Hope this answer’s not too long and rambling!
When breathable bivvy bags first appeared in the 1980s (before that there had been nylon, silk and cotton bags), they were made from Goretex, bottom and top, and although just a bag, was pretty revolutionary for the time, especially for scrappy nights out were before having a wet or frozen bag was part and parcel of the experience (alpine and expeditions). They provided a huge boost to both personal and sleeping bag protection (for emergencies and planned nights), from rain, dampness, snow or tea. Back then the outer fabrics of bags were either basic nylon or cotton, so bags got wet quickly. People generally were not too tricked up on technology so had low expectations of such fabrics, the standard being PU and neoprene, so the improvement was huge.
The early adopters for bivvy bags were lightweights (people who wanted the minimum of gear,), climbers and the military, that huge army market one reason most bags were green. For a while bivvy bags were a license to print money I expect, being easy to make and easy to sell (I worked in a shop selling green outdoor gear, a good market to be in before soldiers got such stuff issued by the Queen).
I can still remember my first night in a bivvy bag, laid beside a stream somewhere in the Yorkshire Dales, watching the satellites passing overhead, an experience few people ever actually have - to literally sleep under the stars. Since then I must have had several hundred nights in bivvy bags, in all sorts of strange - and scary- places (the pic to this article a low point). Over the years I’ve had dozen’s of bags, from my first Mountain Equipment one (very cool blue and yellow), lightweight models by Rab (Survival zone), PHD, Alpkit and BD, in fabrics such as eVent, Epic and often sHittex. I’ve had bags that leaked yet kept the sweat in, bags that were too small, and bags that almost killed me (you’ll need to check out my show on Youtube to know that story). These days I make my own bags, both single bags and double bags, the reason partly connected with your question.
Somewhere along the line, most top-end bags stopped having breathable bottoms, the reasons numerous. First off it’s cheaper, after all, why sleep on a £200 Gortex bedspread, when a £30 one will do the job just as well, and so keep the bag affordable. Yes, a bivvy bag is a great piece of kit but when it costs the same as a tent, people won’t buy them. Another reason for non-breathable bottoms is that mud and grime tend not to be good for bags in the long term, and neither were sharp stones, thorns and glass. I once had bivvy bag with a tiny slit in the bottom (which I finally fixed with seam grip and tape) and I was always surprised how much water got in there.
Although waterproof most breathable fabrics also don’t seem to cope well when stuck in a puddle all nights long, especially when pressed down hard by a sleeping body (tests say they should… but try it), while a PU one tends to be fine, like a tents groundsheet, which will keep your dry even when pitched in a shallow pond. While on this subject, for those who’ve never used a bivvy bag, it’s worth pointing out that sleeping out, directly in the elements, should always be the last resort. Walls, rocks, caves, snow holes, even bushes and depressions should always be utilized to increase your comfort/survival. A second issue is that in direct rain you will get wet, either getting in or out of your bag and even on winter alpine climbs it’s often impossible to stop spindrift getting in a bivvy bag, meaning in poor/bad conditions such a bag will only give you one night’s grace, as after that your sleeping bag will be wet. Most often, on multi-night adventures, the bivvy bag will be used in partnership with tarps/bashers (how the military use them), tents (in tough mountain conditions the inside of a tent can quickly become hostile to an unprotected bag), bothy/huts/public toilets/bus shelters/doorways (very often such places can be damp and dirty, requiring a bivvy bag…. think Snowdon Cafe at midnight, stuck in an alcove looking for protection from rain and wind.
Another reason for a nonbreathable bottom was that some people put their sleeping mats inside, sometimes to protect them from the ground (inflatable mats), and sometimes to keep them dry and clean (again inflatable mats, as closed-cell foam mats mostly travel on the outside of bags) . This again meant that you were laying on £200 of underlay. A second reason for an inside mat was that it stopped you rolling off it in the night., which is easy to do, especially on slope.
Another reason was a PU bottom tends to be much lighter, shaving a few hundred grams of the final weight, which tends to be good for sales, especially if your selling in the lightweight market, where both weight and bulk is vital (think bike packers).
Personally, I was pretty shocked how bad a half and half bag were in terms of condensation when I first used one (Black Diamond big wall), as the bottom was just pissed wet through when used on top of the mat (I laid on my mat as the ground was very spiky and I didn’t want to make holes in the bag). My partner was using a Wild Things fully breathable bag and was fine. Putting the mat inside the bag reduced the problem, but not as much as a fully breathable bag, but then made it difficult to sit in the bivvy bag, as you do on alpine climbs (eating, drinking and unfortunately sleeping!). I quickly realised that these bags were being made for end-users, not like me, and weren’t fit for purpose, but by this time it seemed most ‘climbing’ bags had disappeared (the Wild Things bags was the last great dedicated alpine bag I saw).
