Argon wind shell jacket (custom)
1.6 ounces! (small). This jacket is available as a super simple and super ultralight pullover or with some added adjustment. It's constructed from Argon 90, which is a 15d fabric that has a little more durability than lower denier shell fabrics and has been tested to have 40 CFM breathability.
The functionality of an ultra light wind shell should not be understated. I have replaced many of my light/mid weight insulation layers with wind shell layers due to the shocking amount of warmth provided at such a low weight. Not only does it block wind and prevent evaporative and convective cooling, but it holds a pocket of warm air around your body that provides decent insulation. At the same time, Argon 90 is one of the more breathable shell materials, allowing you to be active in it. At high intensity activity a more breathable layer is desirable, but at moderate activity in cooler weather, a layer like this can provide great warmth with good breathability at a "basically nothing" weight and "almost nothing" size. It also has a DWR coating that will shed light rain.