'Extra Warm' is 1.5 times warmer than our regular 'HEATTECH.' Based on CLO values, which indicate the thermal insulation of clothing. Comparison with standard 'HEATTECH.' 100% cotton on the inside, brushed for extra softness. Ultra Stretch fabric conforms to the body for a perfect fit. Versatile crew neck design with a Tshirtstyle collar for easy layering. Knitted fabric keeps its shape even after repeated washing.