In most stock passenger vans, the floor, walls, and ceiling are all vented to prevent moisture buildup. The floors use a heavy fiberous wicking material to draw the moisture away from the metal so that it can be evaporated through the carpet. Even though not obvious, the side walls are vented top, bottom, and around all of the windows. The headliners are usually perforated, or made of breathable fabric, as well as being vented around the perimeter.
The holes in the ribs are designed to help this ventilation scheme, as well as to prevent moisture build up inside the ribs from condensation, because if they weren't present moisture would build up inside the sealed ribs.
I may or may not be using the term condensation correctly here. What I am referring to is when metal sweats do to the difference in temperature, humidity, and possibly even barometric pressure between the two sides of the material. The result of which is moisture occurring on the inside of a seemingly sealed object such as the skin of a van. This same process occurs in homes with metal framed windows. Under the right conditions we can be talking about a sizable amount of water in a single day, and due to the size of a van we can be talking cup fulls.
If the skin of the van isn't vented in some way via either direct venting, or some type of wicking material to transfer the moisture to the interior, it will become trapped between the vehicle skin and the interior walls/insulation leading to rust, mold, mildew, and dry rot if it can't be dissipated.
This is the exact reason that many camper and conversion vans, and many RV's & TT's in general, are riddled with mold and dry rot problems. Fiberglass will sweat just like the metal will. To my knowledge there is no way to stop this process, besides building in such a way as to compensate for it.
This is just one of many reasons that I have come to the conclusion that a stock window van with factory finished floor, walls, and ceiling is the best base to start a camper van with. All of the hard work is already done, no extra insulation is needed. It is easy enough to add curtains to prevent heat loss/gain through the windows if you choose to do so, and the opening windows will provide you with the necessary ventilation.
I agree that if you're starting with a stripped van you're probably going to want to add interior floor/walls/ceiling, but if you don't do it correctly, with proper ventilation to the inside skin of the vehicle, you WILL have mold, mildew, rust, and dry rot problems in the future.
Someone asked if anyone has torn out the interior of a conversion or camper van. I have rebuilt many over the years. RV's & TT's too. They are typically full of black mold and dry rot, many are sopping wet behind the interior paneling, and it was not caused by leaks, it was caused by condensation.
If you're building your own rig, history is the best teacher. Don't repeat the failures that others have made before you. Don't create an environment to trap moisture and grow mold and rust.
There's nothing more important than your health and well being. We are living in these rigs, not just occasionally taking them on camping trips. If you get mold, you're breathing that every day and every night. It's just not worth the risk. You can't smell carbon monoxide, but it can kill you. You might not be able to smell the mold either, and it's effects might be slower, but why take that risk.
