When shopping for a sleeping bag, insulated jacket, or insulated pants, you will have a choice of insulations:
- Down, which is a commodity product measured by fill power, e.g. 800-fill; or,
- Synthetic, which is normally made of interwoven plastic fibers and which may be marketed as Primaloft, Climashield, or a proprietary version like TNF Thermoball.
Given current technologies, I believe that down is still the superior choice for performance. It’s warmer for its weight and more compressible, and has a much longer lifespan, making it a better long-term investment. Premium sleeping bags, in particular, can nearly be family heirlooms.
Synthetic insulations have one decided advantage: they are less expensive. They also retain more warmth when wet, on a relative basis. However, when the thermal superiority of down insulation is accounted for, synthetic’s wet weather advantage may be a wash — a damp down sleeping bag can be just as warm as a perfectly dry synthetic bag of the same weight, because it’s original warmth was so much better.
While synthetic insulations are frequently marketed as being “warm when wet,” this claim defies the laws of physics. If your synthetic sleeping bag or puffy layers are wet, the thermal mass of that wetness will steal your body heat. Period.
Down insulation can get wet from both precipitation and ambient humidity. Here are four techniques to mitigate these factors:
1. Water-resistant down
Down can be treated to be made more water-resistant. The process adds little cost to the down, and actually improves loft. Water-resistant down — which may be branded as DriDown, DownTek, or others — will not be warm when wet, but I find that it holds up better in humid conditions and that it dries faster than traditional down.
2. Pack liner
I protect my sleeping bag and puffy layers using a Brute Super Tuff 20-gallon Trash Compactor Bag made of 2-mm plastic. Scented bags can be aired out, if it bothers or concerns you.
In very wet environments, I may use two bags: one to protect everything that I will not need until camp (e.g. sleeping bag, sleeping clothes, pad, stove, food for the rest of the trip) and a second bag for things I may need throughout the day (e.g. today’s food, puffy jacket, rain gear). If my shelter is wet, I keep it inside the backpack but outside the pack liner.
3. Mid-layers and sleeping clothes
The mid-layer is worn between my hiking shirt and rain shell in cool-and-wet or cold-and-wet conditions, which preserves my down jacket for rest stops and camp. The fleece dries quickly and buffers moisture more effectively than a down- or synthetic-insulated garment.
The sleeping clothes guarantee me nighttime comfort, and prevent me from having to sleeping in my wet hiking clothes, which would then wet my sleeping bag.
4. The reset dry
This technique is my favorite and the most effective. If I get whacked with wet weather and become either uncomfortable or near the safety limits of my systems, I will take the first opportunity I have to dry out. Depending on the location and time of year, the time necessary for a reset dry will vary. On a sunny, warm, and arid late-morning in the Mountain West, it can be done very quickly. If you’re hiking the Appalachian Trail in February, you may need a motel room.
The reset dry is easy: expose ALL of your stuff to the sun, breeze, and dry air. Hang it in the trees, lay it on the grass, drape it over a boulder, pitch it haphazardly, etc. When things look and feel dry, pack it all up and keep moving.
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