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Waterproof breathable membrane on cold weather sleeping bags

Any experience with marmot's Membrain sleeping bags?  

I bought a CWM -40 sleeping bag a while back and didn't notice that it had this feature.  

Personally I have avoided waterproof sleeping bags as I primarily use a tent and am more concerned with moisture from my body than from outside.  If I need more protection I'll use a bivy.

My main concern is that on multi day trips I will have a lot more issues with moisture in the insulation than on a non waterproof bag.  Has anyone used membrain or similar bag and noticed any difference in moisture buildup over the course of multiple days or is the difference in breathability a non issue. Obviously moisture will be an issue in cold weather regardless, but I'm curious if a non waterproof bag will do noticeably better.  

the bag I use has something similar, a proprietary mountain hardware membrane.  

it's a balancing act, and it depends on a few factors.  as  you hint at, on a longer trip in cold weather, moisture evaporating off your body while you sleep can infiltrate the down insulation and limit its loft.  that can occur whether you have a waterproof/breathable membrane or not.  it tends to happen at the outer edge of the insulated, closest to the cold air, and around your face where moisture from exhaling tends to collect around the opening.  

for shorter trips, this hasn't been a big issue for me.  over a longer time, week or more, it's possible that the slightly less permeable outer shell traps more moisture in your insulation and creates more of an issue.  

the flipside is that you're often in close quarters sleeping in cold weather and sometimes dealing with snow intrusion or rime in a tent...and certainly in a snow cave or igloo.  the waterproof/breathable outer shell does a far better job with those factors, or a spilled cup of tea or soup, than a no-membrane shell.

you can eliminate the concern about moisture invading your insulation by sleeping in a vapor barrier liner.  if you want to evaluate that, I have included a link to a good essay about them. it's not the most comfortable way to sleep - you strip to bare minimum layers and end up with a pretty moist/damp night.  it takes some getting used to, and it only makes sense in very deep cold on longer trips:  also, here is a a low cost liner: ; and a high-cost liner:  


In any winter camping situation for which the CWM was designed for you stand a greater chance of getting the bag wet from inside moisture rather than from without. The only time I have had a problem with outside moisture was if my sleeping bag rubbed up against the wall of the tent. But, again in extreme cold that this bag was designed for it is extremely rare to get wet from outside.

I have found that, in seriously cold weather, any type of waterproof breathable fabric becomes a vapour barrier and that when placed on the outside of the insulation layer creates a mini glacier inside the insulation. This is true for me whether as a hard shell or for the outer fabric of a sleeping bag. I have found that even after only one night of use and with a regular polyester shell on my sleeping bag, that it becomes full of frost. For this reason I always use a VBL to protect the down from internal moisture.

I have used PU coated nylon, sylnylon, Cuban fibre and a host of other fabrics for a VBL and keep returning to simple large garbage bags taped together.

I will on occasion use an eVent bivy bag when sleeping without a tent, but I always bring a VBL for below freezing temperatures. Using a VBL will also increase the lower end of the temperature rating for any sleeping bag by about 10C or so and can be a real lifesaver for extended cold weather camping. Wearing a thin synthetic baselayer inside the VBL will make a world of difference, too.

I've done a bunch of camping in temps of -30 or colder and am aware of the fact that inside moisture is the main concern. On longer trips I've found that I can dry the bag reasonably if it is a dark color and I lay it out in the noon sun every few days During a lunch stop. 

I had amother -40 bag that I sold in favor of the cwm. When I bought the cwm I failed to notice that it had a waterproof membrain. 

What I really want to know is if I will have a noticeable increase in condensation issues with a waterproof bag. 

Unfortunately I am outside the 30 day return policy for moontrail but if the bag will be noticably worst in terms of moisture I may attempt to since it is unused so far. 

In ofherwords what I really want to know is if im not planning on using a vapor barrior should I sell or attempt to return the bag. 

Im aware of normal condensation without a membrain and can manage that but don't want to increase it. 

What ever waterproof breathable membrane we choose for the outside of our sleeping bag, it still won't be as breathable as untreated, non-laminated fabric. Condensation therefore will accumulate at a higher rate within a waterproof breathable shelled sleeping bag than a simple nylon or polyester one.

That being said, the CWM is a great cold weather bag and I would keep it.

November 25, 2020
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