Sleeping bag/quilt with WPB cuben shell

There are two main questions here:

1) Is a WPB shell a good idea for a sleeping bag?
2) Is WPB cuben a good choice for a WPB shell?

A WPB sleeping bag shell is better at avoiding external moisture (ie. rain, melting snow) but worse at dealing with internal moisture (vs. a non WPB bag) since breathability is reduced. I believe the general wisdom here is that WPB is not a good idea, but there are different ways to approach this. With care and skill, one can reduce the risks from external moisture with a non-WPB bag, but internally generated moisture will always a big concern with winter use – an issue which WPB bags amplify. So a WPB shell eliminates a controllable problem but amplifies a difficult to control problem.

Body generated moisture often freezes within the bag as it gets further from your body, leading to an accumulation of moisture over time and a loss in loft if not remedied. This is often why people use vapor barriers, which you could employ within a WPB bag. Most people either go on trips that are short enough that accumulation isn't a big deal (ie. 1-2 nights) or for longer trips they take advantage of warmer periods (ie. sunny afternoons, campfires, hot tents) to dry the bag. With a WPB bag, it's going to be tougher to dry since the breathability is lower, so you'd be at a disadvantage here.

Often when I winter camp it's not too cold when I first head to bed, maybe 20 F. In these temps it might be 30F in my tent, which is a situation where my body can heat the bag enough such that vapor can exit before it freezes in the bag. Thus if the bag has good breathability, I can reduce some of the moisture that was accumulated the night before. With a WPB shell, it's less effective to take advantage of less cold periods to reduce the moisture content in the bag. Once it's in there, it's very difficult to remove on the trail. Accordingly, I prefer to take care not to get external moisture on my bag (no WPB shell), which allows me to take a lighter and better breathing bag.

As briefly mentioned, WPB shells are heavier. I think WPB cuben is around 1.4oz/yd, while the lightest shell fabrics (ie. 7D nylon) are about half of that.

Regarding the second question (ie. WPB cuben a good WPB choice?), I imagine it is. I've never had difficulty with even the lightest fabrics for sleeping bags, so I doubt that durability would be a concern. Breathability is another issue, and think the WPB cuben is generally regarded well in this respect. It's is pretty darn expensive though, and its also not readily apparent why its better than something like Pertex Shield+. I imagine Pertex Shield+ is lighter, less expensive, just as breathable but potentially less durable which I don't consider to be an issue for sleeping bags.

A lot of people prefer to use a second layer/blanket over their sleeping bag such that the moisture can exit their bag and collect in this less crucial – and more easily dried – layer. If you do longer trips in the winter, it might be better to go with a non-WPB bag (higher breathability) and add a thin synthetic blanket on top with the money/weight you save vs. a WPB shell. This setup wouldn't be as effective against external moisture obviously, but it would be far superior for internal moisture which is generally the more relevant concern for longer winter trips where the hiker has the maturity/caution/skill to avoid dunking their bag in a snowbank.

Thanks for your quick and detailed response, Dan!

I see your point about having to protect the bag from internal moisture. My question then is could that be remedied by simply using a WPB inside material in addition to using WPB material for the shell? For example you could have WPB cuben inside and out. If the down inside the bag ever got wet.. well, I imagine it'd be hard to dry. But if you had WPB material inside and out and taped the seams (if that's possible on a sleeping bag) then would water ever get in…?

I think you brought up a good point about simply adding waterproof/water-resistant/vapor-barrier layers inside and out of the bag. It seems what you're saying is: that way, one can still have the breathability and easy-to-dry-ness of a lightweight nylon bag, but with the benefits of increased water resistance inside and out of the bag. I can totally see how this would be a more flexible and cheaper alternative than using a WPB material on the bag, particularly a relatively expensive WPB material like the cuben variety.

Pertex Shield+ does seem like a good alternative. I see that's the material Feathered Friends uses for some of its bags. However for better or worse I don't know of any company that makes or would be willing to make a quilt-style bag with that material (part of why I'm interested in WPB cuben).

So given the disadvantages of WPB materials (less breathability, slower drying, whatever else I missed), I now wonder why companies like Feathered Friends make their expedition bags out of WPB material. (I'm a complete newb to winter camping/backpacking so I wouldn't know.) Any thoughts on this?

My bother has a old North Face sleeping bag with Gor Dryloft outer shell. Dryloft is a version of Gortex with enhanced breathability and was water resistant instead of water prof. He has had the bag for close to 20 years and he never has complained about condensation inside the bag. However after all the use he has put on the bag the down in it (900 fill power) has lost some loft. But not enough to force he to get a new one.

While not about sleeping bags, the author of this post found that his old dryloft bivy never had condensation in it. Dryloft fabric is a type of highly breathable Gortex designed for sleeping bags. However when he replaced it bivy's made for DWR fabric he instantly had condensation almost every night.

http://40yearsofwalking.wordpress.com/2011/07/04/the-bivy-condensation-conundrum/

I have also never seen complaints of condensation from people using Event bivy's. However I have seen a lot of posts about condensation on Backpacking light from people DWR bivy's. It also appears the Uber bivy has very few issues with condensation.

