The E-Bivy from MSR is one of their most basic shelters designed for fair weather bivying, for use under a tarp or for emergency shelter.
The E-Bivy is constructed from a very thin and light 30D Ripstop nylon which weighs in at only 250g with the stuff sack, making it one of the most featherweight bivy's on test. What you sacrifice for the weight savings are features and indeed waterproofing.
Space inside the E-Bivy is plenty for a sleeping mat, person and bag and length is sufficient for a 6ft 2" tester. The hood is able to be pulled over the head from the neck as well as over the top to provide a little extra protection from the elements but there's no drawcord or zip adjustment.
Although the Silnylon top material is not officially classed as waterproof rain beads up quite nicely on it and it easily weathers a passing shower. Thankfully the Durashield bottom is fully waterproof saving you from getting soggy from the ground up.
The breathability of the E-Bivy is excellent, as you would expect from a non-waterproof bivy, we didn't experience any condensation issues on a night when other bags didn't fare so well.
If you know that you're going to be afforded the protection of a tarp or are sure of fair weather then the E-Bivy's diminutive size and no-frills approach make it a potentially appealing option although for the £100 price tag we would have expected more in terms of features, waterproofing or preferably both.