Ever thought about using polyicynene, also known as spray foam. With batts you still run the risk of mold and with extruded poly styrene you may get squeeking and you would still need vapour barrier. I am installing the spray foam which double as vapour barrier but I am also putting in an air exchanger (a home made one) controlled by a humidistat to control the "sweating" or condensation inside the bus. Double pane windows help with condensation as well but they are VERY expensive. If its humid where you are I would use vapour barrier. There two types of building envelopes (the structure that encloses you). Conventional- meaning vapour barrier on the warm side of your insulation which is obviously meant to stop the transfer of moisture laden air through your wall. The second is Hygroscopic- this is where the moisture laden air is allowed to pass through the building envelope. This is generally how straw bale, chordwood, and sod buildings are constructed and it does seem to work in certain applications.
Recently my aunt in Scotland was telling me how many of the old buildings are just constantly growing mold in them. This is because the walls are Hygroscopic but the moisture levels are so high it gets trapped on the surface of the wall and hence molds. Up here in Canada pipeline workers live in RVs and after three years of full time living they have to scrap the RV because so much moisture freezes into the walls then thaws inside the walls in the spring and subsequently rots out the poorly built structure, all as a result of no vapour barrier. There is a company called Versa-Foam among others that sell large quantity spray foam for self doer applications. In a lot of words, what I am saying is, if you want vapour barrier, insulation, no squeaking and no rot or mold in a really humid area spray foam is a great way to go. If your only using your skoolie short term and find spray foam too costly or too much work then as a second choice I would use extruded polystyrene and tape all the joints with an approved building tape.
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