I bought this to wear under a bicycle helmet and my motorcycle helmet. The cap does not add much heat and with air passing around it does a pretty good job preventing my liner from becoming soaked. My head is not huge (medium hats, medium for my bell star helmet) I think I would be happier if the hat had sizing but it seems it fits okay and would be fine for someone who wears a large in say an underarmor heatgear baseball cap (which are a tad loose on me). I like it and may even buy another later on. I won't blame the product but I thought I would get one as pictured black/grey but got a black/black. The color is fine under a helmet but if I was trying it alone for outdoor use black my defeat the purpose of wearing a hat intended to cool your head in the sun and heat.