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I was looking for inspiration when I came across your wagon. Its jaw dropping. In regards to vapor barriers, the styrofoam is its own V.B. and you shouldn't have 2 V.B.'s in a wall because moisture can get trapped between them. I don't think its going to hurt your trailer, its just so well built. I like to cut styrofoam a little smaller and "calk" it to framing with a bit of spray foam when I use it to insulate, then its 100% draft proof