The invention provides a water vapor permeable membrane comprising a dense layer and a support layer that are adjacent to each other, wherein said dense layer contains voids with a void length of 0.1 µm or less and said layer has a thickness of 0.1 µm or more and 2 µm or less while in said support layer, void (a), i.e. the void with the largest length in the 2 µm thick region measured from the boundary between the dense layer and the support layer into the support layer, has a length of 0.3 µm or more and void (b), i.e. the void with the largest length in the region ranging between 2 µm and 4 µm measured from said boundary into the support layer, has a length of 0.5 µm or more, the length of said void (b) being larger than that of said void (a). The invention provides a water vapor permeable membrane having both a high water vapor permeability and a low air leakage.
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