Bivy or Inner?

I don’t find condensation too much of a problem w/ my eVent bivy and when I’m using it, it’s typically in conjunction with snow shelters.

That said, a double walled tent would be even less of an issue.

Most moisture will come from inside the bag. A very breathable bivy (seen a suggestion of a silk overbag) could help shift the dewpoint out from the down but a VBL may be a better choice.

A double wall helps on a smaller tent but otherwise condensation that freezes on the tent is not your worst enemy.

If I ever get a bivy it will be with an eVent top.  I’ll use it only in snow shelters unless I know I’m sleeping in a single wall tent (or some flea-bitten hut in a foreign country).

I’d never use it in an AT shelter B/C I will never, EVER  again sleep in one of those mice-ridden snore shacks. Did I mention never again?

I had a negative condensation experience in the negative teens with a duomid shelter and a bivy. I’ve since gone VB and a quilt over my WM bag. No moisture issues from the inside and the over quilt is easy maintenance for external moisture which is dry unless it melts.

I have managed just fine on 3 and 4 day trips with a down bag and a water resistant, breathable bivy under a MLD DuoMid in the relatively mild Sierra Nevada where night time lows barely reach 10 degree above F with relatively high humidity. I tried an event bivy but it was too warm. :-)).  is your longer trip involve any base camping? On sunny days under my yellow DuoMid there is the ability to dry things out from solar radiation

Bottom line. I would think you would experience incremental degradation of the ability of the down bag to keep you warm. This is why many folks use the two bag system, a down inner and a synthetic outer. Andrew Skurka wrote an article about the impact of condensation after one of his long winter treks. Use Google to find it or check out his website.

I only experimented once with VBL but it seems you would want to try that system out a few times  on shorter trips before going out on a longer trip.

My layering system goes like this warmer to colder

light bag or quilt and foam pad and tarp

add bivysack either DWR or PTFE to keep out drafts, bring extra fuel for hot water bottles for warmth and to drive moisture out of the bags.

add second foam pad and synthetic over bag,

switch out tarp and Bivy  for VBL liner and Mid, probably have avi shovel too for building wind breaks of snow and increasing the living space under the Mid to keep away from the frosty fabric