HEATTECH EXTRA WARM features greater heat-retention properties than regular HEATTECH, and as a result is up to 1.5 times warmer. HEATTECH EXTRA WARM is made from a special fabric that features a unique knit construction and a napped inner surface for protection from the cold, making it an ideal choice for Winter sports or rugged outdoor activities, or just those who fancy that extra bit of warmth throughout the day.