Can you create an unvented cathedral ceiling from above?
I would like to build a unvented cathedral ceiling from above on the main floor of a split level house. Climate zone 7.
Currently the house has 2×8 rafters with R19 Batts, a vent space (I believe useless because of the orientation of the rafters and the low slope) and around 3/4” plaster/lathe on the inside covered with tongue and groove pine paneling on 3/4” furring strips. The slope is 2/12, and the rafters run perpendicular with the slope.
Ideally I would like to insulate from above so that I don’t need to make a mess of the finished and renovated interior. The roof is nearing replacement time so it seems to make the most sense.
A rigid foam over roof seems the easiest, however we have a transom window tucked under the soffit where the two story and single story portion of the house meet. The bottom is approximately 7” from the current roof deck.
So far bulk filling the rafter cavity with spray foam from above seems the best option? Is it possible to lower the amount of spray foam in anyway?
The only assembly I can think of was to build a new over roof out of 2×4 lumber 24” OC perpendicular to the existing rafters. Use 3.5” mineral wool batts against the exterior side of the existing plaster and then bulkfill with 6” of closed cell spray foam. This would leave a small 1” gap between the roof deck and the insulation, but it would not be able to vent at all. I would love to limit or not use spray foam but I feel like in this application it is a necessary.
Any thoughts or ideas are greatly appreciated!
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