inner window; secondary window
Refers to an additional window installed on the interior side of an existing window, often for thermal insulation, soundproofing, or condensation prevention. Contrasts with 外窓 (そとまど, outer window).
See also: 外窓
I installed an inner window as a measure against the winter cold.
Closing the inner window reduces outside noise considerably.
Compound of 内 (うち, inside) and 窓 (まど, window). The reading うちまど follows standard kun'yomi compounding.