Low slope compact unvented roof
I know variations of this question have been asked and answered but now I’m confused and a bit worried before I close my ceiling in.
I live I rural Virginia in the foothills West of the Shenandoah Valley. Winters rarely colder than ~ 20 deg (more typically mid 30s), summers can be quite hot and humid – as high as mid 90s with 90% humidity, more typically mid 80s.
I built a 24’x36’, 2×6 and block wall building for woodworking with a 2.5 pitch roof, 12” deep I-Joists, no attic. Ceiling is ~ 14’ high on one side, 9’ on the other. Its built into a hill – one end is ~ 7’ below grade with 2” rigid foam on the exterior block walls below grade and R13 on interior firred walls. R19 on all above ground walls. I opted for a non vented roof given low slope and little space for air gap inside joist cavities. I sealed all seams and gaps around the perimeter above the walls on the interior side with closed cell spray foam, then added R39 batt fiberglass insulation to the ceiling. I also sealed all seams, sill pate, penetrations, etc in the walls with closed cell. I made it as air tight as I could. It has an asphalt shingle roof, but no other insulation. I am putting a tongue and groove ceiling in for good looks and to aid drying to the interior (ceiling is not in yet).
HVAC (minisplit) is also not in yet. Exterior is done, except 11’ wide main door which I will build once the shop is operational. I have plastic sheeting covering the opening, but not sealed… have to get in and out. I was installing drywall on a very hot and VERY humid day. At the hottest part of the day, I noticed significant condensation accumulation at the highest side/edge of the ceiling… the first 4-6” of batt insulation was WET. I put two dehumidifiers inside, added several fans, and there was no more condensation on subsequent hot days. My hope and belief is – once the space is fully conditioned and sealed, I won’t have a problem.
But before I install my ceiling, I would appreciate your thoughts. Would I be better off pulling out the batt and get the entire underside roof deck between joists sprayed with closed cell instead?
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