The material features moisture-absorbing and Heat of Absorption functions. Extra Warm ‘HEATTECH’ is lightweight yet 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.