Lightweight yet warm fabric with Heat of Absorption and Heat Retention functions. "HEATTECH" Extra Warm 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 20°F to 60°F. This is a guide when worn in combination with other clothing. The most comfortable temperature range will vary from person to person.