Ok thanks, Baltic birch has come up before as a good lightweight option. Attaching it shouldn't be an issue. I could take a better pic later but this is what I have now. I'm in a warm temperate area so I'm just trying to figure out the simplest/cheapest insulation that won't be useless or have moisture issues.
First priority is a roof vent for light and circulation. I'm assuming if I avoid the ribs I should be fine with it anywhere. Obviously I don't know what I'm doing so I've been holding off to now. Pic shows the level of "skill" I've got but it works for now while I learn more.