I’ve used the Montbell Breeze Dry-Tec for several years. I think it must be the wide-long because it’s too big for me and I’ve thought about cutting it down. Oh well.
It does transfer moisture to a moderate degree, but not like the Goretex Pro in my kayaking drysuit. Not generally a problem as far as condensation; more of an issue of clammy in still air. Will not keep bag dry in heavy dew situations, and not because of leakage (but then I’m guessing any fabric would have the same problem).
It does extend the temperature rating of my bag and allows me to use my bag like a quilt (as I almost always do) down to lower temperatures. Theoretically it would allow me to carry a lighter bag. I also like having it as a reserve to keep the bag dry in blowing rain under my tarptent. I just keep my bag in the bivvy fulltime (protects it from leaking pack cover), and I put my pad under the bivvy.
But what I most particularly like about it is its utility in wind (and mosquitoes) when I sleep tentless, which is most of the time, especially if that wind is not cold–without the bivy, I’d have to be bundled up in a zipped down bag to avoid the wind, and that can be miserable. The bivvy is also useful for keeping gear dry, out of the blowing sand, or from getting lost, e.g., pants. (keeping water bottles from freezing?)
One thing I cannot endorse is using it in rain instead of a tent (except in emergency). Pretty miserable to feel that cold water accumulating just on the outside of that thin fabric, and it will get clammy in there (and how do you break camp in that scenario?). True with any bivvy, I imagine.
I strongly recommend you avoid all bivvy designs that use coated non-“breathable” fabrics on the “bottom”; an hour or two into your sleep and that “bottom” is just as likely to be on top, collecting condensation. I’ve built some that way and they are near-useless.
But I do like the very simple design of the MB bivvy–no zipper needed, just a big opening with a drawstring. I use it upside down. When it gets windy or real cold, the “hood” can just flop over me while still venting underneath as needed. That’s the way I also use all my sleeping bags (I have VB liners sewn into my bags). When it’s cold, keeping in that energy-packed exhalation goes a long way to keeping me warm. Never could understand why everyone wants to lose all that good heat, and get more sore throats in the night from cold air sucking out moisture.
