In reply to martin s:
www.polartec.com says...
Polartec Classic products are available in a range of weights to provide the right level of insulation for most outdoor activities. Polartec Classic 100 weight fabrics are the lightest and are excellent for a warm first layer or lightweight sweater. Polartec Classic 300 fabrics are the heaviest and provide a warm thermal layer for very cold outdoor activities. Halfway in between are Polartec Classic 200 weight fabrics, providing outstanding warmth and versatility.
Thermal Pro fabrics are the most visually dynamic and technically advanced of the Polartec insulation fabrics. The 100% polyester velour, pebbled, or shearling surfaces create air pockets that trap air and retain body heat, providing outstanding warmth without weight. These fabrics offer excellent breathability and dry quickly.
Thermal Pro High-Loft offers exceptional compressibility - up to 40% more than standard fleece.
I've got a Thermal Pro top and it is bloody warm. Need a windproof in the slightest breeze, though!