Surprised no one brought up how a layering system should work. We are familiar with the “3 layer” system.” Base layer, mid layer, and shell. This is a “heavy work” system, heavy work being backpacking during the day. In colder weather the base layer wicks moisture from the skin, the mid layer (fleece for me) transports the removed moisture as vapor through the fleece, and a wind shell helps keep us warm in windy conditions because fleece is highly breathable, which is why it allow water vapor to move through it. A rain shell helps keep us dry in rain. We may not wear all pieces at one time, which is the beauty of layering, we can mix and match the three layers.
But there is a fourth layer — insulation for times of “light” work or times we are “at rest.” Light work is usually doing chores while one is in camp. It should also keep us warm in camp when we are basically at rest. In really cold weather, the insulation piece should be worn during rest stops while backpacking (heavy work).
Down is usually the best 4th layer as it provides the best insulation per ounce of weight. You should almost never wear down while doing heavy work (backpacking), although I do so in unique situations occassionally.
If I will not need fleece during daytime “heavy work,” I don’t bring it. If I don’t need fleece during the day, I will bring a lighter down jacket for night. In winter I will bring a fleece mid layer, and a heavier down jacket for periods of rest or light work.
I’ve been using this system for around 40 years. It was Patagonia who really dialed in the layering system for us backpackers and other outdoor pursuits, and it was Patagonia who actually educated the public.
Below is a link to my blog, where I discuss layering systems in great detail, and share Patagonia’s influence on modern day layering systems. I think it is worth a read.
Disclosure: I am not any sort of clothing expert, so keep that in mind if you decide to read the article. If you need car repair advice, I am an expert on that subject :-)
http://popupbackpacker.com/understanding-layers-using-patagonia-clothing/