I've made a bunch of versions of this jacket: http://www.backpackinglight.com/cgi-bin/backpackinglight/myog_event_jacket.html#.VknvRdKrSt8 I've finally found some fabric that works pretty good – the Pertex Shield from thru-hiker.com – $17 per yard – 1.8 oz/yd2. This is pretty good at being waterproof when it rains, yet breathable. I wonder if the Pertex Endurance (1.4 oz/yd2) would work? I've used generic WPB from owfinc.com and questoutfitters.com, which was waterproof, but even when just hanging around camp at 50 F, it would get wet inside. I used M50 from thru-hiker. 0.7 oz/yd2. When it rains, the clothing underneath gets wet, especially at the hood and shoulders. Yet this is somewhat manageable if I wear synthetic, and it only rains a little. And if it's just misty or windy it works really good. I see the advantage of windshirts. One thing, the fabric is so light, it easily snags if it rubs against a branch. And it's so light it'll rip. I used epic silicone encapsulated. It seemed like it worked, but I'm not sure, but then one trip it was rainy and the clothes underneath got wet. (West side of South Sister – very windy, snowed a little, yet I survived because here I am). It's also very light so it snags easily. The generic WPBs and the Pertex Shield have the membrane, so they're stiffer, so they don't snag so much. I got some 2.5 layer eVent from BPL, which was a one time deal. owfinc.com has some 2 layer eVent which might work. After a year or so, it started delaminating at the shoulders and hood, and water leaked through when it rained. The body oils from sweat degrade the membrane. Maybe eVent is more susceptible? My newest "great idea" is to have a lining at the shoulders and hood. Regular linings just hang there, but I have the lining actually carry the weight of the jacket. Then, the outer WPB layer is cut 2 inches bigger, and sewed to the outside. Then, there's a small air gap between the WPB and the lining. This minimizes oils getting onto the membrane. The air gap allows for air to circulate so any water can evaporate – maybe the membrane has some water condensed on it at times, but I don't know about it so it doesn't matter. Inside view: 
The liniing is a light brown supplex. Maybe a little lighter nylon would be better. There's also a strip to make a sleeve for elastic around the waist. Outside view: 
Closeup: 
The outer WPB covers the sleeve seam which is a weak point for waterproofness. It also covers the shoulder seam. You can see a gather to the right of that to "use up" some of that extra 2 inches wide that the WPB fabric is.
