| Position | Over base, under shell |
| Job | Trap warmth (insulation) |
| Types | Fleece, down, synthetic |
| Role | The system's 'thermostat' |
A mid layer is the insulating middle layer in a clothing system, worn over the base layer to trap warmth. Common mid layers include fleece, down, and synthetic-insulated jackets. You add or remove the mid layer to regulate temperature, making it the adjustable ‘thermostat’ of the layering system.
Choosing one
Breathable fleece for active use; a warmer insulated jacket (down or synthetic) for cold and rests.
In the system
The middle of the three layers, over the base layer.
Frequently asked questions
What is a mid layer?
A mid layer is the insulating layer worn between your base layer and outer shell. It traps warm air to keep you warm, and you add or shed it to fine-tune your temperature as conditions and activity change. Fleece and puffy jackets are the most common mid layers.
What makes a good mid layer?
Enough warmth for the conditions, breathability for active use, and packability so you can stow it when not needed. Active pursuits favor breathable fleece or active-insulation pieces; cold rests and belays favor warmer down or synthetic puffies.
Fleece or a puffy as a mid layer?
Fleece breathes well and works while you're moving and sweating, but is bulky and not very warm for its weight. A down or synthetic puffy is much warmer and packable, ideal for cold and for stops, but less breathable when working hard. Many carry both.
Sources
- Layering basics — The Mountaineers