Keeping your sleeping bag dry in freezing temps

While there are others here with much more experience and hopefully they chime in, all I can suggest is don't give up on the VBL too quick. Waking up in a pool of sweat sounds like you were too warm. A VBL may work better for you if you sleep "comfortably cold" by appropriately venting so that you don't overheat and sweat yourself out

Just below freezing is way too warm for a VBL.

It depends on where you are – in the Sierra you can put out your bag in the sun and watch moisture rise off the lining, even in the middle of a snowy winter. It can be 30f at night and 80f during the day in summer.

I never worry about it even with plain old down. i have not had any problems with losing so much loft that the quilt failed to perform.

If it really is too cold to dry anything out during the day, I think the laws of physics limit you to the VBL solution–if you can't take water out, you've got to not put water in.

If you're pooling sweat, maybe you've got too much insulation and need to either take less bag or vent yourself better.

A backpackable wood stove is another option, typically used with a floorless pyramid or tipi type shelter. You can theoretically dry your bag and clothing with the dry heat it provides, though I'm not yet experienced with how well this works in practice. I will be trying it out next season. My small Wifi stove from Tigoat weighs 27oz with a 6.5ft pipe.

The breathable bivy and synthetic overbag are also good options. Both will likely move the dew point outside the down bag, though perhaps the overbag more reliably. A summer weight bag with 2.5oz Apex would probably do the trick, and is fairly easily made. I believe it also adds 10-15 degrees of warmth.

An insulated snow cave or trench can also move the dew point away from the bag, but tends to increase the humidity of the air as well.

I wonder if a snow trench may be an optimal solution, because the insulation of the snow would help move the dew point away from the bag, but the open ceiling would then vent that vapor, helping to pull more vapor from the bag. A tarp over the top of the trench could be used in inclement weather, preserving some air flow.

This is all speculation mind you…I only recently got a winter set-up together, so, next year for me…

I know that this seems counter-intuitive, but you need to make the inside of your tent better ventilated. I know it will be cold, but that air movement will carry off a lot of the excess moisture that leaves the sleeping bag. Therefore, your sleeping bag stays somewhat drier. Ultimately, that leaves you warmer while you are in the bag. However, it leaves you colder when you are out of the bag and sitting in the tent.

Also, it helps if you have the warmth rating of the sleeping bag to be correct for the temperature that it will be in. For example, if I expect the cold temperature of 0 F, then I take a sleeping bag rated for -10 F. I would not take one rated for -40 F.

–B.G.–

A stove would be nice, but an issue if there is no fuel due to heavy snowfall.

I do think an overquilt is a good option.

Use a VBL. The physics are inexorable. As Todd noted above,"If it really is too cold to dry anything out during the day, I think the laws of physics limit you to the VBL solution–if you can't take water out, you've got to not put water in." Stephenson's Warmlite makes only VBL sleeping bags. Their website discusses the benefits of VBLs.

VBLs keep you warmer and protect your insulation. And contrary to conventional wisdom, they work at any temperature. I use a VBL in the middle of summer at 600 feet elevation, with temps plunging to the upper 50s. Yes, I get a little condensation around my feet. No problem really. I wipe the condensation out.

I use pretty much all the stuff you mentioned.

I like to add more heat, hot water bottles, force more moisture out of the bag.

I have only slept in temps of around 20f and up , at those my take is that ventilation does help a lot.
Last winter on two consecutive almost identical nights , on the same spot, I had a rather damp first night (wet under fly and collapsed sb foot box) because by mistake I had my top and bottom vents closed, the second night both were open resulting in a dry bag and dry fly in the morning.
BTW, having had a good look around in the morning, the other tents that were all sealed up (we were on snow ) were wet in the morning, later to have folk drying their bags in the sun.

If you're spending $ and weight on bivies and VBLs why not just switch to a synthetic insulation bag? Several of my cold weather backpacking friends seem to prefer synthetic for that reason.

Tips:
Unless you're below 0 F, go ahead and change your clothes including underwear and socks. clothes can bring a fair amount of moisture into the bag.

Make sure you don't breathe inside the bag. Drape a scarf/shirt over your nose to keep it from freezing but don't tuck the scarf in into the bag. That way the scarf won't pull moisture into the bag.

If you're sweating, unzip the bag and move a leg in/out to regulate temp.

