Abstract: Solar heat is utilized for heating a door structure whereby to minimize the escape of heat from within the building or other structure upon which such door is used. The door comprises at least two flexible sheets, one thereof constituting the principal door structure and the other constituting the heat generating means. The second sheet is spaced outwardly of the first sheet and parallel therewith. Both of the sheets are supported in any convenient manner such as on rollers either or both of which are preferably spring loaded for counterbalancing purposes. The inner sheet is normally opaque to normal light and heat radiation, such as by being made of steel, and constitutes the principal door. The other sheet is in one embodiment transparent to shortwave electromagnetic radiation but opaque to longwave electromagnetic radiation and may preferably constitute a piece of transparent flexible plastics material.