A running vest uses insulation to warm your core, which in turn keeps the rest of your body warm.
Popular running vest insulation comes in a few different forms: down, synthetic and a mix of the two. But all insulations work by creating loft to trap warm air next to your body.
Duck or goose down is a natural running vest fill that insulates well, weighs next to nothing and packs down small. Synthetic fill creates loft like down but is engineered from manmade materials, like polyester; synthetic insulation is better at keeping you warm even if it gets wet, which is something natural down struggles to do.
Fleece is a type of synthetic insulation for running vests that also traps air between fibers, which helps keep you warm. Fleece running vests are great for crisp early morning runs in the fall.