Flexible transparent barrier films are required in various fields of application ranging from flexible, transparent food packaging to display encapsulation. Environmentally friendly, waterborne polymer day nano composites would be preferred but fail to meet in particular requirements for ultra high water vapor barriers. Here we show that self-assembly of nanocomposite films into one-dimensional crystalline (smectic) polymer day domains is a so-far overlooked key-factor capable of suppressing water vapor diffusivity despite appreciable swelling at elevated temperatures and relative humidity (RH.). Moreover, bather performance was shown to improve with quality of the crystalline order. In this respect, spray coating is superior to doctor blading because it yields significantly better ordered structures. For spray-coated waterborne nanocomposite films (21.4 mu m) ultra high barrier specifications are met at 23 degrees C and 50% R.H. with oxygen transmission rates (OTR) < 0.0005 cm(3) m(-2) day(-1) bar(-1) and water vapor transmissions rates (WVTR) of 0.0007 g 111-2 day(-1). Even in the most challenging environments (38 degrees C and 90% RH.), values as low as 0.24 cm(3) m(2) day(-1) bar(-1) and 0.003 g m(2) day(-1) were found for OTR and WVTR, respectively.