Top Hat.
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This is an electrical top hat. When electrical wiring is first being installed in a new house the wall outlets and switch boxes are installed inside these plastic top hats. Their purpose is to maintain the integrity of the vapour barrier seal in the wall, which prevents moisture from condensing inside the walls in winter. Cold climate stuff, I guess. Standard in Canada.
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Here’s another view of the plastic top hat. The electrical box goes inside and the outer flanges are used to tape the hat into the hole in the vapour barrier, completing the seal.