The difference in silnylon and Tyvek is that silnylon is "NOT" totally water proof and tyvek is. Most silnylon is rated at around 2lbs of pressure per square inch and if any pressure is put on the silnylon it "WILL" leak water. I have all types of silnylon tarps and ground sheets and the stuff is really good quality. I have a 1.1 oz. silnylon tarp (10X12) that I carry with me most of the time in case of emergency.When I first made this tarp I pitched it in my back yard and left it there for around three weeks. It got every kind of weather that you could throw at it and it never leaked. I know this because I had all sorts of older gear under the tarp the entire time that it was pitched in my back yard... Silnylon is mostly what I like to refer to as "hydra phobic". That is that it does a great job of repelling water but it is simply not water proof. You can coat nylon with different coatings to make it water proof but then that greatly increases the weight of the tarp therefore defeating your purpose of the light weight tarp. I guess what I trying to say is that all your silnylon will work better over you than it will under you... Any way I just thought that I would add that ..Hope this helps...Happy Trekking...sabre11004....

The first step that you take will be one of those that get you there !!! <img src="/forums/images/graemlins/wink.gif" alt="" />