When I think of thermal insulation, two basic types come to mind:

• solid plates like I used in vapsy:
  and
• the wool type (mineral wool or glass wool for example)

Solid plates are advantageous because they can be part of the support structure, while wool will most probably need some kind of outer shell.

In Vapsy, vermiculite plates with 25 mm thickness were used and they did a decent job. Easy to cut and drill, reasonably robust and not too brittle. I guess I'll use them again at least for the lower part of the chamber where the saturated vapor will reside:

For the upper part, it might get a bit fancier:

If an outer shell is required for e.g. wool insulation, I can also exchange wool for something else like expanded glass granules:

They come in various grades and would form a packed bed. A packed bed has the added feature that it can be cooled after soldering:

There would be some problems though, like natural convection in the packed bed resulting in a probably unwanted heat loss, and also transporting hot air towards the fan when it's not blowing, which the fan might not like. Also, the fan would only begin by cooling the insulation, but not the chamber. Needless to say that this wouldn't be particularly easy to build, either.

The arrangement shown above is a kind of counterflow arrangement where the cold inlet air hits the coolest part of the chamber first (I've written about the temperature profile within the chamber in a previous log). I could also move the fan to the bottom and make it blow upwards through the insulation, resulting in something like a parallel flow because the cool air hits the hottest part of the chamber first. If this was a common single-phase heat exchanger I'd probably say that the counterflow arrangement would result in a higher overall cooling heat flow, but since phase change within the chamber will take place, things are probably a  bit different. Moving the fan to the bottom would also eliminate the risk of hot air reaching the fan by natural convection.

Simply using more vermiculite plates for the upper part is probably simpler, but I'd loose the ability to have active cooling for the chamber. I could also just try it without insulation first and add a fan for cooling.

Regarding thermal conductivity I found these values:

• rock or glass wool: 0.03 ... 0.05 W/m
• vermiculite plates: 0.06 ... 0.07 W/mK
• expanded glass granules: 0.06 ... 0.12 W/mK
• expanded clay granules: 0.10 ... 0.16 W/mK

Bulk density and heat capacity might be important aspects as well (for cooling the insulation with a fan), but my gut feeling is that the resulting bulk heat capacity will be in the same ball park for common materials.