Putting in a doorway through a load bearing wall. The wall is 7 inches thick, so I opened it up expecting to see 2x6 studs. Instead I found 2x3 studs on each side of the wall separated by chipboard - essentially two half-thick walls with the chipboard in between. The studs are supporting a 2x3 overhead plate on each side, again separated by the chipboard. Chipboard is not glued, just stapled to the studs. This wall appears to be supporting only the ceiling joists, nothing above them. The second floor is supported by an entirely separate support system that hangs several inches above the downstairs ceiling joists. I am able to see this through an access hole in the upstairs floor. There is about a 1-1/2 foot plenum between the downstairs ceiling and the upstairs floor. The main 6x10 house beam runs parallel to and above the wall I am working with, but I don't believe it is being supported by the wall. The joists run well below the support beam. I need to determine what size header I will need across my 39 inch opening, which involves the removal of two studs. This appears to be some kind of modular house thing, but I'm not sure. Is anyone familiar with this type of construction? Thank you.