While I don't know what the hydrostatic head or reathability specs were for Dryloft. I would guess that WPB cuben is probably as close as your are going to get to the original Dryloft fabric. I would encourage you to try it. And let us know how it works out.

Thanks for the encouragement Steven :-) So it looks like at least in some folks' experience WPB fabrics are breathable enough for body moisture to leave the bag and not get trapped in the down and make it wet.

Here's what outdoorgearlab.com has to say about the advantages of WPB fabrics, in case anyone's interested (from http://www.outdoorgearlab.com/Sleeping-Bag-Down-Cold-Weather-Reviews/Buying-Advice)

"There are several reasons why a waterproof-breathable material is better than one that isn’t. First and foremost, down loses nearly all of its insulating properties when it’s wet. Keeping it dry is therefore paramount to a warm night. Condensation is your enemy. As your body heats up and perspires it creates condensation both below (between the pad and tent floor and between the pad and bag) and above you (small ice crystals will rain down in a poorly ventilated tent). Being able to brush off frost is key to a dry bag, especially on multi-day trips. The Snowbunting’s waterproof shell material, and a 1.5-inch flap that covers the zipper, make it sufficiently weatherproof for foul, unprotected bivys. We tested the water resistance of several bags by leaving them under our house’s drip line for 36 hours of mid-winter melting. We were stunned by how well the Petrtex repelled light snow and steady dripping. A frozen puddle formed on the bag in the morning. The fabric strikes an ideal balance between water resistance, weight, and breathability."

Hi Stephen,
To some of your points:

"But if you had WPB material inside and out and taped the seams (if that's possible on a sleeping bag) then would water ever get in…?"

This is probably the worst option possible. The WPB fabrics do not block just any water, but only liquid water. The water vapour produced by the sleeper would pass the inner WPB layer, condensate inside the insulation and will be trapped there. Due to the reduced breathability of the shell, it would be very difficult do dry such a bag afterwards.

"I now wonder why companies like Feathered Friends make their expedition bags out of WPB material."

Firstly, the fabrics used by some manufacturers is usually more breathable than WPB ones used for shells, so it is questionable what of the points discussed here apply to it. Secondly, on a longer expedition, you have to solve the problem of the condensation inside the insulation regardless of what materials were used in the bag. This usually means using a vapour barrier. This is not mentioned in the marketing talk, because it would deter the buyers (who would buy another bag from someone who is not telling them they're supposed to sleep in a plastic bag inside it). A WPB outer shell of course helps to cope with external moisture, but there are other ways to avoid it.

More general comments:
You can find custom quilts (but this applies to bags as well) made of cuben (some air air intake, such as a small breathable panel, must be included to allow the down to loft) on BPL forum. This is a better option from a functional point of view, but the comfort range is reduced because you can not avoid the vapour barrier effect at higher temperatures when you might not need it.

Unless you exactly know what you want and how it will work for you, it is better to keep the system modular and thus more versatile: to have separate breathable bag, a vapour barrier and a bivy.

I have tried something similar to what you are talking about. Not a quilt made out of the WPB Cubin, but I had ZPacks make me bivy made with WPB Cuben for the top and normal Cuben for the bottom.

I will add that I also have an eVent bivy and DWR bivy. My experience is that under certain conditions, ALL these materials will develop condensation. There are many threads that talk about the condensation issues, but I think both Dan and Jan are bang on – especially Jan’s last para:

"Unless you exactly know what you want and how it will work for you, it is better to keep the system modular and thus more versatile: to have separate breathable bag, a vapour barrier and a bivy."

When I had ZPacks make me the WPB Cubin bivy, I knew that I would need a Vapour Barrier to make it work.

Thanks guys for the tips. Very helpful, at least for me.

I found two other BPL threads from past years that echo these comments:

http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=58014

http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=38676

Would still love to hear if anyone has experience with WPB cuben on down bags or clothing.

"As your body heats up and perspires it creates condensation both below (between the pad and tent floor and between the pad and bag) and above you (small ice crystals will rain down in a poorly ventilated tent). Being able to brush off frost is key to a dry bag, especially on multi-day trips."

This quote from Outdoor Gear Lab seems to be missing the largest component of perspiration: from your body INTO the bag. Solving this is the largest issue and the real key to a "dry bag". When it's cold enough that you're brushing frost off your bag, even a DWR generally works fine for external moisture.

Have a look at GoLite's sleeping bags/quilts. They use WPB at the head and foot ends – the areas you are most likely to get wet via your breath and contact with the tent walls. By using regular shell fabric for the middle 80% of the bag you still have a lot of area to allow breathing.

"While not about sleeping bags, the author of this post found that his old dryloft bivy never had condensation in it. Dryloft fabric is a type of highly breathable Gortex designed for sleeping bags. However when he replaced it bivy's made for DWR fabric he instantly had condensation almost every night."

"I will add that I also have an eVent bivy and DWR bivy. My experience is that under certain conditions, ALL these materials will develop condensation. "

Funny since Dryloft is the old name for Windstopper-fabrics in insulated gear and WS and eVent are very similar.