I was in Walmart yesterday and found what looked and felt like a bolt of silnylon. Upon doing the "breath test", it seemed like you could force some air through it, but it was very difficult.

As I said, it had about the same hand as sil, but since air could be forced through, it obviously wasn't silnylon. I compared it to some light 1.1 oz uncoated ripstop they had in stock (it was a jackpot day) and in comparison it seemed to have some sort of coating on it (given it's shiny appearance), or perhaps it was just calendared? If it's not silnylon, in all likelihood what is it?

It was very light, and

A couple months ago I bought 3 different rolls of stuff that looked exactly alike, one was super slippery, and the other two were a little different in their texture, and were definitely coated with something, they were all the same shade of gray, but only the super slippery one passes the breath test, just like the silnylon sea to summit dry bags I own. So what I really want to know is what's the other stuff they're selling that doesn't pass the breath test?

You can do a forum search to discover that many MYOGers have purchased the 1.1 oz. silnylon from walmart, and that it is indeed silnylon. I bought 15 yards of it, and it is completely waterproof. The actual weight is 1.3 oz. with the coating.

As for the other stuff, I'm not sure, but it seems to repel water decently and for only $1/yd. you can't go wrong.

I just finished a bivy project using both walmart fabrics, but haven't had a chance to test it yet. In the end it weighed a total of 5.0 oz.; pretty good for about $6 worth of fabric.

One more thing: I've also read on this board that for testing breathability of fabric it is better to suck air through the fabric than to blow through it. Just a thought.

Well, I went ahead and bought some of the "mystery fabric" and upon weighing it, it comes out to 1.2 oz. 1.2? That's a new one...

In comparison to the 1.1oz uncoated fabric that I bought at the same time (like I said, it was a jackpot day in the bargain bin), it has a much more "crinkly" hand, but not as stiff as actual silnylon. I'm wondering if it is just a thin pass of silicone on one side?

I'm going to try some water tests with it tonight. If it isn't reasonably waterproof, then it will probably be used in a bivy and/or windshirt project.