don't think its water proof its just suppose to help wick up very small amounts like a little condensation i think. but with all good luck there will be no condensation and won't have leak problems.
i just didn't want to put insulation or wood right against the metal that was all i didn't care what it was but the rosin paper was pretty inexpensive i think $11 for way more than enough to do the bus. i'm sure tar paper would be just as good or plastic.
well i'm off on the hunt to construction sites to salvage a little more wood so i can keep working away.