Bivy vs Synthetic Overbag

I attach a XXL RAB Xenon jacket (synthetic) as a cover for my down quilt. I’m 5’9″ and it goes from my shoulders/neck to  past my butt.  I have used this combination in the Pacific Northwest, in the clouds, in mixed condensing/freezing temperatures (maybe down to 30°), in a Zpacks Solo Plus.

I stayed warm. My down bag stayed dry.  My Xenon dried quickly the next day in the noon sun.

YMMV

If you use a VB liner then you won’t have any problems with internal moisture condensing inside the sleeping bag. However I wouldn’t personally wear merino anything inside a VB bag, experience tells me that you do need to use a  synthetic base layer with a VB system. Experience also tells me that every day or so I need to use an alcohol bath to stay fresh and the sleeping set needs a squirt of isopropyl in the plastic bag for the same reason.

Read Andy Kirkpatrick for the definitive answer on mountain bivvy systems but Full Goretex bivvy sac all the way would be my recommendation based on the few I’ve done, plus a small tarp to keep off the spindrift and / or a specialised summit tent like the BD Bibler series

I use a synth overquilt on my ski tours and have pondered the same question. I think that the best choice depends on your usage. I have a MYOG bivy also, made with some fabric i got from Milesgear (makers of the Uber Bivy). I really like how that fabric performs both in warmth and in breatheability. Seems to get me close to 10 degrees extra. My quilt, also MYOG with 2.5 climashield, seems to get me 15 to 20. Bivy a bit lighter than quilt. So I take the quilt, because i will use it under a mid type shelter. But if I expected to be out in the open sometimes without the mid I think I would go bivy instead because of being better sealed from wind, and being able to have boots, etc. Inside the bivy.

I also don`t think i would use any other type bivy – anyother top fabric I mean.

So far I have been impressed with the performance of the Robic 0.7 shell on my overquilt but it is not a substitute for a real bivvy bag. I plan on using it inside a tent. Having said that it will squeeze  inside my Goretex bivvy bag but my bivvy bag is much bigger than most people use, it will fit a FF Snowy Owl inside it. Nunatak custom and sized big

I’ve been thinking about this lately too.

Personally, I think the ideal system would be a very lightweight bivy (like borah gear) with a thin layer of synthetic sewn on to the outer shell.

My limited experience with bivies tell me that they increase rather than decrease condensation, but the synthetic layer should help with that.

I have used variously a Rab Goretex bivy (too heavy >1kg), a midweight storm bivy and an MLD Superlight bivy. I sold the Superlight because it is a little too specialised (useful as a UL hiker’s bag only) and probably wouldn’t survive a sitting bivy on a rocky ledge. I’m looking at lightweight alpine bivy bags (~400g) but it does seem like the quilt will be more flexible. It’s a good point about the dew point; my idea is to basically sandwich the down bag between the VBL and an external layer to keep it as dry as possible. I’m sure the VBL will help a lot with keeping moisture out of my insulation but there is still a lot of moisture in your breath that I fear might condense using a bivvy bag. As I will be using a mid all the way, I will probably take a quilt. I can carry a little more weight on the bike so it’s not a big problem to carry a bivy bag and quilt if the quilt won’t work as an outer layer in an open bivy,

The combination of a VBL, down bag and a synthetic over-quilt gives you a lot of flexibility and options to handle a lot of different conditions, especially if you’re sleeping in a shelter. Seems like a no-brainier if you already have all three of these items. However, if you’re sleeping exposed in spin drift snow, then it make sense to swap out the over-quilt for a large WP bivy sack. In the VBL I prefer to wear only base layers as they become clammy to slightly damp due to condensation, but YMMV.

@James

You said: my idea is to basically sandwich the down bag between the VBL and an external layer to keep it as dry as possible. I’m sure the VBL will help a lot with keeping moisture out of my insulation but there is still a lot of moisture in your breath that I fear might condense using a bivvy bag

If I understand you correctly, you are thinking of using a down bag, then placing a VBL over the bag, and finally a bivy or synthetic over the setup. My limited understanding of VBL tells me that this is going to lead to a very wet down bag. Even if your breath is fully exhausted outside of this setup, our bodies also expel moisture. You use a VBL in very cold conditions INSIDE the bag to keep that moisture from reaching your bag and possibly freezing there. However, you seem to be thinking of putting the VBL over the sleeping bag, which will make it wet from your body’s persperation and unable to dry at all, possibly making it worse than without a VBL.

