Abstract: The border element of a double glazed window unit of the casement of awning type includes an elongate unitary body formed entirely of vinyl plastics; its cross-section defines: a glass-receiving base portion including a web and a pair of legs surrounding the outer marginal edges of the glass sheets; a first enclosed hollow portion integral with the opposite face of the web and defining a first insulation chamber; and a second enclosed hollow portion integral with one of the pair of legs and with the front wall of the first chamber to thereby define a second insulation chamber. A window unit is thus provided where the metallic components used for the support and the operation of the window are enclosed within these insulation chambers and where effective insulation between both faces of the window unit may still be maintained.