Abstract: A film sheet having a pressure valve portion with one or more holes passing partially through the sheet film on one side of the sheet film, and a thin portion covering each of the one or more holes. The thin portion is preferably constructed to rupture when a predetermined maximum sustainable pressure differential is applied across the sheet film. In one embodiment, sheet film has a first film layer; and a second film layer, having one or more through holes, laminated to the first film layer. In another embodiment, the sheet film has a first film layer, a second film layer having one or more through holes, and a third film layer having one or more through holes, wherein the second film layer is laminated on one side of the first film layer, and the third film layer is laminated on the other side of the first film layer so that the one or more through holes of the second film layer are aligned with the one or more through holes of the third film layer.