I had a Feathered Friends sleeping bag made with a zipper to fit this bivy and have enjoyed using it. The bivy has two sets of zippers. Your sleeping bag zips into the inner zipper of the bivy and the outer bivy zipper closes the both as a whole. I really like this set-up. It makes your bivy and bag into one unit. The bivy bottom is utterly water-proof. It is made from a tent bottom like material. The bivy upper is Mountain Hardwear's Coduit SL material. It is VERY breathable. It will withstand some amount of water, but a good dousing will soak it through. Overall, I recommend the bivy. I think it makes excellent protection your sleeping bag when you may only have partial protection from the elements. I have used it for tarp camping with much success.