New answers tagged freezing
You should seal that hole to keep vermin from passing through and to insulate. Any traps (the "u" or "p"-shaped drain fixtures that maintain a water-seal to prevent sewer gases from venting into your home) should be prevented from freezing, but those are usually located under the sink fixture outside of the wall. Some bathtubs and showers may be an exception ...
Note that some freezers, paradoxically, had actual heaters for the outside walls to reduce condensation. That has mostly gone away as insulation has become better and energy efficiency has become a major selling point.
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