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Is it Better to Buy a Sleeping Bag with a Gore-Tex Shell or to Use a Bivy Sack?

Is it better to buy a sleeping bag with a gore tex shell or to just use a bivy sack?
Is it better to buy a sleeping bag with a Gore-tex shell or to just use a bivy sack?

Had a reader contact me asking whether it is better to buy a winter sleeping bag with a waterproof/breathable shell fabric or to just use a waterproof/breathable bivy sack instead?

The idea of covering a sleeping bag with a waterproof/breathable shell fabric is appealing because it would mean that you don’t need to carry a bivy sack to sleep in a snow shelter or worry about getting internal condensation on the outside of your sleeping bag when you touch your tent’s walls at night.

Sleeping bags with waterproof/breathable shell fabrics:

Less Breathability, Not More

But experience has shown that covering the exterior of a sleeping bag with a waterproof breathable shell tends to trap more perspiration inside the insulation of a sleeping bag than one a much lighter shell fabric. This perspiration gets trapped in down or synthetic insulation, reducing the insulation’s ability to trap heat, so you’ll sleep colder in a bag with a waterproof cover. In the worst case, the insulation will freeze inside the bag and then melt when you get back into it.

Contrary to what you’d expect, waterproof/breathable fabrics are actually far less breathable than most of the non-waterproof shell fabrics used on the exterior of sleeping bags today.

You can get the same waterproof benefit by spraying a DWR coating on the outside of these lighter weight,more breathable fabrics, which will repel water droplets that fall onto the outside of the bag, causing them to bead and roll off, just like a rain jacket. Most sleeping manufacturers already do this at the factory. But if the DWR coating wears off, you can reapply it at home using Nikwax TX Direct or similar products.

Waterproof/breathable bivy sack with fully taped seams
Waterproof/breathable bivy sack with fully taped seams

Taped Seams

One of the key differences between sleeping bags covered with waterproof/breathable laminates and a bivy sack are the seams. Most sleeping bag manufacturers don’t tape or seam seal all of the seams in their bags, which is really required for true waterproofing. Think about all of the tiny needle holes in the baffling of a down bag. Taping or seam sealing them all would be very costly.

However, most bivy bags made with waterproof/breathable fabrics have taped seams or can be easily sealed with seam sealer. You really can’t do the same with a sleeping bag.

Trip Specificity vs A Systems Approach

Waterproof/breathable sleeping bags are heavier and much more expensive than ones with a regular lightweight shell fabric. They also tend to be very destination-specific and less general purpose than buying a regular sleeping bag and augmenting it with a lightweight waterproof/breathable bivy sack on the trips where you need it.

Personally, I prefer investing in a system of components that I can mix and match depending on the requirements of different trips. For example, if you buy a bivy sack you can combine it with different sleeping bags or sleeping quilts, depending on different destination’s trip requirements.

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11 comments

  1. Philip, yes. I don’t care to use either. They tend to trap in water, and, do not breath enough to make them effective at perspiration release. You are much better off with a regular lightweight nylon cloth. DWR treatments are problematical. I don’t use those either. But most of my trips are of the one and two week kind.

    Doing one one or two night trips, they work OK. You really do not spend enough time for condensation to really reduce a bags insulation.

    There is one area that is really not well examined. Drying at night due to body heat. While excess insulation can cause a frost line/condensation inside a bag, usually body heat has a drying effect on down and lofting effect if it is cooler. If it is too warm, dampness from perspiration is increased, leading to less lofting. So down has a wider comfort range. If matched to the conditions, it stays much dryer…again due to your body heat and perspiration. Often, a damp bag will dry by morning, simply sleeping in it. If impeded by a DWR or other WPB fabric, this can get quite uncomfortable(cool) and not dry at all. Often, in a bivy, you get colder as the night progresses, not warmer.

  2. The moisture from your body is going to condense somewhere, if the tent is warm enough it will be on the outside of the bag. Unlike summertime, it wouldn’t evaporate from there. So the question is where do you want the moisture to condense. One of the most effective systems I have users was a down mummy bag inside a wide cut synthetic bag. The inner bag was always dry as a bone. Too heavy and bulkier but if I was going to be outside for many nights in the winter I would strongly consider it.
    Weigh your bag before and after a multi night trip, you will be surprised at the moisture weight gain

    • Heavy and bulky, but viable, especially if you pull it in on a pulk.

