A shutter with a frame having side members and top and bottom members, and louvres mounted between the side members, and a control for controlling and tilting the louvres, and louvre seals at the top and bottom of the frame, the louvre seals being loosely supported on the top and bottom of the frame, the top frame loosely supporting the top seal in a vertically dependent position, and the bottom frame loosely supporting the bottom seal in an upright standing position, and which render the two louvre seals swingable to allow sealing contact by the top and bottom louvres, when the louvres are tilted closed, and which seals swing back to vertical when released by the louvres. In a preferred form the blade seals are incorporated in frame extensions, with the frame extensions being swingable on the frames, and being biassed to hang vertical when out of contact with the louvres .