Condensation issues arise when you have hotter humid air meet a colder surface. The type of material doesn't really matter, think of cool mornings where there's dew on everything. Homes are usually built with a vapor barrier to reduce this issue.
As far as what type of insulation to use, it really depends on your budget, and what you plan to do with the bus. generally it goes as follows:
Good: batting- batting is usually the cheapest option, it's better than the factory insulation but is the most susceptible to mold growth due to condensation. If you were to use batting i would recommend using a vapor barrier to try to keep the moisture in the insulation layer down.
Better: Foam board- this is usually more expensive than batting but is more resistant to moisture, working with foam board can be a pain trying to match it to the curves of the roof. While it is more resistant to moisture i still used a vapor barrier with mine.
Best: spray foam- spray foam offers the best r value in the same area as the other options, it also acts as a vapor barrior in itself. other benefits of spray foam is it can get into all the small nooks and crevices that you wouldn't be able to get to with the other options, it also acts as a sound deadener as it attaches directly to the panels reducing vibration and sound transfer. With all the pro's of spray foam comes a matching cost, this will be the most expensive option. Most people find a pro to do it although there are kits you can buy and your local menards, lowes, ect.
There are a ton of factors that go into choosing what type & how much r value you need. A person that never plans to see cold weather won't need as much insulation as the one who spends winters boondocking in canada. I would recommend reading through other builds and seeing how others have insulated. There are a ton of insulation threads out there with a lot more information than I've provided, this is just a general run down from what I've gathered from my research.
