Need help with vaulted garage cieling

You need enough insulation so the temp differences are not that different where vapor will condense. Corrugated steel or metal, vapor will ride the air leaks and vapor will condense into water. So you need more insulation inside and lots of space where air can move and vapor to escape or dry out. Since your rafters look shallow, I would aim for at least 2" space between the roof sheathing and the insulation. Add one or two short chimney on the ridge with weak power exhaust fan.

Add xps foam boards in the rafter bays with that air space in mind then add as much as your budget over the rafters. 3", 4" boards, as much as the budget. Then seal all joints so inside air doesn't leak into the rafter bays. Eternabond tape is most excellent adhesive. I also think gorilla duct tape (black) has very good adhesive and life. I used to have a pool and used the tape to hold pipe insulation to the ladder bottom rung. After about 3 summers in the water, tape still had some adhesion to itself. House wrap tape (acrylic, brand name such as tyvek) may also be good but never used it. Warranty on such tapes is 10 years. As such, adhesive on those tapes may be only good for about 10 years.


That video is matt risinger, I think. He seems to think about things and do a good job. IF foam boards directly on the roof sheathing, it can create moisture problems if indoor moisture is too high and goes through the ceiling/roof materials. Vapor condenses between the sheathing and foam boards.



Insulation DOES NOT stop temp transferences at the surface of itself. Energy gain/loss WILL happen no mater how much/kind of insulation. Insulation slows down the gain/loss. So no matter how much insulation there will be condensation point and that place needs to dry out.


Adding. Tarpaper (heavier or more layers) is a good material because it creates gaps between layers and also absorbs (and holds) moisture. By itself, though, it's not enough. Video straps to make that gap is another good thing.