Posted June 02, 2008 - 07:46 PM
You will need a vapor barrier. The outer walls and the cold and the inner walls and the warm will create lots of sweat and dew droplets. This isnt going to bother the outer wall, but it can soak up into the insulation. The foam will usually have a vapor barrier on one side, but ideally, a vapor barrier on the inside of the walls, but on the outer skin side of the insulation. If there is several sections, it must be overlapped to allow water to shed all the way down to the bottom. Keep the area at the last 2" of the wall (the very bottom) insulation free. And make sure if any water does collect down there, it has a way to run out on the ground. The ceiling is going to be tricky. Vapor barrier will need to overlap so sweat can shed off and then overlap on the vapor barrier in the walls. Sweat from the roof will run down each side, it must be able to run all the way down the walls and drain out the bottom. If you have the aluminum outer skin, the sweat is going to be heavy, especially if its heated and you are in the cold. Anytime you have a temp difference, ie outside vs inside, you will get condensation.
And if you can use foam, slightly thinner than the wall thickness (example, wall thickness is 2" this, then 1/5" think insulation, with the gap between the outer wall skin and insulation. This is also where the vapor barrier will be to protect the insulation. Any wood from the floor that may come into contact with the sweat, in areas not exposed to people, hit it with some of that Jasco wood treatment to keep it from rotting also.