Abstract: Window thermal insulation device for thermal insulation of a window opening area, including a plurality of linear members arranged in successive end to end relation for forming a perimetric selectively sized and shaped enclosure frame defining a window space therewithin, a like plurality of interconnectors for interconnecting the adjacent ends of the linear members, linear locking grooves in the linear members and extending therealong peripherally outwardly of the window space, a pliable plastic thermal insulation sheet extending across the window space of the frame and having its peripheral margins disposed correspondingly in the locking grooves of the linear members, a like plurality of linear cover strips having linear locking tongues projecting therefrom and disposed correspondingly in gripping contact with the adjacent portions of the peripheral margins of the plastic sheet and in releasable locking engagement with the adjacent locking grooves and arranged for maintaining the plastic sheet in substantial