Abstract: An interior storm window assembly, mounted to an interior building wall over a window opening containing a prime window therein is provided, having double hung, vertically movable window pane members received within opposing, self-locating vertical channels and pivotally attached to adjustable weight balancing elements within those channels. The frame structure of the window assembly, including the peripheral rail about each window pane member and the vertical channels, is formed from plastic material, such as vinyl, having low thermal conductivity, and a rigid, reinforcing rod, preferably of metal, is inserted within the plastic rail at the bottom of each window pane member. The vertical channels apply pressure inwardly to those peripheral rails to releasably retain the window pane members in vertical orientation as well as urge the peripheral rails laterally into sealing contact with the vertical channels.