Insulating my tiny cabin

I have a small cabin 10x20. It is all 2x4 walls and ceiling. I bought it as a shell and had it delivered. It's just a weekend getaway place.i have already installed R13 batts in the walls and put up exterior 4x8 sheet siding on the inside. It has a vaulted ceiling with a small loft where my kids sleep. The roof is metal. I am getting ready to put tongue and groove pine on the ceiling. It is not vented. The builder of the cabin wrapped the cabin in this white plastic stuff, even the ceiling. I think that stuff is for condensation or something. I was thinking about just putting more batt insulation up on the ceiling but want to make sure that I won't have a moisture problem. I hear it with a little wall mounted propane heater and used a window air conditioner in the summer. Attaching some pics so you can see that white plastic stuff

Mike, Its a bit of a cluster at this point without knowing what the exterior wrap the builder put on. You have a bunch of vapor barriers going on there and while insulating is usually a good thing, who knows what the net result is going to be now. Some would argue that insulating the ceiling would allow the exterior edge of the 2x4 to get very cold now as a result of the lack of heat flow to it. Between the moisture that you will be generating in a confined space and the exterior temp of that 2x4 edge, it is not uncommon to see condensation. If the builder just included housewrap as a standard, you could insulate and include a Class 1 vapor barrier on the ceiling to prevent any dissolved moisture from migrating into the cavity. Tough to make a definite recommendation right now without know what the builder put on. Either way, its a bit of a conundrum. You might just be better insulating it with un-faced batts and covering it with some insul-web so that it can dry out to the inside.