I was waiting for other posts to add to this discussion before offering my dribble. I have some experience similar to yours Soumodeler.
These are some other options to be considered that were not shared. I thought they were viable options as we're not all doing the same trips or have the same "outdoors" backpacking/camping style. We all don't have to stubbornly haul 70-90+ lb packs going
so over the top on gear or refusing to consider logistical eases to enjoy winter hikes/camping trips either. HYOH.
Sure sounds like breathing condensation was the main culprit so excellent advise was offered by Downeaster although could loss of loft been more than that? Was compromised loft just noticed in the ZP quilt or did you observe it also in the WM Versalite? Perhaps, loss of loft didn't only occur because the down got compromised from above as you breathed onto the quilt or into it only from your breath? Did you breathe into the Versalite too? Perhaps, vapor from your body or internally(from within the system) or because of other factors compromised loft too?
--- Loss of loft only in the ZPacks quilt and only at the very top, near where my head was. The Versalite had no problems as it appeared all the water stayed on the shell and froze there, except for the inside where it was slightly damp. I did breathe into the Versalite many times as I was lower down in the bag reading for a few hours before going to sleep. I only noticed the ice later on.
On what day out did you start noticing this? Was it day 1 or days into the trip?
--- This was a single night trip.
What tent did you use? How were you ventilating it? Was there any wind? What was overhead(what kind of tree canopy, etc). Did you camp in/on any of Springer's grassy areas?
--- TarpTent Notch, with mesh inner wall. Plenty of ventilation, with a decent breeze blowing throughout the night. I was near the shelter, so no grass and light tree cover.
BTW, I've had the exact same scenario occur in O*- minus 15* f temps with rime on the top 1/4 head area of the top quilt layering a EE 850 fp non hydrophobic down quilt over a WM 10* Versalite or Marmot 15* Helium(2005 version) namely in dry Utah, northern AZ in the Skylands on a GET completion, on a Sierras winter trip and on a few eastern winter trips while cowboy camping or A frame config. tarp sheltering. Avoiding frost/rime is not just about better ventilation. Still have some occasional rime but much less than in the past. Fortunately, most of my trips aren't that long duration or they are broken up with a day in town or with access to a dryer or acceptable weather to somewhat dry gear every 7 days or so.
When I was experimenting with layering bags and quilts in winter weather, before I decided on exploring and going with other approaches, I found I need not be concerned as much when I layered a borrowed EE Prodigy 40* APEX over the Versalite or Helium per EE's suggested option for 0* and under winter weather. The Prodigy also was employed on 3 season wet and mixed weather treks.
Did you bring anything wet into the tent? Were you or your worn apparel damp, even lightly so, when you got into your stacked sleep system? Very common on winter trips to dive right into the sleep system being damp(sweaty yet cold) or wearing damp gear. Yeah, mountaineers and climbers bring wet stuff into tents and possibly even sleeping bags but how long do their activities typically last or how far are they traveling each day? Might be better to separately bag damp(not totally drenched) stuff allowing to dry inside the bag from body heat.
--- The only wet gear I had was my slightly damp hiking layers, which were stored under the sleeping pad at my feet. Bone dry clothes on going into the bag.
How long were you in this stacked sleep system each night? The longer you're in it, very very common in winter backpacking, the more the loft can be compromised. Diving into a sleeping bag in winter the minute the sun goes down at 4:30 p.m. can make for a long time in the bag. It's one of the reasons why I try to hike after sunset in winter. It brings a different dimension to experience. I like that.
--- In tent at 4:30, noticed ice around 11.
On what day out did you start noticing this? Was it day 1 or days into the trips?
--- Day 1, only night spent out.
Did you cook in your tent?
--- No
Is your Zpacks quilt using .70 ventum with the DWR? How old is the quilt or how many nights using the quilt; what's the state of the DWR? IMO, that Zpacks shell fabric is not best for conserving loft under your scenario. I've seen the loft compromised too many times with ZP bags/quilts under too many scenarios not to
mention it. You can actually see the down through the fabric it's so wispy;pros and cons to that under different scenarios.
--- I don't think this one has DWR. It is an older model, well before the new vertical baffles.
Were you crushing any of your sleep system layers? ie; through wearing to much sleepwear inside it, the WM Versalite, or not accounting for the extra size dimensions needed in the ZP quilt to layer? This can impede "moisture" going up and out through the layers.
--- Not that I was aware of. No puffy inside bag, only thin layers. ZPacks was draped over Versalite, so no compression there.
Possibly, a synthetic quilt of appropriate size to top layer or to employ as a one bag synthetic for long trips without access to drying is an option to be considered?
At some pt, like when you're out for a very long trip with no access to dryers or don't want to wait around expecting acceptable drying weather to dry(?) a heavy down winter sleeping bag to really dry through(in frigid winter w/ no wind ???, tricky), or bring an overkill of apparel, or an overkill temp rated down bag, which is not everyone's backpacking style but may be acceptable for one more inclined to be a camper/hang around type than a hiker, maybe a synthetic bag is best?
Or, how about incorporating the latest hydrophobic down insulation versions in a winter sleeping bag(you don't have it in the ZP quilt I suppose?) potentially bringing you across the finish line on your wet or cold winter trips? Don't look at hydrophobic down as a loft conserving panacea but as an option to extend loft life that might be appropriate for the length of your trips, your scenarios, access to drying, hiking/backpacking/camping style, etc?
How about placing a scarf or other piece of gear over your nose and mouth or shell around the head area outside on the bag/quilt to capture the condensation in it? I did this to help mitigate the rime on the mentioned layered sleep systems in winter.
Have you explored bags/quilts that have a different fabric or treatment in areas prone to wetness or condensation like in the hood/shoulders and foot areas?
Letting down get wet, or even having loft compromised, and then jamming inside a pack with potentially other wet stuff is not good especially if you're doing it early on during extended winter trips, in remote locations, the down bag(s) are your main insulation...Dry or at least pat down the top quilt to make it dryer before packing.
Considered these options a few yrs ago.
https://backpackinglight.com/forums/topic/101595/
https://backpackinglight.com/forums/topic/55089/