Baselayers break down into two major categories; those made with natural fibers like silk or wool and those made with synthetic fibers like Polypropylene. Many people have preferences on what they wear, however, few people know that there’s actually a reason for wearing either a natural garment or a synthetic garment.
Natural fibers, like wool, silk and cashmere breathe more effectively than synthetic fibers. They allow moisture and air to pass through them at the vapor level which allows you to stay dry and comfortable longer than you would in a synthetic piece. On top of that, natural fibers are also reactive to the individual user, allowing for a greater temperature range at which you can wear them without overheating.
This is an excellent video from Icebreaker about the difference between synthetic and wool fibers.
Synthetic fibers, on the other hand, are significantly more durable, which is a huge plus in the right scenarios. They also dry faster than most natural fibers. When companies are producing synthetic baselayers they use a combination of treatments to ensure that the layers wick appropriately. This generally involves using a combination of hydrophobic (water repelling) and hydrophilic (water attracting) coating that help move moisture (sweat) from the inside of the layer to the outside. When you stack synthetic layers together you get a combination that creates quick and efficient moisture management (and dry layers).
So, when do you wear, what? Ideally you want to target your layers at your activities. For backpacking, hiking, or multi-day biking trips that fall in summer, fall, or winter, I always, always stick with wool. Personally, I much prefer the breathability over the quick dry benefits. Both materials will keep you warm in motion so when I have long drawn out periods of prolonged low-intensity movement (like backpacking) I’m not worried about sweating through my layers and I know I’ll have ample time to dry any rain moisture out. On the converse, when I’m sailing, kayaking, climbing or dealing with high moisture environments (rainforest, Appalachian spring, monsoon season) I definitely prefer to roll with something synthetic. In these situations, I know that I’m going to get wet so I’m looking for clothing that will dry quickly when I get into camp and stop moving.
The take away from this is to tailor your equipment to your trip. There is a huge abundance of technical layers available and with that users have the ability to pick and choose what’s going to work best.