Originally Posted by JohnandMandi
My fault for getting off topic but...
Hypothetically, if there are three coats of Por-15 on the entire floor would it eventually succumb to moisture from above or is the main concern now it rusting from below?
Secondly, is an asphalt based product enough to "seal" it properly to said thrice coated floor?
John, preventing rust and preventing the migration of moisture are two different problems, although moisture can cause rust if long term. Even if the floor was made of stainless and therefore rust wasn't going to be an issue, there are reasons I personally wouldn't want condensation under the subfloor and/or the floor insulation. One of the things I hate about most older RVs is that they tend to smell musty, and that affects me big time. That's why I would want insulation installed so that moisture can't get to the cold surface to condense.
When I used the word seal it was in the context of preventing water vapor in the air inside the van from being able to find its way to the coldest surface. The metal van floor will get cold on cold days, and if the air inside the van gets humid due to occupants, the only way I know to prevent condensation is to use enough insulation to keep the inside surface above the air's dew point.
Plywood alone will serve as insulation, but depending on outside temperature and inside humidity, moisture could form on inner surface (this is unlikely). Water vapor in air can also travel between joints in plywood segments or around perimeter edges and then condense on cold metal van floor (more likely if not sealed properly).
To your second question if I understand the context properly, we often used asphalt based sealers to join sections of insulation on walls, floors, and pipe in very cold applications to prevent moisture from getting to cold surface between the different insulation sections. But the sealer is not insulation itself. Asphalt based sealers were very popular a very long time ago to seal foamglass insulation in large freezers. It was used it walls, ceilings, and particularly under concrete floors in cold rooms and freezers. With proper construction it lasted for many decades. But again, the sealer itself doesn't insulate, it just fills the gap between insulation segments to prevent moisture from penetrating through to cold surface.