Down, synthetics and wool are the most commonly used insulation materials in winter coats. Each has its own advantages, but the way they keep you warm is similar: your body generates heat and warms the static air trapped between the fibers and filaments of the material. "We know that air is the best insulator on the planet," said Hsiou-Lien Chen, an OSU textile researcher and associate professor. “If we can increase the space for trapping air, the insulation of the material will improve.” Here's how each method works and how to determine which one is right for you.