Again, I may be misunderstanding your planned setup, so I apologize if I did.

I think I was unclear; I am planning to use the VBL inside my down bag, with the synthetic quilt on top of the bag to add a little warmth and keep it dry from drips and condensation on the inside of my shelter. I’ve been using my Duomid for the last month or so in temperatures around 10°F and it is always very frosted up in the morning. Not such a problem unless the sun gets to it and it starts to melt.

James, may I ask where your ski tour will be?

In the Caucasus, the plan is to ski along the proposed Transcaucasian Trail route in Georgia, then hike it in Armenia.

<p style=”text-align: left;”>Sorry, my mistake. I shouldn’t read before breakfast. I processed that info completely wrong indeed. My apologies! Happy New Year!</p>

In the Caucasus, the plan is to ski along the proposed Transcaucasian Trail route in Georgia, then hike it in Armenia.

WOW!

Now that’s a trip I’d like!

RE: Bivies under DuoMid in winter.

I have used both eVent (IntegralDesins now Rab) and water resistant (BPL made by OwareGear) bivies for winter snow camping in the Sierra Nevada below tree line. I never experienced any condensation inside the water resistant bivy. I found I could not really use the eVent bivy since I got too warm during nights of 10 to 20 degrees F and most of the time was sleeping on top of it.
Like James S unless I keep the door wide open and or pitch the DuoMid high, I wake up every morning with a layer of frozen condensation inside the DuoMid. The trick is to get up and out while keeping the ice all off your sleeping gear. I am never out for more than 4 or 5 mights. I can see how on a longer tour, this frozen condensation would be important to manage in order to keep your down bag dry. I found I am able to shake or brush any dropped ice off of my water resistant bivy before the ice melts.

Sounds like an awesome trip. Please write a trip report for us with photos.

I will post an article on my site here: http://www.spektakl.co.uk

I’ll also post a gear list and feedback on the synthetic bag if anybody is interested or planning a similar trip. I assume interest in the TCT will grow (for better or worse) as the trail development continues. The most difficult element of planning so far is negotiating the bureaucracy of post-Soviet states and planning a route through unrecognised countries!

How high will you be going and how much wind are you expecting? Depending upon that I would bring a solid inner. If you encounter a windy night above the tree line, spindrift will cover everything inside your tent, which is quite unpleasant. I might be wrong, but I doubt that you will get a duomid spindrift-tight in strong wind – unless you make a snowcave out of it.

@ James S. Awesome website and stunning photos. Thanks for sharing!

Thanks for the kind words Bruce, I aim to keep the site updated this year also. The planned route is mostly between 1500m to 2500m, some of the passes up to 3000m. I can build a berm around the Duomid if necessary and keep it storm pitched. The synthetic bag should provide decent protection from spindrift. I once had an unpleasant night in the Romanian Carpathians in which my poles were frozen closed and the mid had to pitched using a tree branch! Needless to say, we were fairly well coated in spindrift by the morning. I am considering taking an MSR Advance Pro (1.5kg single skin alpine tent) instead of the Duomid as it is about the lightest/smallest tent with that level of weather protection.

I’d be very interested in feedback on the use of the synthetic bag over the down one Vs the use of a VB.

James, Im just curious what is your down bag is rated at and what is your MLD apex quilt is rated at? Also what are the temps that your expecting at night on your tour?

My down bag is nominally rated to -14°C (7°F) but I have had it overfilled so probably a little lower. Inside I will use a Western Mountaineering VBL, which should add a few degrees. I haven’t ordered the quilt yet but will most likely go for the 48°F Spirit. Mat will be a NeoAir XTherm on top of an Evazote groundsheet (possibly a Zlite too, although I find the XTherm quite warm). Average lows in March are around -6 to -8 °C (20 to 17°F) with cold nights around -15°C. I’ll also have an 800 fill down jacket and felt boots for extra insulation when needed. I’m planning to take the same gear on detours to some of the big peaks along the route, but that will not be until mid-late April so temperatures will be higher.

Having used several floorless shelters for snow camping, i highly recommend adding snow flaps to the bottom edge of yours if you go that way. They work quite well to seal out spindrift and wind, especially if the daytime teperature gets above freezeing so that the flaps can freeze in well. Plus they give you a more solid pitch overall. But I hear you on that MSR, I saw a good review on it and it seems well designed.