Ahh condensation, my old nemesis.
Here in the PNW, the amount of condensation is ludicrous.
My experience, is that putting the tar paper in helps a little. After all, what you want, is to keep the roof from getting below the dew point, as it's being cooled from outside the truck, insulation helps. If you use insulation though, you'd better use something that won't collect moisture, don't use fibreglass for instance! At some point, the roof will still get below the dew point, and then the condensation forms on the insulation if it's exposed, or head liner, or both.
So, what I've found most effective, is to keep the humidity in the truck down, driving the truck every day does this, as the heater (especially the fresh air) dries the truck out. Now, I understand that not everybody drives every day. When I'm not around, and my truck is sitting in the car port all lonesome, I leave a small (500watt) electric radiator in the cab. Now, it's not on all the time! I just use an outdoor timer, and have the electric rad and the block heater come on at the same time, for a few hours, just before dawn, and just before dusk. The block heater is nice too, as it keeps the darn block from getting all rusty from condensation too, as its warmed up. Having a block heater is great for winter starting, but the best part is, if you leave the truck plugged in for a few hours before you start it, the intake manifold and carb get warm, and won't freeze up as often. Where I live, any time the temperature gets low, and the humidity gets high (all winter!) the carb will freeze up a few minutes after the truck starts.
In my last truck, I had an old in car heater under the dash, so didn't need to use the electric radiator. I haven't found one "cool" enough looking for the panel truck yet, and as I'm not often gone, I use the portable heater.
Nothing is as un cool as driving around having the roof drip on your head. Ugh.
