The Question (see pic below): Can I safely sister 2x8s to a 2x4 king post truss/open rafter system to achieve an insulated open ceiling/vaulted design.
The Details: I have a garage that is 18 feet long by 14 feet wide. I am planning on converting this slowly to a finished shop space and would prefer open rafter/vaulted ceiling for aesthetics. I also want the option to convert to ADU and am going to the headache to rough in everything to code. Energy code requires R49 unless it's an joist-vaulted or single rafter design, in which case you can use R38. Since I am leaving 1-2 inches of space for the air gap between the roof and the insulation, I think I can just add 2x8s sistered to the 2x4 rafters then use 6 inches of thermosheath R13.1 (per 2 inches) to get R39.3 and still have enough room for the air gap.
The entire king post truss system is built entirely out of 2x4 except the king post, which are 2x6s. There are 4 trusses spaced 4 feet and then rafters spaced 24 inches (the last truss is 2 feet from the back wall). I'm located in Spokane, WA.
I'm just worried that the weight of the 2x8s will exceed the load for the roof, especially where there is open rafter. Is this the case, or am I OK? Any other thoughts on how to achieve this?
Here's a pic of what I'm dealing with:
Here is what I am going for, ultimately, understanding that when it's all said and done only the tie rafter and the king post would be exposed.