The Thermalite top is an extra layer of warmth for your body. Made from a material that stretches in 4 directions. Worn underneath the foam, it will reflect heat inwards while wicking away moisture. A warmer alternative to rashguards. You can use it as an extra layer under your foam or as a stand-alone top on warmer days to protect you from the sun (it has a UPF 50+filter). The Thermalite has a loop to attach shorts. The T-shirt will provide you with excellent protection from the cold and wind.