Find Lightweight Bivy Bags with the features you're looking for.
How Much Lighter are Lightweight Bivy Bags?Bivy Bags are not by any means a required item for most backpackers, and it's certainly possible to practice even lightweight backpacking without a bivy bag. But in combination with a tarp tent say, a lightweight bivy bag can keep you dry and warm on a stormy night without the weight and bulk of a tent, which will usually make for significant weight savings. But like any piece of backpacking gear, there's a big difference between the typical and the lightweight, as you can see over at the list of list of lightweight bivy bags.
The Integral Designs Salathe Bivy, for instance, is a claimed waterproof bivy bag (really unless it's made of plastic it's not 100% waterproof) that weighs in at 950 grams, or 2.1 pounds. The Terra Nova Equipment Moonlite Bivi on the other hand is another claimed waterproof bivy bag that weighs in at a slim 180 grams, or .4 pounds.....
- Intergral Designs Salathe Bivy - 950 grams / 2.1 pounds
- Terra Nova Equipment Moonlite Bivy - 180 grams / .4 pounds
Common Features of Lightweight Bivy BagsIf you're carrying a bivy bag then you're probably already invested in lightweight backpacking, but even so there are some differences between the heavy bivy bags and the lightweight bivy bags.
- Fabric Choice - The problem you'll encounter with lightweight bivy bags is the same one you'll encounter with lightweight tents, which is that you need the bivy bag to breath while at the same time keeping moisture out. With this in mind, most of the weight savings that you'll see in lightweight bivy bags come from the choice of fabric. Some will use different fabrics for the top and bottom, making the bottom of a more durable fabric in the theory that it will be in contact with the ground and suffer more abrasion and moisture exposure as a result. Of course, it's debatable how effective some of these lighter fabrics are at keeping water out. Water Proof is a relative. But if you choose your campsite well and have some form of shelter overhead it'll be difficult to get wet.
- Bug Netting - Another common feature on a lot of bivy bags is bug netting for the face opening, which allows you to breath and be free of bugs. And though this bug netting doesn't weigh much, some bivy bags do forgo it. If you're camping somewhere that bugs aren't a problem this isn't really a concern, but if the bugs are out, one idea might be to use a bug head net that you would wear during the day on the trail as a supplement to the lightweight bivy bag at night, giving the bug head net two uses. The list of bivy bags page lists which bivy bags on the market come with the bug net feature.
Drawbacks of Lightweight Bivy BagsLightweight Bivy Bags are a great way to go backpacking, provided that you know what you're doing. In the summer a lightweight bivy bag makes a good alternative to a tent, but when the weather turns ugly you'll be wishing you had that tent. One system that seems to work well for people is to use a lightweight bivy bag in conjunction with a tarp tent, which provides good three season protection from the bugs and the elements while still being lightweight.
Like most lightweight backpacking gear, the drawbacks only become a problem when you don't understand the limitations of your lightweight backpacking gear. Being lightweight, this kind of gear is less forgiving of mistakes, but with a little knowledge and experience you wont be making mistakes.