    • I”ve done this on a couple winter trips: used a 3-season down bag with a Polarguard over bag. A nuisance because the over bag had no zipper (to save weight), but warm and pretty effective at keeping the down bag dry. I dried the Polarguard bag on sunny days, and it didn’t seem to gain much moisture weight (did not weigh before, after though). I had a bivy bag to go over all, but ended up not using except for a couple nights.

  3. I think you hit all the main points. Waterproof/breathable shells on a sleeping bag can’t do the job because there are just too many seams. They reduce breathability, leading to moisture accumulation in the insulation unless the sleeper is using a vapor barrier liner and/or is meticulous about drying the bag (try that during winter trips!). Also, laminate sleeping bag shells rustle annoyingly every time a sleeper moves inside.

    I would suggest that even a bivy sack should have the minimum moisture resistance to accomplish the job for the season and trip. Moisture acclimation inside the bivy can still a problem…

  4. Bivy sack for sure. More modular. Remove to max breathability of the bag. Put on to keep the elements out, especially if not using much of a shelter like a tent…

  5. I like the bivy bag over a sleeping bag (when required). As others have noted it’s a more versatile, flexible system. What I’ve noticed is that in cold conditions frost forms on the inside surface of the bivy bag. In the morning it’s a simple matter to turn the bivy inside out and knock the frost off before stowing the bivy. As you note, if the shell were integral to the bag, the frost would form inside the insulation, and be impossible to get rid of without drying the bag in a warm dry location.

  6. I can’t say that I’ve ever experienced moisture retention in waterproof breathable shelled sleeping bags used for one or two nights, (though I’ve not re-weighed them after the trip).. On multi-day trips in really cold climates with no means of drying a sleeping bag a vapour barrier liner solves the problem, or I’ve also successfully used a second shelled sleeping bag unzipped as an over-quilt, and that worked. For me, the waterproof shelled sleeping bag is most useful as an insurance policy on alpine climbs where a bivy is not planned, but might occur.. A three season shelled bag offers the lightest warmest protection for such events. I agree that for most applications, a separate bivy bag offers most flexibility.

  7. VBL.

    (I don’t intend to take this thread down the VBL rabbit hole but since winter & vapor was brought up…..)

    If winter camping means below freezing, then IMHO the best, mid & long term solution for sleeping bag(s) is a vapor barrier liner between a comfort layer and your bag(s) + outer-clothing-added-for-warmth. Then you won’t need the (often heavier & more expensive) Goretex bivy. Plus your clothes-to-be-dried will dry faster.

    Some folks feel fine in just their all-covering, always-on base-layer which they then dry as they travel. Some prefer a comfort sheet if the VBL feels ‘wrong’.

    The tradespace is how long can you air out (i.e. let water sublimate in sun & air) your bag or sheet. At the extremes… If you can hang your bag in the dry, sunny, air all day then you’ve no need for a bivy or VBL plus you’ve max comfort & warmth. If you have zero dry time, then suffer temporarily the frozen sheet but gain consistently warm bags.

    Remember, it takes a few nights for most to adjust to sleeping humid…. and to see if you want a sheet. VBL’s also slightly reduce water consumption.

    For more info, see http://andrewskurka.com/2011/vapor-barrier-liners-theory-application/

  8. Among several benefits of biv sack is elimination of need for groundsheet. I don’t think a w/b shell serves this role adequately at all. Carrying such a unit on a day climb would probably be overkill.

  9. Cold enough and I’ll use a VBL sack inside my sleeping bag. Bivy or not – problem solved.

    For those that aren’t comfortable in a full length VBL sack a climbers bag length VBL sack may do the trick. This especially when wearing a hooded jacket or on shorter duration trips.

    If the appetite for the VBL experience is a thin one I find a down inner bag with a synthetic over bag works well. Depending on conditions, bag ratings, etc. water vapor from within moves through the down insulation and evaporates outside (warmer), freezes on the outside (colder) or freezes in the outer layers (quite cold) of the synthetic over bag.

    I’m a bit on the larger side at 6’4″, 225 lbs. and a restless side sleeper. Deep Winter backcountry treks, using a pulk, see me with the following sleep configuration. I use a BA Encampment 15F Long with a BA Wedgie Bag Expander as a synthetic over bag. My inner bag is frequently a Montbell Down Hugger 800 #1 Long. Next inside comes a DIY VBL bag that’s large enough to accommodate my shifting about but not so large that it’s not effective as a VBL.

    If I wanted to spring for a separate 0F or “expedition” rated bag I’m sure I’d probably save some weight. I’d still use the VBL sack to preserve the integrity of the bag’s insulation.

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