Since there were 3 rolls of insulating fiber left over from building the kiln, I decided to use them as added insulation over the first chamber. It took two of the rolls to cover the kiln, with one roll left over for future repairs. This new layer of fiber alone should help a lot with heat loss during firing, and it will be covered with more clay/straw mix, pretty much doubling the insulation over the first chamber.
I found a place in Karatsu that makes a clay/straw/sand mixture that they sell as the base layer for tile roofs. They can ship one ton of the stuff to my studio for about $90, or two tons for $120. REALLY worth it considering the work it takes to mix that much mud and straw. Also, it has much less shrinkage than the regular clay mix I used for the first layer. I made a 100mm test tile to test shrinkage, and the stuff only shrank 3mm from very wet to bone dry.
Here, the first stage is done, with some bricks holding the fiber in place, just in case some high winds come through. I’ll apply the mud in the next week or two, hopefully before the firing happens.