One more layer of protection means valuable extra time to escape the heat in case of a fire. UPR Racing Supply has SFI and FIA rated Under Layers available from Alpinestars, Sparco racing, RaceQuip, and... Read moreG-Force. Under Layers come in different styles and are made from a variety of heat resistant fabrics. Also called “Fire Proof Underwear”, “SFI Nomex Racing Underwear” and “Nomex Long johns”, they all perform a similar function which is to give you another heat resistant thermal barrier in your racecar. Sparco and Alpinestars both offer very breathable moisture wicking fabrics to help cool your body as the perspiration evaporates from your skin. It’s a common misconception that wearing a t-shirt under your racing suit is cooler than wearing SFi or FIA rated Fire Retardant Under Layers. The fact is that cotton T-shirt does not breathe as well as racing under layers and they will retain moisture creating more of a sauna effect in your racing suit.