Silk is all you need under Omnitherm when conditions are mild and temperatures are above freezing. No fabric feels better against your skin, especially when you are sweating. A natural fiber, silk will absorb moisture but has a low density. That means that when you sweat, silk keeps moisture on your skin to cool you. But when you stop sweating, silk dries quickly.
Silk regulates moisture without adding insulation and has an amazing ability to avoid odor.
Put your silks on over your expedition weight synthetic when conditions are cold and windy. When it's cold, synthetic fabrics like polypropylene or polyester work the best. Synthetic fibers do not absorb moisture so they are easy to keep dry. Omnitherm's ability to regulate moisture keeps them even dryer and greatly enhances their insulation. Polyester fibers have a much higher melting point.