Synth down bag inside emergency bivvy – extra warmth

That bivy will trap a lot of moisture and you might get wet. I think you would be better off buying 2 yards of pertex .9 or .7 and making a sleeping cover from that. It will act as a wind shell basically. If you draped it over you inside the sleeping bag you would stop or slow down all of the body heat rising up. I have done this with plastic poncho and it helped quite a lot. Very comfortable too, not clammy. There is only one way to find out, if we were talking about 20deg or below the sol bivy might work better? Vbl is common, but there is a lot of variables to get it to work right.

I give up. What is synthetic down?

I know what synthetic insulation is, and I know what down is.

–B.G.–

Now you've stumped me. The bag has downtek fill, which is down, with DWR treatment. Does that make a difference in terms of moisture build up when using a vbl or a bivvy like I've described?

I've heard of people doing this successfully with the SOL Escape Bivy, since it's (somewhat) breathable, but the DWR on the down won't make up for condensation if you use a cheap, non-breathable bivy. It will help a bit, but the loss of loft would still easily exceed the extra warmth from the bivy.

Perhaps you want to look into a water-resistant bivy? I recently made one on the cheap out of silnylon and tyvek. It won't add any insulation, obviously, but it cuts out drafts, which can make a big difference.