Narrow frost band along the bottom of the inside glass

One useful distinction is which side of the glass the moisture is on. Frost on the room-facing surface that disappears as the pane warms fits surface condensation. Haze or moisture trapped between the panes points instead to a problem with the insulated glass unit’s seal. Frost along the inside edge alone doesn’t establish that, so the surface-versus-between-panes check helps separate an indoor condensation issue from a glass-unit concern.

Frost extending onto the sash, especially at one window, is a useful pattern to note, but it doesn’t by itself distinguish an air leak from a locally colder surface. Similar frost across several windows during very cold weather with high indoor humidity is more consistent with widespread surface condensation. Neither pattern proves the cause.

A useful comparison is the room’s dew point against the temperature of the glass. If the glass is at or below the dew point, moisture from the room can condense there; it freezes if the glass is also below freezing. Colder outdoor temperatures can lower the glass temperature enough to cross those thresholds, but that alone doesn’t explain why the glass is so cold.

The windy-day comparison may help with the sash question: hold a narrow strip of tissue near the sash joints and watch for movement that changes with the gusts. That would support air getting through a joint, though a still strip doesn’t rule out a cold edge from heat conducting through the frame or glass. Avoid treating the tissue test as proof either way.