'HEATTECH Ultra Warm' fabric with Heat of Absorption and HEAT RETENTION technology. This is our warmest 'HEATTECH' yet at approximately 2.25 times* warmer than our regular 'HEATTECH'. *Based on CLO values, which indicate the thermal insulation of clothing. Compared to standard 'HEATTECH'. *Recommended for temperatures of -20°C to 10°C. This is a guide when worn in combination with other clothing. The most comfortable temperature range will vary from person to person.