"Staple tyvec to the bottom of the trusses, and put enough 1x2s across to support the insulation. (1 every 6"?) Fill the space between the trusses with cellulose insulation."
I am currently researching insulation solutions for our wood-floored cottage that is also up from the ground, so glad to see your design and questions here!
I am posting because I found some very important safety information regarding the use of STAPLES to attach FOIL type insulation.
New Zealand, my neighbour country, has banned foil and staples in underfloor insulation because some people have been killed when repairing underfloor insulation due to the metal in the staples and foil coming into contact with electrical wiring.
We have used earth-wool in ceiling and so far I plan to insulate floor with a combination of this and maybe some expanding foam in parts.
Good luck with your project, I like how the greenhouse is attached to the kitchen and goes to the ground...
here is a link to the New Zealand site: https://www.energywise.govt.nz/at-home/insulation/underfloor-insulation/choosing-underfloor-insulation/