Cathedral ceiling ventilation baffle
I have a 12/12 roof over an attic that I am making into a storage room. I added 1/2″ plywood onto the joists to make a rough floor. On one half of the roof a second roof was built over it at about 10/12 to extend the roof out over a porch. All three “roofs” meet at the ridge. I am building ventilation baffles between the rafters with 1×1’s as the spacers and 5.5mm underlayment plywood as the baffle panels. The sheathing is old 1″ boards on the rafters with 1/2″ OSB over that on the single roof side. On the double roof side it is 1″ boards throughout with OSB only along the upper 24″ or so. Below that it is just the 1″ boards with lots of large gaps for air. There are lots of gaps between the old 1″ boards throughout the roof. It will be 3-1/2″ Roxul between the rafters and then 2″ of XPS under that and then a 6mil poly sheet. Approximately R-25. I’m in central Vermont and I know that is below standard, but it is for storage and is too low to ever be a living space. The floor of this space has R-19 fiberglass in it covered in plywood now since it used to be the ceiling. Here is my question: On the side without the added roof I’m doing it as shown on one of your pages, but on the side with the added roof it seems that no baffles would be needed under the original roof layer since the space above that (the space between roof 1 and roof 2) is wide open to the outside world for ventilation. Is that reasonable? If any moisture leaks up into the 1″ boards I would assume they will dry out from the top. Or, is 1″ wood too thick to allow adequate drying. Lots of air on top so it will not be stagnant.
I hope I made the structure clear, what do you think?