Abstract: The invention relates to an apparatus for venting a building such as a greenhouse, by means of a sheet member which can be mounted in one or several openings of the building. The sheet member comprises a frame with four sides wherein are provided on at least two horizontal controls, which are formed as first rack bars. The frame has a horizontal edging and several parallel slats provided on at least one side hereof with a layer made of windproof and elastic material. A pinion is mounted in rotatable position opposite each guide on the edging, said pinion comprising a first and a second gear, the first gear being in mesh with the mentioned first rack bars, and the second being in mesh with teeth on the second rack mounted in movable position on the edging.