PPT Diagnstico e tratamento das infeces neonatais Uso Racional de Ultralight Thermal Vest. Sometimes the best layer is the one you barely notice. Our Ultralight Thermal Vest adds lightweight insulation under shells when conditions cool off, helping your outer layer work harder without adding bulk. On its own it becomes an easy vest for brisk mornings or late evenings, and because its so light, it slips into a bag almost as easily as it slips into your layering system.