So, I have been reading here and googling looking for info on where to leave an air gap in the insulation process. I have my sheet metal interior walls out from under the windows and ready for insulation. I have rigid foam for this. Does the air gap go between the outside metal wall and the foam or between the inside wall, which will be wood, and the foam. I have read opinions on both ways but was seeking opinions from seasoned skoolies on which way is best on a bus?