Abstract: A window unit is provided, comprising: a frame body having an upper edge, a bottom edge and two side edges defining the perimeter of the frame; a pair of side mounting flanges, each comprising a substantially vertical plate and each secured along a respective side edge of the frame; an upper mounting flange comprising a substantially vertical plate and a ledge extending outwardly from a lower edge of the vertical plate; whereby the upper mounting flange is secured along the upper edge of the frame by means of the ledge such that the vertical plate of the upper mounting flange is in a first vertical plane behind a second vertical plane of the vertical plates of the side mounting flanges.