One questions we get a lot in the SYMG office is regarding down vs synthetic insulation. These are fill choices for all of the sleeping bag and “puffy” jackets. Let’s explore the differences of each and discuss situations where each one excels.
Down fill is very light and compressible. It is typically given a “fill” rating between 500 and 900, indicating the quality of the down. The higher the number the lighter, more compressible and more expensive the piece will be. For any given temperature rating, down is lighter and more compressible than a synthetic fill. It also has a longer lifespan. The major drawback of down is that if it gets wet, it is utterly useless and takes a very long time to dry, even in ideal conditions. A waterproof dry-sack will keep your bag dry even when dunked in a creek, but humidity is more difficult to contend with, and accidents do happen.
Synthetic fills are man-made hollow fibers that trap heat. Their major benefits are that they retain up to 60% of their warmth even when wet and are less expensive than down for any given temperature rating. The downside is that they are heavier and less compressible than down.
So, which to use? I like to think of my gear choices in terms of a system. In this case, we want our system to be lightweight, take up a minimum volume in our pack, and we need insulative value even in the event that our gear gets wet. Packing a synthetic bag and jacket will certainly keep us warmer if wet, but will be heavier than we’d like. All down runs the risk of having no insulating pieces if it gets wet. So, the combination that works best is perhaps a down sleeping bag and synthetic jacket. Since the bag is heavier and bulkier than the jacket by nature, it’s a good candidate for down. Meanwhile, the synthetic jacket will work if the down bag gets wet.
Especially in the sunny Sierra, this system works great. However, there are times when other systems are preferable. For instance, on kayaking trips or backpacking in rainy, wet forests of Olympic National Park, you may prefer to have all synthetics since there is a good chance of getting your stuff wet. On winter mountaineering trips, all down might be best because there’s little chance of getting them wet (everything is frozen). This minimizes weight and space, making room for all the extra winter gear.
