SUL winter setup: SW Tunnel + WPB bivy?

> What happens when the dew point is inside the tent, outside your down bag. Condensation occurs on outside of your bag and destroys down

Rarely happens in practice in my experience. What does happen is that the water vapour makes huge hoar-frost on the inside of the inner tent. I've seen 1/2" ice crystals above my head some mornings. But the frost was on the inner tent, not on my SB cover.

I suspect, but do not know, that using an eVent or propore bivy inside a single wall tent would often result in disaster. The frost would form on the inside of the bivy and transfer onto the SB.

Cheers

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Rodger is very experienced, IMO I would listen to him.

If your using an EPIC bivy under those conditions it will freeze to the bivy. I would look at either an eVent or uncoated nylon overbag, if you go that route.

What kind of winter conditions are you talking about? -30 the dew point will be inside you bag, period. Unless your useing a stove for heating.

-5, like your talking about, Rogers right, you will usually heat up a two man double skinned tent enough that it will freeze to the tent and not in the sleeping bag.

I would NOT use a bivy here as the only purpose that it would serve is to try and keep falling frost off of the bag, something the shell can usually handle.
Even a plain uncoated bivy under these conditions will likely attract frost as it provides an aditional barrier; keeping the freazing point nearer the bag and making the moisture louse energy, cooling and freezing as it moves through the bivy.
But you may find that it may not. Please let me know how it goes.

I think that your misundertanding whats going to happen when you use a full VBL. Your going to see almost no moisture entering the bag from your body, rather most will escape into the tent from your mouth – or into the down, if your breathing into it. Becouse most of the moisture is going right into the air, it will either freaze to the tent walls or be vented out.
You dont really need a WPB cover for your bag under these conditions. In fact, if your so worried about getting your fill wet, consider a fully WP but NOT breathable shell for the inside AND outside of your bag. No issues with frost falling on the bag, AND a weight savings on your VBL clothing/liner, and no issues with your breath wetting out the down.

sorry, but if your worried about frost on the bag, I'd go for the 100% WP but not breathable bag, or plan to ocashionly dry out the bag during sunny days.
Plan on a slightly warmer bag then you'll need to accommodate some loss of loft in high humidity environments. Once it gets below -15C humidity tends to drop off significantly. Also, your bags shell should be able to handle frost forming on it from the air, and if your using a full VBL you don't have to worry about water entering the down from your body. In this case, why are you paying through the nose for a WPB bivy? Just get another VBL and use it as your overbag.

Thanks Roger and Robert for your comments.

Even a plain uncoated bivy under these conditions will likely attract frost as it provides an aditional barrier; keeping the freazing point nearer the bag and making the moisture louse energy, cooling and freezing as it moves through the bivy.

good point. thanks for clearing this.

Hi, The answer seems to be a "very light" sleeping bag made completely out of 100% waterproof Cuben Fiber.

I agree with the vapor barrier concept and have used it quite a bit for the feet and hands. I think this can also be an effective tool for a sleep system. However, there are specific drawbacks to a vapor barrier bag.

When using a vapor barrier bag such as the all-cuben bag described above or a vb bag liner, it means that any clothes being worn will be greatly impacted by the body's moisture. A thin layer could be worn but not much else.

A standard UL technique is to sleep in all of your insulation clothing to increase the warmth of your sleeping system. For me, this is a huge component of my system. An all-vb bag would render these garments very wet by morning.

If your goal is to keep all of your down 100% dry no matter what but you also want to wear insulation clothing to bed, I'd recommend VB clothing that would be worn inside of the insulation layers. This would eliminate the problems inherint in a vb bag system.

Once this approach is taken, the bag could also be constructed out of cuben to minimize weight. But I'd prefer a bag made of highly breathable nylon so I can push the moisture out. This also adds versatility- vapor barriers are not the most comfortable solutions, especially as temps rise.

To see the power of VB, just try this simple test. Put one foot in a thick wool sock and the other in a sealed plastic bag. Hang out at see which is warmer. The power of vapor barrier is often overlooked- I'm glad to see the discussion here.

Doug

> The answer seems to be a "very light" sleeping bag made completely out of 100% waterproof Cuben Fiber.

However … this idea has two problems:
1) getting the air out of the down when you try to pack the sleeping bag up
2) getting the air back in for insulation when the bag is unpacked.
:-)

Cheers

However … this idea has two problems:
1) getting the air out of the down when you try to pack the sleeping bag up
2) getting the air back in for insulation when the bag is unpacked.

Add a valve.

Thanks Roger. Yor comment has just given me an idea for my DAM project.

Instead of a valve how about several strategically placed air inlet/outlet "ports". These could be placed at head & feet locations and could be made of a fine mesh (down proof?) mosquito netting. This assumes that the baffles holding the down are also made of a similar mesh–to allow air movement. Once the bag is "inflated", the ports could be covered by a cuben or w/p closure. I personally love using a VBL, and do sleep in my regular (non-VBL) clothing. Haven't had any major moisture problems yet. I do open the bag (as needed) to allow extra venting/cooling–otherwise the VBL can trap too much heat and moisture. If you perspire heavily a VBL is not for you.

With the right design, I believe a cuben sleeping bag could be the next big weight reduction for a certain type of UL hiker. Example: My current setup is a Marmot Hydrogen bag (1# 5oz) and a separate VBL silnylon insert (6oz). With a cuben version, I could skip the separate VBL and yet still have a much lighter sleeping bag.

Steve, that a cool idea. It will work great. Much less of a hassle then a valve. I am now just waiting for Bill to make cuben down bag.

I'm wondering if this old idea, of the discontinued Pneugear Cocoon 4, might come in handy as you think about your sleep system using cuben:

http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?display_style=nested&forum_thread_id=4105&nid=29352