Proper layering is one of the highest-impact outdoor skills, and understanding the function of each layer, rather than just stacking warm clothing, leads to better comfort across changing conditions.
The base layer’s job is moisture management, moving sweat away from skin — synthetic or wool materials perform this function well, while cotton retains moisture against skin and is generally discouraged for active outdoor use for this specific reason.
The insulating layer’s job is trapping body heat, and its effectiveness relates to loft and trapped air rather than simple thickness alone — a well-designed lighter insulating layer can outperform a bulkier, less efficient one.
The outer shell layer’s job is blocking wind and precipitation while ideally still allowing some moisture to escape — this is where the trade-off between waterproofing and breathability becomes most relevant to understand for your specific conditions.
Adjusting layers actively throughout a trip, removing a layer before you start sweating heavily rather than after, is the practical skill that makes the system actually work in changing conditions.