Hanging your bag before and after bed will probably help dry it more than you think. Cold air can be extremely dry, helps evaporate out that moisture.

these are time problem methods used by folks in the moutains/polar regions

also note that even if its cold the sun can help dry the bag … a dark colored bag helps this process

;)

Another vote for some kind of VBL. You can use a VBL bag inside the sleeping bag, but that would mean that the moisture would be retained in any clothing you'd be wearing. That would not be good if you wear any down clothing to bed. Instead, try a VBL shirt and cap over your base layer. Various companies make them, such as RBH Designs and Stevenson's Warmlite.

If you are sweating, take off a layer, do something to not sweat. If it's just the normal moisture that always comes from healthy skin, it can be managed with ventilation of the shelter.

Thanks for all your responses!

To summerize:
Most people would prefer a VBL. After reading all your experiences and opinions I agree. Looking at the phisics this would be the only real solution.

A synthetic bag would I think have the same problem, it is just not affected as much by water as a down bag. A stove would be nice, but heavy and would require some extra skills.
A very good point by James was made, to have a sleeping bag that is a bit to cold for the situation. When you are not to worm, you sweat less, so less of a problem. You would be able to add wormth by adding layers.
Drying your sleeping bag in the sun is worth the efford, even in very cold condition.

Conclusion:

Sleeping bag should always be with a non-waterproof shell. So just a lightweight nylon with dwr.

When sleeping in -10 degree C (14 degree F) or colder: Use a VBL
When sleeping in -10 to 0 degree C (14 to 32 degree F): Use a lightweight nylon overbag. Since the dewpoint in these temperatures is outside the bag, all you need is a layer to keep the water from you sleepingbag shell.
In -5 degree C (23 degree F) and up: you don't need an extra layer, provinding you have enough ventilation.

Have we now come up with the ultimate conclusion that answers dozens of forum threads? :-p

Nope. :>

I don't use a VBL because I've never needed one.

Would it be possible to dry out the bag by tossing in a hot Nalgene water bottle? This would be done in the morning after waking up. Only the Nalgene will be inside the bag without a human (who would otherwise add perspiration).

Niek, the other consideration is time

If you’re going to spend just a few nights it’s probably not a problem

If you’re doing a many day trip you’re more prone to water in your sleeping bag

In the warmlite description, he talks about multi day arctic expeditions where day after day, the sleeping bag accumulates more and more frost

Wow. A lot of old threads are being resurrected here at BPL.  The reason for the frost on the outside of the bag is perspiration from the human body moving through the insulation, condensing on the outer surface of the bag and then freezing.  The main material risk is compromise of the insulation’s ability to insulate.

Andrew Skurka discussed this issue here at BPL in his series of articles about his winter walk across the midwest. (use your Google search) The answer of course is a vapor barrier layer.  As Jerry says, for shorter trips, one, two or three nights or perhaps up 4 nights, this is not a problem since your bag while comprised is still doing enought of its job to insulate you.  Can you dry out the bag during the day? Well maybe during sunny spring days when you are not moving camp.

As an alternative, I would actually look at a lightweight synthetic quilt to go over the top of your bag.

While you can try and control your sleeping temperature and try to not sweat too much, the same way you do when walking during the day, in this situation there is no way to really get rid of the condensation completely. All you can do is control where it forms or what happens to it.

The condensation will form when the water vapour from your body touches something cold. On a normal camping trip, the inside of your tent is a little warmer, and the outside air a little colder, so your fly becomes that barrier between hot and cold. It’ll be cold from being exposed to the outside air, so as the water vapour rises and touches it, it’ll condense to water drops.

But when the temperatures are really low, even the air inside your tent becomes cold. You are all warm wrapped up inside your sleeping bag, and so the place where the warm air meets the cold surface is the outer material of your sleep system. The water vapour will condense to water drops just on the inside of this and thus wet the insulation of your sleeping bag.

People have suggested a VBL but while I’ve never slept like that, personally I feel I would find that a little uncomfortable, having the moisture trapped inside with me. Instead I have a synthetic quilt I use in the summer if I know I’m camping at a campfire of some kind (easier to throw in the wash and get the smell out than down) and I put that over the top of my bag. Now the cold surface becomes the outside surface of that synthetic layer. It can still condense, but you’re at least controlling it and forcing it to so so there rather than the down bag, so it won’t affect your warmth or comfort significantly.

Plus it has the advantage that the extra insulating layer means you can take a slightly lighter bag, overall the weight shouldn’t be all that much different.

Synthetic overbag idea is well-documented of late. Here are some references:

Sleep Systems for Winter Backpacking – BPL Winter Trailhead

Overbag Moisture Management – Timmermade

How to layer quilts for sub-zero camping – Enlightened Equipment