Abstract: A valve assembly clamped to the upper end of a vent pipe includes an elongated sleeve with a band clamp and sealing gasket at its inner end for ready clamping to the vent pipe. A butterfly valve plate is nonrotatably fixed to a pivot shaft in unbalanced relation, with the shaft extending through the sleeve and having a transverse operator arm. Semi-annular seat means provide a sealing seat at upper and lower surfaces of the valve plate, on respective opposite sides of the pivot axis; and a spring or weight urges the valve plate to the seated position.