In another thread, I'd mentioned reading about people making their own tents out of LDPE with strategically placed "transparent duct tape" to carry the loads and that I was considering this method for making a backpack. Another member, far more experienced and no doubt wiser than me cautioned that the LDPE would stretch and pull away and tear at the edges of the tape. I'm sure this is true, however me being a newbie and not knowing my limits and being full of folly, I took this good advice as a challenge, and I set out to prove it could work.
The image above is of a 4.5" square of my test fabric. I started by cutting the shape out of a plastic trash bag. Then, I layed strips of the tape edge to edge at a 45 degree angle to my cuts until the whole thing was covered. Next, I put another layer of tape over the first, at a 90 degree angle to the first layer of tape (still 45 degrees from the edge of my patch). Finally, I trimmed the excess tape from the edges of the patch. The tape strips are 1" wide because I'd cut my 2" roll in half before I even planned on this expiriment, otherwise it'd be 2" strips. It's very tough, I can pull and tug on it all I want without deforming it. If I try to use my fingernails to tear it at the edge, it does deform a little, but it still doesn't tear. I had a friend in a post office weigh it for me and it came out to 0.02 ounces, so that means about 1.28 oz/yd.
(4.5/36) ^ 2 = 0.015625 square yards
1 / 0.015625 = 64 of these patches in one square yard
0.02 * 64 = 1.28 ounces per square yard <=EDIT: 0.02 is wrong! Should be 0.2, 12.8 oz/yd^2!
Since the scale only measures to the 100ths, I suspect it's not super accurate, but even if this patch weighed as much as 0.03 ounces, that comes out to 1.92 oz/yd, still very light.
One downside, is that the tape doesn't stick very well to itself; it has to be made this way so it comes off the roll. But, it sticks very well to the plastic trash bag, so if I did it over, I'd sandwich the plastic bag between the two layers of tape, and not only would it stick better, but it'd protect my fragile vapor barrier from the inside as well as the outside.
Another downside is that it sticks to dirt at the edges of the tape where the adhesive leaks out, so if you insist on super clean stuff, this is not the material for you.
Will it work long-term as pack building material? Only testing and time will tell.
EDIT: My math is wrong! This material is actually WAY
heavier! It should have been 0.2 ounces for my test patch, not 0.02. This means it weighs over 12 oz/yd^2! I'm sure I would have caught my mistake earlier, but I haven't worked with silnylon, cuben, or any of the other traditional materials, so I have no reference for what it should feel like.