Lightest Breathable Waterproof Material?

I think M50 is the current champion. Cubic Tech claims that they are working on a breathable version which does have the capacity to trump M50 but it isn’t a product as of yet. In case you missed the thread, you’ll want to check this out:

Suluk 46 – bonded cuben fiber/ momentum 50 bivy

Momentum is pretty waterproof and very breathable – thruhiker.com .5 or .9 oz/sq yd2 versions or owfinc.com has .9 oz/sq yard that's supposed to be Momentum that's cheaper

What about the material this reaingear is made of?

http://www.backpackinglight.com/cgi-bin/backpackinglight/driducks_rain_gear.html

These are the two lightest I have been able to find. Let me know if you find any others as well.

1.6 oz/yard 2-layer: http://www.backpackinglight.com/cgi-bin/backpackinglight/myog_bulk_fabric.html

Not exactly sure what it is, but 1.6 oz/yard is really light. I don't know anything about using 2layer vs 3layer in a bivy though.

The next lightest at 2.5 oz/yard I have found is the 2.5 layer w/p/b here: http://www.questoutfitters.com/h20.html#Waterproof/breathable%20Fabrics

edit: Good point Daryl. Here's extra info about using Propore (driducks). It is pretty light (and cheap!), approx 2.37 oz/yard. You can score a few poncho's on ebay still I think. That is the only source I have found.

http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=21443

http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=21902

I bought two driducks ponchos and cut them up. The fabric weighs 2oz per yard. I made a hooded pullover rainjacket with a 9" zipper. The finished weight is 114g (4oz). This material is not very durable but may be the lightest easily available waterproof breathable. The two layer event is also 2oz per yard but is very hard to find.

I talked to them on the phone about quest 2.5 layer 2.5 ounce fabric

They said it was probably just as waterproof but maybe not as breathable as eVent

Pennine Outdoor in the UK has a "P2B Feathereight Breathable Nylon" weighing 70 to 90 grams per square meter. It is listed under "waterproof" fabrics. Sent them an eMail inquiry, but got no answer.

The Momentums have a DWR coating, but isn't calling them "waterproof," a bit of a reach? If you want to call them waterproof, you might also want to look at a 1.36 oz. polyester that Fabrics'n'stuff in the UK sells that has as hydrophobic a DWR coating as any I've seen. Water will not sit still on it. But, using any of these DWR treated fabrics as the only protection between us and the elements = bad idea IMO.

Yes I know that M50 is the light but it is only DWR finish. That's fine under a tarp but this bivy is for use without a tarp. Maybe should have put that at the start. Although I now wonder if you could treat M50 with Nikwax or similar to make it WPB?

Extreme Textiles in Europe offers this at 55 grams:
http://www.extremtextil.de/catalog/Fabrics/breathable/Micro-Ripstop-laminate-dull-55g-sqm::1428.html?XTCsid=30a75b7924d501b085f97e40edaf4979
It seems to be about the lightest so I may use it.

That's if I make the bivy. I'm going to calculate the weight first to see if it's worth it. Though I really do like being in just a bivy when it's not hammering it down non stop for days on end.

I'm planning on putting in a large removable netting so that on clear, buggy nights you could string open the mouth of the bivy for good circulation and still have the netting to protect you.

This suggestion is outside the scope of your original question but would you be better off (lighter is what I really mean) using a M50/CT0.3K.08 bivy in addition to a CT0.3K.08 tarp? Combined they would weigh in the neighborhood of 4 ounces (source: Suluk 46) and never wet out in even the more torrential downpour.

Chris you are absolutely right, it would be the lighter option by far. There are only two problems.

1) Price – this is the ever eternal problem. The cuben is twice the price of silnylon or the WPB fabric.
2) I really enjoy being in just the bivy. Sure when it's been raining for 40 days and nights it does start to lose it's appeal. But there's something about it that I just love even more than being under my tarp.

Not sure if it's even my most pressing project as I already have a bivy and tarp.

I have Suluk's bivy, I ordered it customized with a 36" centered (top) zipper, which is really nice. I used it on my JMT 25 day hike and it was fantastic. about 3 oz. I like the bivy, even with a tarp tent because it adds these features:

1. breaks wind (reduces windchill temps about 5 degrees)

2. keeps you centered on your sleeping pad (the pad is inside the bivy) — otherwise I'd wake up in the middle of the night finding myself on the ground off the pad.

3. protects down sleeping bag against tent-wall condensation if you touch tent wall inside the bivy.

4. protects sleeping pad one layer more than groundcloth/tent floor does against abrasion/punctures.

5. acts as a top sheet if it is too warm to sleep inside the sleeping bag at start of the night. Many times it's too hot to sleep inside the sleeping bag but it is too cold to sleep on top of the sleeping bag without something on top of you. The bivy does a great job as a bed sheet in those situations.

"1.6 oz/yard 2-layer: http://www.backpackinglight.com/cgi-bin/backpackinglight/myog_bulk_fabric.html"

That is eVent by the way, you can't buy that anywhere

It's white which may not be the best color

It's 2 layer so you have to add a liner fabric

It would be good for a sleeping bag

Maybe you could use it for a bivy and not worry about lining it?