by means of corner connectors not shown which are l shaped and inserted accordingly into cavities in the frame profiles the direction in which air flows through the depicted air supply shutter is from left to right in fig2 shutter flap 100 is pivotably mounted in the lateral frame profiles 30 40 by means of two axle pins 110 111 immediately adjacent the lower frame profile 10 axle pins 110 111 are disposed on the inlet side of the frame from the horizontal swivel axis defined by axle pins 110 111 shutter flap 100 extends slantingly upwards and is tilted in the direction of the outlet side shutter flap 100 is embodied as a multi chamber hollow plate in order by means of the insulation thus achieved to prevent condensation from forming on the shutter flap 100 and in order to provide beneficial thermal insulation in the closed state to this end shutter flap 100 consists of a wall surface 101 facing the inlet side in the closed state and a wall surface 102 facing the outlet side in said state between which wall surfaces a cavity is formed that is subdivided into several sections by transverse and longitudinal ribs 103 104 commencing on the inlet side 3 of the air supply shutter the inner wall of the upper frame profile 20 extends upwards with a small angle of incline in the direction of the outlet side 4 of the air supply shutter the slightly upwardly sloping inner wall section 23 of the upper frame profile thus defined transitions at a line 24.