Kit
The packable layer test: does it earn the litre it takes?
A jacket you leave at home because it is bulky has a warmth rating of zero.
Every piece of kit competes for the same thing — the volume in your bag and the willingness to carry it. A shell that packs to the size of a grapefruit gets taken. A shell that packs to the size of a loaf gets left on the hook, and the warmest jacket in the world is worth nothing on the hook.
So the honest way to compare layers is not grams and it is not fill power. It is warmth per packed litre, and then the second question: how fast does it dry if it gets wet.
Synthetic insulation loses to down on warmth per gram and beats it badly on wet performance and on price. If your weather is reliably dry and cold, down is the better buy. If your weather is anything else — and for most people it is — synthetic is the one you will actually reach for, because it keeps working damp and it survives being stuffed rather than folded.
A practical rule for a day out: one packable wind layer always, one insulated layer if you will stop moving for more than twenty minutes, and nothing else. Most people carry a third layer they never take out of the bag.
Written by the Ultramax USA team. This is general information about footwear and clothing, not medical advice — see our legal notice.