Polycro is waterproof. Tyvek is not. However, Tyvek is water resistant, more puncture resistant than Polycro, durable and lies flat easier than the (pardon) gossomer-thin Polycro. 1.1 (1.35) oz sylnylon is not waterproof, either, but it has a higher head than Tyvek. I'm not sure about comparative puncture resistance. I have used Polycro and find it tedious to use compared to either Tyvek of sylnylon.
BTW, my favorite silnylon is the double coated type which has a head comparable to polyurethane. It is supposed to weigh more than the simple silicone impregnated nylon, but my scales don't show as much difference as the manufacturers report - 1.4 oz v. 1.6 oz.
Are you sure you are not talking about fiber-reinforced vynil? The Krinkly stuff available in blue at any hardware store?