This is our heaviest base layer, at 250g/m2. It combines our 18.5-micron responsibly sourced Merino and recycled polyester technical fibers in a remarkable construction that positions the polyester next to your skin for maximum wicking while the wool faces out for superior insulation. With its Long sleeves it is the ideal garment to layer beneath one of our AmFIB jackets. Thumb loops keep the sleeves in place as you pull on your jersey. Men's Merino Thermal Long Sleeve Baselayer