Those fabrics are zp (Zero porosity) which means that they are typically impregnated with a plastic resin... Usually melamine or polycarbonate (mylar) as a liquid (melted) that soaks the fibres and is then rolled flat and thin between metal rollers. Should be waterproof and totally NOT breathable.
The sailcloth is really neat stuff, and often direct source can be cheaper but quantities are usually huge. Kitebuilder often has sales on seconds or overstock that is cheep. I bought a bunch at less than $4 a yard....
Like someone else mentioned I'd use it for tarps, but probably not a hammock.
Last edited by Rapt; 09-05-2007 at 13:29..