I'm installing a microwave/range hood. To preserve usable depth in the cabinet above, I'd like to use 3.25 x 10 duct through the cabinet, and transition to round duct in the attic. However, a ceiling joist is blocking me from running the duct straight up. Is it allowed by code (and not problematic any other way) to build an offset which would move the duct 3 inches to the side, within the cabinet?
If it's allowed, can it be done with simple parallel bending as shown in the sketch, or would it require actual curves?