Trailer Ceiling

I have a 45foot trailer were i have alot of my shop stuff in drillpress,hydpress,lathe,mill, shaper and other stuff. The walls have plywood on them but i would like to heat it some what so i need to do something with the ceiling. Has anyone done this before? Do i need a vaperbearer? what would be the ceapest way to do this i hope to build another shop in the next 3 years?

Trailer Ceiling