Snow hole sleep setup (quilts???!!)

So I'm going to sleep in a snow hole, I don't want to have to buy a new expensive sleeping bag and a bivy bag, I can't afford that. What I can do is use my current 3 season down bag with a synthetic bag over the top to gaurd my bag against damp conditions and boost the bag into acceptable. Currently I'm torn between a full bag, the Rab solar ultra 1, or a quilt, sea to summit glow GW1.

Full bag will be warmer and more protective against wind, the quilt is waterproof lighter and cheaper.

What do you think? Very interested to hear about anyones past experiences with weird winter sleep systems. 

Edit: To add a bit of info, it will be 3 days, 2 in a true cave and one possibly in a trench. I will also be sleeping in all my layers which will be a wool hat, ME eclipse as my baselayer, an active insulation piece and a 250g fill down jacket. Legs are Light wool long john's and 80gsm ME compressor pants. 

The quilt is made to go over sleeping bags the combo would be approximately a 30degree+40 degree bag and I will be in the alps at around 4000m in late Dec/early Jan. 

I use a down mummy bag under a quilt and it works but the quilt needs to be big enough to cover everything and the strap system needs to be pretty good too keep everything in place otherwise moving around will leave big gaps to let draft in. I couldn't find a width dimension on that sea to summit quilt but enlightened equipment sells synthetic quilts in a wide size that have a good strap system and it's lighter for about the same price. Not sure what shipping would be to your side of the pond though. https://enlightenedequipment.com/revelation-apex

The rab bag would work as long as it's big enough inside but I'm not sure how to determine that without trying it in person.

Hello,

As said, the top quilt needs to be big enough. I also recommend it be long enough to pull over your head, or you may want a puffy cap. A major heat loss point is the head if it is not covered to the same degree as the body.

Do not expect a summer quilt to add more than 10-15F to the rating. On a positive note a synthetic top quilt more or less eliminates the problem of the down bag getting damp from condensation.

Thank you

How warm is your summer bag? Snow cave or trench? One night or multiple nights? Solo or with a partner(s)? All will affect choice of gear.

For one night, I don't worry too much about down getting wet. For multiple nights, I'm starting to think about vapor barriers, when during the day will we get sun and have a chance to dry things, maybe bringing a synthetic bag, etc.

But I think just as important a question is what's the total sleep system? I've spent nights of single digit (F, not C) temps in a Feathered Friends Vireo (45F in the torso, 25F in the legs) with puffy pants and a Grade VII parka reasonably comfortably. On a cold climb, I'm going to have the puffy pants and parka anyway, so I can get away with a lighter (very light in this case) bag knowing I'm already wearing additional insulation. Not quite an answer to your question about synthetic quilts and whatnot, just an example of making a lighter bag work as an alternative with a bit of pre-planning and systems thinking.

As far as quilts go, I'd echo the sentiment about having a good strapping system to keep the edges down. I've found it more effective to run the straps all the way around the sleeping pad rather than just around your body. Keeps the edges of the quilt a little lower and helps reduce drafts.

I have a 20F down quilt from Enlightened Equipment and a 50F synthetic EE quilt. I've tested this down to ~10F and been very happy. There is a chart somewhere (maybe even on EE website) that shows my 20 + 50 = 0F. Definitely want the wide quilt over top and your strap setup to be fit checked. I also wear a hat and big fleecy balaclava. Additionally I usually lay a jacket or any layer on top of my pillow to further wrap my head up should I need. I do sleep warm and I consider the temperature ratings to be perfect for me, so take my experience with a grain of salt. I'm pretty sure my experience is not an open bivy, so take any wind into account.

Found that chart.

https://support.enlightenedequipment.com/hc/en-us/articles/115002770588-How-to-layer-quilts-for-sub-zero-camping