Mid Layer
A mid layer is the insulating garment between your base layer and your shell. Its only job is to trap air, which is what keeps you warm.
It is the layer you add and remove most often through a day, so it should be easy to pack and quick to put on. Fleece is the default; a light insulated jacket is the cold-weather upgrade.
Fleece or insulation
| Fleece | Insulated jacket | |
|---|---|---|
| Warmth for the space | Lower | Higher |
| When damp | Still works, dries fast | Down collapses; synthetic copes |
| Packed size | Bulky | Small |
| Breathability | Excellent | Limited |
| Best for | Active use, damp conditions | Standing around in the cold |
For a trip crossing climates, a fleece is the more useful single mid layer. Add the insulated jacket only when the forecast goes below about 10 degrees C.
Fit
A mid layer should fit close enough to trap air near the body but leave room for the base layer underneath, and it must fit under the shell without compressing. This is the main reason to size a shell up rather than down.
Weight
Fleece is described in GSM. Around 100 GSM is a light grid fleece that breathes well; 200 GSM is the classic travel mid layer; above 300 GSM competes with an insulated jacket for pack space without matching its warmth.
Related terms
- Down vs synthetic insulation: choosing the fill
- Sizing between two sizes: the layering allowance
- Base layer: what goes underneath
- Glossary: all terms