On a project that we are currently working on, we are removing a deck over living space and converting it to a low sloped shed roof. We plan to leave the existing membrane(Bituthane with a protection sheet of some kind) roof to keep the interior protected while we work on the new roof. We will have to open up the end where the seat cuts for the new rafters need to sit on the plate but plan to leave the rest. There are recessed lights in the room below so I am sure that there is a lot of air getting into the existing setup. Our intention is to not have to remove the ceiling in this space( we will likely convert the existing cans to LED units that we can seal to the drywall to limit air intrusion). I am thinking about installing sheets of polyiso over the membrane once we are ready to sheet the new roof to help keep the sheathing above dew point and detailing with closed cell foam around the heels of the new rafters. I am wondering if I should be concerned about the new “attic” space that will be created between the foam and the new roof deck. I could potentially put a gable end vent on one side but the other abuts the house. The pitch is only 2-12 so I would be concerned that trying to have a vent at the top against the head wall would be vulnerable to wind driven rain. I have attached a picture with a pointer indicating where the foam would go. I am on the Eastern Shore of MD.
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