I just got back from an overnighter with my Driducks bivy. Unfortunately, the conditions weren't very challenging to testing the fabric--or so I thought at first. Lows in the upper 40s, moderate to light breeze the whole night, dry air. There wasn't any dew on top of the tarp, bivy, or anywhere around the campsite in the morning.
I went to sleep expecting lows in the mid-upper 30s, so I was wearing an insulated jacket, and a 20 degree down quilt. Unfortunately, the lows stayed above 45 degrees the whole night, so I was plenty toasty. This is important to note, though. Actually, I was too hot to the point that I started sweating several times during the night because of my laziness which kept me from just de-layering (I just hate waking up at 2am shivering, so it wasn't so much laziness, just wanting to prevent that from happening...). Anyway, come 2am in the morning, I woke up and realized that the dip down to 35 degrees had never happened. Instead, I was simply covered in sweat. The inside of the bivy top (which my skin was not in contact with) was also slightly moist. It turned out that all the sweat that was evaporating off my body was trying to get out of the bivy, but the top fabric wasn't able to keep up. Using my camp towel, I dried off the fabric, and de-layered to properly cool down. The rest of the night I was perfectly dry, and the condensation did not re-appear.
I wish I had a Goretex or eVENT bivy to compare it to. The only other waterproof bivy I have experience with is the Montbell Breeze-dry Tech bag cover, and the extent of my experience with that is that in highly humid conditions, there was internal condensation, but in highly dry conditions there was no internal condensation. Further testing is required with the Driducks bivy to see how much of a factor my sweating played in the internal condensation I saw. I think it was the single, direct cause, but would like to confirm it on a night when I layer a little more appropriately for the actual conditions.
Clear plastic bottom and duct tape. I'm envious I didn't think of that. With such a simple and inexpensive construction, a bivy like this should be within the buying power of just about any UL backpacker out there.