‘Rash’ layers are made from 100% Lycra a UV protection material. Lycra draws moisture away from the body and allows it to evaporate rapidly, cooling the skin especially on those hot summer days. It can also been worn under neoprene as a rash guard.
If comfort is your primary objective rather than warmth, then the Pro Brushed Lycra range is for you. The soft brushed inner provides better thermal regulation properties than normal lycra whilst maintaining a luxury feel against the skin.
If it’s warmth you’re after then PolyPro is what you should be looking at.