Fabrics for pieces that will keep you warm.  Fabrics accomplish this task by trapping your body heat within the fabric.  In general, thicker fabrics = warmer layers, but the more efficient the fabric is, the more heat can be retained in a thinner layer so quality of fabric can be an important consideration.  When considering a middle layer, you can have a range of options from lighter weight fabrics for warmer conditions to thick and beefy layers for extreme cold, keeping in mind that as you move, you will warm up and will likely adjust your layers as the conditions and your activity level change.  We have written a couple of blogs that provide more information:  Layering For Warmth and Packing for Multiple Climates