Hi
@rod725020 and welcome!
A greenhouse is a very humid environment many days of the year. If you have never owned one, you will find out after you build it, so make sure you do it the right way, or you're in for a heap of trouble later on with mold and rot. Before picking any insulation or taking advice from someone who may not be knowledgeable enough to give you a good answer (like me), I would check about fiberglass insulation and moisture.
Do your research first. Check YouTube, read articles, speak to people that do insulation for a living.
For me. I know I would want a vapor barrier and would probably use pink insulation board and even double-bubble, double-sided reflective insulation. One to insulate the wall and the other to reflect any heat away from the wall and back into the greenhouse, but there are variables in every greenhouse setup.
~Jim
