I am considering adding hard-wired smoke detectors to my house. Due to other in-process remodeling projects and the structure of the house, I have access to get behind the drywall at each location that needs a detector so this would be a relatively easy job.
After watching some videos and reading up on how to do this project I understand what needs to be done except that I am unsure about the electrical boxes for each detector.
The videos and tutorials all used single-gang old work metal boxes. I found a related question here: How can I install a wired smoke detector without compromising the fire rated sheetrock? The accepted answer suggests using metal octagon boxes with a fire protection layer added. However, this house has no fire protection: no OSB layers above the walls, no fire-rated caulk at intrusion points, no fire-rated drywall, etc.
Do I have to use metal boxes or is plastic acceptable? Should they be single-gang or octagon ceiling boxes? Do I need additional fire protection around these boxes? Given that the rest of the house has no fire protection does this even matter?
The house is located in the U.S. in a state that adopts the latest NEC unaltered every three years.