Now it’s worth noting that often what looks like drenching is actually only a small amount of liquid and that the bag will quickly dry unless used this way for days and days. The average person exhales 200ml of fluid through their breath at night, and so not breathing inside your bag is always vitally important (one reason for ‘zipping yourself in’ won’t keep you dry). The actual liquid coming from other sources varies, and someone whose warmth is well controlled and is ‘just right’ will sweat very little. Stick on loads of clothes, make them damp (wet socks stuck in your armpits for example), have a heavy winter bag, and you could sweat a great deal.
So how to deal with this issue?
Well, the obvious answer is to buy a fully breathable bivvy bag. You can buy several full breathable bags (I’m guessing people know the difference between breathable and ‘breathable’), but these tend to be made in less breathable fabrics (cheaper fabrics mean you can make the whole bag from the) that although good for single nights, or a basic bag covers for damp camping, do not really constitute full ‘bivvy bags’ as I see them. The best example of this design would be the Alpkit Hunka, which I’ve used a lot when combined with a tent or flysheet, but would not consider good for exposed bivvys for more than one night, one reason being that such bags can not be sealed uptight.
I’m not keen on putting things inside a bag, as often your mat may already be wet (foam mat on side of a rucksack), which is a bad way to start building that little safe cocoon. And so you’ll probably have to use a full-length inflatable mat (useless for alpine climbing unless you’re on snow), which you stow inside your rucksack, again another compromise. One option is to create a sandwich, with a foam mat on the outside and a thin tent underlay mat on the inside, cut to size, then folded up inside your pack (cut into sections and tape, so it folds down).
Another problem I’ve found with mats on the inside is that they just don’t fit well, that you end up in a tight sausage of fabric, which tends to show the bag is a sleeping bag cover, not a bivvy bag, which should be big enough to move around in (take off boots, sort gear under the cover of the cowl, piss in your mate’s cup etc). Make that mat a very thick one, like the Exped down or synth mats and you can have a problem. For this reason, I often go for XL bags, as they have more space, plus you can sit up in them on ledges etc while inside (spend a day or two sat in a bivvy bag and you’ll know what I’m talking about)
If you can’t help but sleep with the PU side against your bag (mat on the outside), and you want to reduce dampness, then consider sleeping directly on the PU, with the sleeping bag formed like a blanket, as this tends to (counter-intuitively) create fewer problems (try sleeping in your bivvy bag like this at home to see how it works).
So back to your question. Snow holing is amazing, one skill that can really save your bacon in the mountains, but more importantly allow you to live even in the most horrific environments in luxury (well, squalid dirtbag style luxury) (article here on ’Snow holing for climbers’), vital for trips to the greater ranges. One of the big issues with such bivvys is that they’re super damp environments, so everything needs to be carefully looked after, your sleeping bag most of all. You have a good sleeping bag, a good bivvy bag with eVent on top and PU on the bottom and one of the warmest mats you can buy. For such conditions your best set up would be a layer of plastic as a groundsheet (cut open survival bag, or something from the garden centre), then a closed-cell foam mat, then your bivvy bag with your inflatable mat inside, then your sleeping bag. The double mat system is both to do with the issues covered above, but also to reduce to a minimum heat loss into the ground, as often after a few days in a snow hole, using even 4 season foam mats, you’ll find you’re laying in an icy mould of your body. This set up allows you can sit up and do stuff, keeps your bag dry, and reduces to a minimum any condensation. In your case you have a very thick mat, so maybe you’d need to put that mat on the bottom instead as it would be too tight a fit? If so then use a very thin foam mat inside your bivvy (3 season mat or 3mm underlay)…. or just make do if you’re only out for a night or two, as the Rab Endurance will repel most of the water (dry it up with toilet roll each morning if you find a lot of moisture). If it’s going to be a problem them laying on your waterproofs or other layers (so PU layer, clothing, sleeping bag) will have them capture the moisture, not your bag. Also, avoid wearing tons of clothes when sleeping, eat well and warm up before sleeping (might be worth reading my article:’ Maximising your bags warmth’).
As for me, well I don’t think anyone makes a bivvy bag that suits my specs anymore (there used to be loads, but alpine climbing, like proper balls-out stuff, sleeping on ledges, or on your mate’s knee, is a dying game, perhaps because those that enjoy it the most tend to die doing it, a Russian roulette club) and so I need to make my own. This DIY (Don’t Involve Yourself) option is actually super simple and can be pretty cheap, with Goretex and eVent and other fabrics (plus patterns) to be found on the web. Sewing is easy (done on any sewing machines), with seams either tape or sealed with Seamgrip. I tend to only use my own bags these days, making single and double bags of various designs with the following critical design features:
- Fully breathable top.
- Large enough to be able to take boots on or off.
- Must feature a cowl that covers the top of the bag, so as to keep out snow and allow some ability to cook and eat inside the bag
- Bright colours to make it easier to be seen.
- Reflective tape on cowl.
- Keep it simple
Anyway, I hope you enjoy your trip and you get plenty of snow!