Originally Posted by witoke
I am still convinced that the moisture comes in from underneath. Seems to me that the floor would be rusted randomly all over if it was just condensation. Plus in one of your pictures I can see the same thing I had, a penetration through the floor allowing outside air and water to come in.
We can debate where or how interior floor rust enters the cabin and does its damage---all to only have me refer back to more significant rust inside
than appears outside.
Referring to my own hands-on experience during first inspection of my "new" '03 E250 EB (shown in the above link) the factory rubber & backing pad was absolutely saturated with water; not moisture as in slightly damp---soaked. After placing a hold deposit I asked the owner to remove the mat completely and flip it upside down so the padding was away from the floor, could in theory dry out in the week before I'd return to complete the sale.
When picking the van up (summer months in Ohio) that mat and padding were STILL soaked, like a sponge in fact. The van had been sitting still for nearly 2 months by this time, no chance of tires slinging water against the under body.
Once home I laid the mat out padding side up on my patio where it sat for another 5 days of warm dry humidity-free days. Each day I'd place a hand in several areas testing how quickly it was drying out.
Curiously that same mat & pad is rolled up laying naked (uncovered/unprotected from the elements) on my front porch but even so the padding has not absorbed any moisture, its fairly dry even 3 years later.
Sooooooooooo all this is saying interior floor rust where an absorbent material such as carpet or carpet padding is a direct result of moisture trapped in that material and it never being able to completely evaporate out. This then keeps that moisture against the metal 100% of the time which leads to interior rust.
This condition also happens to the front seating area, exactly the same way, exactly the same results
FWIW that absorbed moisture comes from bare bones cargo-only vans without any sidewall or roof insulation---our vans "sweat"
Other opinions may exist where or how that rust forms but my ersatz "research" and experience tells me I'm onto something.
Even though its my thread on FTE its a good read for anyone dealing with this issue of interior floor rust no matter its severity or state of advance.