The rainguards on our front windows were a quick fix.  We needed to sleep in the van long before we had windows or a vent/fan install planned; which also meant we needed a way to get fresh air and try to limit condensation inside the van. While our sprinter is remarkably drier than the bus was (if it rained outside the bus...it was also raining inside), but it is still the pacific northwest.  The rain guards allow us to roll the windows down enough for fresh air but not enough to allow rain in.  This has to be the most affordable and most bang for the buck of any of the projects coming our way (and it doesn't hurt that the entire install took almost no time (I didn't even have time to finish my project cocktail).