Fleece - Wool

The insulation layer is the second layer in the “layer-on-layer” principle.
You can choose out of a variety of products, depending on the activity
at hand and the weather conditions. Fleece is a very effective insulation
material, since it has high warmth-to-weight ratio and also effectively
transports moist out of the garments. A quality fleece also provides the
benefits of keeping the child dry and comfortable.