Thom mailed me an early prototype to get some feedback on its performance. I've only spent one night in it, but wanted to post some initial impressions and pictures:
The bivy is made from homewrap insulation grade Tyvek and weighs about 8 oz. The seams are bonded, not sewn, and the cut tapers toward the foot. There entrance is a simple opening in the head of the bivy with no zippers or velcro. It is 76" long and has a maximum girth at the opening of 30". There is plenty of room for a full-sized sleeping bag and mattress (I used a Big Agnes Regular (72") AirCore) inside the bivy.
Stiffness: I really like the material. While it is certainly stiff (and I did nothing to soften it), to me, this was desirable as I get very irritated with nylon-top bivies that blow into your face while you sleep or start to sag on you overnight. This bivy stayed in the position I placed it in all night long. I did add a guy-line to hold the hood up, but the stiffness of the material allowed the one-inch bond above the head-area to act like a bivy loop and kept it up, out of my face, despite moderate wind. (No more than 10mph.)
Warmth: Wow. This material is WARM. In the middle of night, I was a bit overheated in my 35F bag and ended up unzipping it and draping it over myself like a quilt. Very impressed with this aspect of the material.
Windproofness: Again, very amazing. This material is much more windproof than lightweight nylon and doesn't flap in the wind and smack you in the face . The bivy has a wide enough circumference so that even with my 2.5" thick air mattress, I was completely below the opening, completely surrounded by the bivy, and the wind passed over me.
Condensation: I experienced no condensation or dampness, although I was on a ridge-top not near water and there was no dew in the morning, so this requires further testing. I'd be surprised if a bivy this warm wouldn't produce some condensation, so I'll put it to the test next weekend.
Waterproofness: Hard to say as there was no rain or puddles on the ground. I've had dew soak through a DWR nylon/silnylon bivy (that cost a pretty penny compared to this one), so if the Tyvek resists dew and water well without suffering from heavy condensation issues, I will be a true convert.
Cost: Clearly, one of the best things about the bivy is how cheaply it can be made.
Noise: Yes, it's crinkly and loud. Noise of materials seems to bother some around here, but I don't really care. When the wind would hit the guyed out head-area, it would hum a bit, much like a tarp might in strong winds. I imagine the Tyvek hums louder and at lower wind strength than other materials. One could soften the tyvek and reduce the noise, but I prefer the stiffness of the material so I'd rather deal wtih it.
Entrance: This is just an early prototype, and Thom has been working on other openings, but I would prefer a side zip or velcro opening. I really liked how the bivy completely surrounded me and wouldn't want to sacrficie that for the ease of sliding in from the side.
Roominess: If you placed the bivy on top of your mattress or use a smaller mattress, the bivy would be incredibly roomy as it is. With my 2.5" air mattress I would have liked just a few more inches of wiggle room to turn over in my sleep more easily, but it was a really good fit. I'm 5'8", 155. Taller/bigger people probably wouldn't be able to get away with the big air mattress inside the bivy as I did.
Aesthetic: I wish I were one of those people who did not care about the, uh, "ghetto" aspect of using a Bivy with DUPONT, TYVEK, and worst of all, NASCAR printed all over it. Alas, I am not. It's not a huge deal, but the bivy would be a lot more marketable if it were made in plain white or even dyed green. That's probably low on the list of modifications, as this is just a first try, but it would be nice.
Compressibility: It could be more compressible, though this isn't a big concern for me. I believe it will become more compressible with usage.
Weight: Would it weigh less if it used a lighter type of Tyvek? But how would performance suffer? I don't know enough about the material to say, but I think Henry Shires' "SubLite" is made from a lighter Tyvek than this and performs decently, or so they say.
All in all, I was very impressed with the bivy and will continue testing and giving feedback.