Layering system at a glance
- Base
- Moves sweat, never cotton
- Mid
- Traps air: fleece, down or synthetic
- Shell
- Blocks wind and rain
The base layer
The base layer sits against your skin and its job is moving sweat away from you, which is why cotton is the wrong material: it holds water and then cools you as it evaporates. Merino and synthetics both work, with merino handling odour better and synthetics drying faster and costing less.
The mid layer
The mid layer traps air, and air is what insulates. Fleece is bulky, cheap and keeps working when damp. Down is far warmer for its weight and fails when wet. Synthetic insulation sits between them. Which one is right depends less on temperature than on how wet the conditions are and how much you're carrying.
The shell
The shell blocks wind and rain. A windshirt weighs a couple of ounces and handles a surprising amount of what people reach for a rain jacket for; a waterproof shell handles the rest and breathes worse. Together the three let you run cold on the climbs and add insulation the moment you stop, which is the actual skill. Most people carry the right layers and wear the wrong ones, staying too warm while moving and then getting cold at the top because their base layer is soaked.
Why cotton is the exception
Cotton absorbs many times its own weight in water, holds it against your skin, and loses essentially all of its insulating value once wet. Worse, wet fabric against skin conducts heat away far faster than dry fabric does, so a soaked cotton shirt actively cools you at exactly the moment you need the opposite.
Wool and synthetics both keep working damp. Wool holds warmth better when wet and resists smell; synthetics dry faster and cost less. Either is fine, and the choice between them matters far less than not wearing cotton.
Managing layers while you are moving
Start slightly cold. Everyone knows this and almost nobody does it, because the first ten minutes are unpleasant. Ten minutes later you are warm and dry rather than warm and soaked, and that is the whole game: you cannot un-sweat a base layer on the trail.
Vent before you shed, using pit zips and a collar, because it is faster than stopping. Then put insulation on the instant you stop, not once you notice you are cold, because a damp body in still air cools much faster than it warms.
What this means when you're choosing
- A base layer in wool or synthetic, never cotton
- A mid layer you can actually walk in without overheating
- A shell that packs small enough that you carry it
The common mistake
Starting the climb warm
Being slightly cold at the trailhead is correct, because you'll warm up within ten minutes. Starting comfortable means sweating through the base layer, which is what makes you cold when you stop.
Common questions
Why is cotton bad for hiking?
It absorbs sweat, holds it against your skin and takes a long time to dry, which pulls heat out of you as it evaporates. In cold or wet conditions that's a genuine hazard rather than just discomfort.
How many layers should I carry?
Base, mid and shell covers most three-season hiking, with a second thin insulating layer for cold trips. Carrying more layers than you'll swap between is a common way to add weight without adding warmth.