You don't, except to prevent rainwater leaks. The vapor barrier should be
on the inside in a cold climate, to prevent condensation inside walls...
It's also easier to find and fix air leaks from the inside, on a cold day,
using a large exhaust fan and cheap siliconized latex caulk that dries clear.
In your case, you don't need to caulk inside or outside. The plastic
sheeting over all the windows takes care of everything. And who cares
if your trailer gets a little water in it anyway? It's probably the
first time the floors have even come close to being washed.
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