The material features Heat of Absorption and HEAT RETENTION functions. Extra Warm ‘HEATTECH’ is approximately 1.5 times* warmer than regular ‘HEATTECH.’ *Based on CLO values, which indicate the thermal insulation of clothing. Compared to standard 'HEATTECH.' *Recommended for temperatures of -5°C to 15°C. This is a guide when worn in combination with other clothing. The most comfortable temperature range will vary from person to person.