Originally Posted by Zephod_beeblebrox2
How do you handle condensation?
I'm thinking of lining the sides of my bus with that solid pink foam from Lowes and putting OSB on the side nearest me attached to wooden battons. The ceiling will probably have a layer of the space blanket stuff with vinyl glued over it. The floor is already plywood.
Water in wall assemblies is a topic that fascinates me, but I'm no expert! There's a lot to learn and at some point a person just has to say "here goes.. I hope I've learned enough." Here's a Building Science
reference you might find interesting. It seems to me that the main idea is to use insulation, vapor barrier, and air sealing in concert to control where in the wall the dew point occurs and limit the amount of water that can be present at that point in the wall. It's also important to design so that moisture in the wall can dry to the appropriate side. I think that part gets interesting because Building Science focuses on buildings which usually have porous cladding, whereas our buildings (buses) usually have metal cladding..
At the foam-to-metal-skin interface: my thought is that it should be sealed really well (spray-on or pourable foam bonded to clean metal) or should have a gap (solid foam board with a space between it and the skin) so that condensate can bead and roll down or can dry in place, but can't get held in suspension like the water in a flattened plastic bag -- a wet plastic bag seems to NEVER dry out. The foam might never be bothered by this, but figure on the metal coating eventually failing and then that trapped water contributes to rusting out from the inside. This is the sort of thing the EIFS stucco people have to worry about, too, and they might use grooved foam or crinkled house wrap to ensure there's drainage space between the foam and building sheating. For my bus I'm thinking about foam board (cheap) spaced off the metal a bit with liquid foam (expensive) poured down to fill and seal the space.
Finally, the floor -- harder to deal with because, being flat, you don't have any gravity drainage to help you. Seems to me that you'll want to do a really good job sealing the basin where the water belongs (sink, shower pan, etc) and use moisture-resistant materials below assembled in a way that they can dry out easily. To me that usually means gaps where air wants to flow through. Think of how nasty and water-damaged every kitchen sink base cabinet is.. If the storage space were open instead of closed up with doors, that wouldn't happen.
The foil facing on that OSB is a vapor barrier. But solid closed-cell foam (the extruded polystyrene pink/blue board) is also a vapor barrier, particularly if joints are taped, so the foil might add cost but little benefit. Having the solid wood wall might be nice for the ability to hang anything anywhere, but I also think Nat makes good points about using plywood straps to save weight and increase insulation if you can confidently plan for things like cabinets that must be wall-hung. Also, if that OSB is double-faced, it's a double-edged sword: any water (or humidity) that does get into the board will have a VERY hard time getting back out. It would have to dry to an edge rather than a face, and it's a very long way to an edge from the center of the panel.