Why do my windows sweat or fog up?

Condensation on the inside of the glass usually means humid indoor air meeting a cold surface, which is common in older single-pane Chicagoland windows during winter. Fog or moisture trapped between the panes is a different problem: it means the seal has failed, and that window is no longer insulating the way it should. Surface condensation can sometimes be managed with airflow and humidity control, but a failed seal is a sign it is time to replace.