Is it 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:
- Western Mountaineering Puma -25 GWS
- Western Mountaineering Bison -40 GWS
- Marmot Col -20 Sleeping Bag
- Marmot CWM -40 Sleeping Bag
- Exped Waterbloc 1000 Sleeping Bag
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 with a 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.
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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