The January burst
Cold here is not an event, it is a residence: weeks at a time when the frost drives deeper every night. That kind of cold does not need an exposed wall and an unlucky night; it finds every pipe that lives at the edge of the heated envelope, and it finds them every single winter.
Where the cold gets in
The rim joist is the front line: the band of framing where the house sits on the foundation, insulated to whatever standard the build year required, carrying the plumbing that a kitchen or a bathroom above needed. Supply lines routed through it, or up an outside wall, or over a cantilevered floor, live closest to the cold. The older the house, the more of these routes it has. A century home in the city can have a bathroom group added in the forties over an unheated back porch, and that group freezes on a schedule its owner could mark on a calendar.
The burst that waits for the thaw
Ice can split a pipe and then seal it: the plug holds while the freeze lasts, and the line only flows again when it thaws. Which means some January failures are discovered in March, as a wall that starts weeping during the first warm week. A supply line that lost pressure in deep winter and “fixed itself” did not fix itself; it is waiting.
The immediate moves are the universal ones: main shut-off first (by the meter, street side of the basement), then heat and open cabinet doors for the freeze, extraction and drying for the flood. What is particular to here is the follow-up: a burst inside a finished wall wets the cavity, and a cavity dried by guesswork in February becomes a mold finding by July.