For years I have been designing an air ship to match the shape and demensions of the star ship Enterprise NCC 1701 and have overcome many problems, however, in order to keep from losing so much helium through diffusion through the outer covering I would like to know if there is an element that repels helium or is less diffusable by it? Is there is top chemist out there who can offer something better than mylar? My ship design is 154 feet wide at the saucer section and about 320 in length over all. With tens of thousands of square feet of outer canvas and about a quarter million cubic feet of helium, helium loss could be my biggest problem. My weight budget allows for about one ounce per square foot of outer covering. so I can have multiple layers. Polyethelene plastic stops radiation due to the hydrogen atoms in the molecule, would this be a clue to a solution?