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.

Abstract: There is provided a roll-type door having horizontally positioned slats which is normally held by operator means in an open position against gravity. Said operator means includes fusible means responding to a predetermined heat level for releasing said door and permitting same to move under gravity in a closing direction. Said operator means includes a device for initiating said door movement immediately after the response of said fusible means to said heat level and further includes a connection to braking means for insuring against too rapid a descent by the door. The door may be manually opened if desired or mechanical means, power driven if desired, may be provided for opening or assisting in opening such door. In the latter case, clutch means are provided for disconnecting same immediately upon breakdown of said fusible means whereby said door will be enabled to close freely in response to gravity subject only to control by the above-mentioned braking means.