hated cutting was Cerafelt by JM. Deadly fibers.

Fiberglas and resin binder burns... Rockwool does not. You can do blankets or boards but you can butt them close together.

Insulation is all about density and you really need to buy from a vendor who services the power, chemical, or steel industry. They know the products.

Sorry to have let this lay so long but THANK YOU to all who have responded. We've been busy working on the 1385 and appreciate the encouragement. The latest posts are here: https://www.midcontinent.org/category/cnw-1385/ ......................mld

Here is an alternative solution: two layers of 1" foil backed rockwool, foil outwards and sealed down the edge with 4" aluminium adhesive tape, which is very sticky indeed, giving a sealed 2" mattress. Once you have got these made up - no dust and no soggy insulation held against the boiler; held on with soft iron wire while you get the cladding in place.