Undersuits provide thermal protection when diving and can range from fleece layers to more high-spec suits with built in heating or anti-compression technology. Your thermal requirements will depend on water temperature, your suit type and how much you feel the cold. Drysuits have their own thermal properties, some are much better than others. Compressed neoprene drysuits are much warmer and require thinner undersuits, but trilaminate membrane suits have very little thermal properties and therefore require an increased level of thermal undersuit.
