Originally Posted by r_w
Did you insulate INSIDE the ribs? Is that possible with spray foam?
Nope, not possible unless you take the outside skin off. I insulated over the ribs. I would be surprised if insulating inside the ribs would actually do any good. Since they are directly connected to the outside metal they act as a thermal bridge from the bus skin to the inside. Foaming the hollow inside of the ribs would not stop heat/cold from travelling through the metal, around the insulation to the inside of the bus. Foaming OVER the ribs, on the other hand, does work. Put as much as you can over those things..
Studs in a house have a similar effect (thermal bridging), but we don't notice it as much since wood isn't as good of a conductor. Here are some thermal images of what I'm talking about.
Inside of house is warm, outside is cool:
Inside of house is cool, outside is warm: