Abstract: A system for mounting thin transparent membrane material over the interior of a window to provide an insulating effect. An elongate narrow retainer molding is positioned along the peripheral frame portion of the window. This molding includes a narrow base portion having a flat surface intended for adhesive and permanent attachment to the frame. Integrally formed with the base portion are two side components which extend upwardly to a top surface which is shaped concavely to define a receiving region. Detents are formed in these side portions which extend over a centrally disposed groove. A beading of circular cross-section is urged with the peripheral portion of the membrane into the centrally disposed groove to provide a non-adhesive form of fixation of the membrane to the retainer molding.