Hi - new to the forum. I'm renovating my first home. It's a 1903 concrete/block two story farmhouse. I pulled all the insulation out of the attic and torn out the sheetrock and old insulation from the upstairs ceilings. Never been so covered in mouse poop. After pulling out the ceiling rock I noticed there might be some potential with leaving the a-frame ceiling exposed. My thought is to insulate then sheetrock. However, I only have 2x6 joists. I'd like to use polyiso, but I can only find 2" thick. Can I glue two sheets together? Do I have to put a a vapor barrier or soffit vents?