I just finished sewing another bivy sack. This one is Momentum fabric on top and coated nylon on the bottom. I made it large enough to cover my best winter sleeping bag, so it is very roomy for a summer bag. I put one 30" zipper down the side, and then one 30" zipper across the top, shoulder to shoulder. As a result of two zippers, the total weight was up to 8.6 ounces.
If I had done the whole thing out of Momentum, made it snugger, and did something lighter with a zipper, I probably could have kept the weight to 5 ounces.