From what I've seen, its basically a honeycomb with a thick plastic coated foil to act as an atmospheric barrier. I came across it from a number of sites, but I've not managed to get any samples yet.

> And how do you maintain the 'very low internal pressure' over the decades that you will need it for?

now thats a million dollar question. I don't know.

https://en.wikipedia.org/wiki/Vacuum_insulated_panel Looks like the thing. So, yeah, very good vacuum. It's apparently decades for the lifespan, but much of it boils down to the materials being used.

> conductivity does not depend on pressure

I think it might be fumed silica aerogel at about 0.1 bar.

This paper describes the use of the same material for building insulation:

https://www.researchgate.net/figure/a-The-thermal-conductivi...

I suspect the key issue will be maintaining that low pressure for the ~100 year lifespan of the building. Perhaps future versions will come with micro pumps so that once per year the pressure can be pumped back down to 0.1 bar. Looks like for about a dollar you can buy a little 3 volt vacuum pump. One per panel out to do a decent job.

It is a good vacuum. They work but are expensive and a single nail destroys the whole panel, but maybe cheaper than the loss of floor space.