Careful chaps......you're talking at cross purposes.
Tyvek is a vapour permeable but water-proof membrane (think Goretex jacket). It isn't the only one on the market, but is probably the market leader. It is a very long way from a building paper, which is a bit vapour permeable, and not very water resistant, and tears (Tyvek is tough as old boots). Each have their place.
Tom, what are you planning to do with this stuff? PM me if you want to keep that a bit quiet....
Mike