Why does a down bag gets cold when it gets wet?
Do you multi-day hike in cold and wet conditions found in places such as Alaska, the PNW, Scotland, or Scandinavia? Then you probably know the terrible feeling of waking up in a wet and cold down bag. It has a terrible impact on your entire camping experience and, in some cases, might get you cold or sick.
Doesn't down come from birds who live near water in cold conditions?
How come down keeps the birds warm but down bag gets cold when wet? Why is it hard to dry?
To understand the problem, we need to know a bit about down.
Down is a cluster of very fine fibers found under the wings of certain birds such as the goose and the duck. It helps them keep warm by trapping air next to the body.
The thing to note is that: 1) these plumes are individually attached to the body and can be individually moved by the bird which helps in drying out and 2) the fibers are lubricated by the bird using body-produced oil.
In a down bag, these same plumes are no longer lubricated so when the down gets wet, these plumes start clumping together. Because they are loose (not attached to anything), they are hard to separate out. Once clumped together, there is no air space between the fibers, so the bag loses its loft and becomes flat.
If you are lucky to have good weather the next day, you dry the down bag in the sun. If not, you are in trouble the next night.
If you are hiking in wet conditions or in snow, consider synthetic insulation.
Synthetic vs Down
Down has the best "dry" thermal performance but in many applications, you require best "wet" thermal performance.
Unlike down which is loose, synthetic insulation comes in sheet form made from fine polyester fibers with a durable silicone finish. That is why the synthetic fibers absorb very little water and don't clump together when it gets wet. Because the synthetic insulation doesn't clump, it can maintain air space and your bag can maintain its loft and keep you warm.
Synthetic insulation has come a long way from being heavy, bulky and barely lasting a season. That's how most people still perceive synthetic insulation because many synthetic bags and insulated clothing still use the older generation of synthetic insulation.
This older generation (used by Primaloft, Patagonia etc) is made from tiny cut fibers that held together using scrim and glue. They require extensive quilting techniques every 6-8 inches which results in cold spots. The fiber can be pulled or torn apart easily.
The new generation of synthetic insulation, Climashield Apex, is made from continuous filament insulation. That means every fiber spans the entire width of the insulation and cannot be torn or pulled apart. It's that strong. For this reason, no quilting is required, and bag has no cold spots. It is also very lightweight compared to down, packs more compact and last many years. That is why USA military, as well as world's best brands, are now using Climashield for their high-performance gear.