hi same problem in my works van. my own van a crew cab 350 mwb high top is ply lined in the middle section thought great when purchased it so decide to ply the rear section but added a 2" polystyrene infill to try and insulate, only to find in the recent cold spell when weather got warmer the strip lights had water in them

went to the caravan centre the guy there said to stop this in caravans the insulation is bonded directly on to roof no gaps

i have fitted two fiamma 450 roof vents

insted of the flettner ones and open up in dry windy days to find this helps a little

also fitted 10 round vents in ply