Vented vs Unvented Roof Assembly
Hi, I am in the process of designing a net zero home with continuous insulation in Zone 6, near Kingston, Ontario.
The home is 48’ across by 44’ deep and will have a 1.5:12 mono-sloped roof. The roof trusses are 28” deep, leaving a lot of potential room for insulation.
The wall assembly will have cedar siding, furring strips, 2 x 2” of Rockwool comfortbatt exterior insulation, taped and sealed zip sheathing, 2×6 wall on 16 centers filled with Rockwool R24 insulation batts. I have a high level of comfort with this assembly and believe it will give the desired R value and moisture management for both summer and winter (please feel free to comment on this if I have overlooked anything!).
The roof assembly however is where I am not certain on the best approach. I am seeking an assembly that will be effective in managing moisture during warm summers and cold winters, protects against the elements and hard driving rain, which we often get in the summers and provides an appropriate amount of insulation to minimize heat loss in the winter and keep cool in the summer. What is critical for me, is a roof that minimizes risk of mold/moisture intrusion and leaks.
I am targeting an R value of over 60 and closer to 70 if possible. From what I have read, once you hit R70 it becomes a point of diminishing returns.
Additionally, the house faces south east and the plan is to have a 10 kw solar array installed on it and ideally with a metal roof.
My question is, given the specifications above, what would be a recommended roof assembly for this home that minimizes risks around moisture and provides the desired R value?
Given the depth of the trusses, I would be interested in being able to use that cavity space to the greatest extent possible. I believe in an assembly where you have insulation on the exterior of the roof sheathing and insulation below, you have to keep the ratio to roughly 60/40 R value split between the two. In this case, I wouldn’t use much of the truss cavity and would add several inches of rigid foam on the exterior, making a very thick roof.
I would appreciate any insight/recommendations on how best to create this roof assembly.
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