The Deep Winter Baselayer puts grid-backed insulation across the front panel and sleeves, where wind chill hits hardest, and switches to a merino wool mesh across the back, where the body generates the most heat. That split construction lets the layer hold warmth without trapping the moisture a hard winter effort produces. What Our Gearhead® Experts Are Saying:"I wore this through a two-hour climb in freezing wind and never felt that clammy build-up under my jacket that ruins a winter ride. My back stayed dry while my chest stayed warm the whole way up."