Originally Posted by
njkayaker
Yes, a helmet could provide enough extra shade but not restrict enough airflow to keep your head cooler. Any additional airflow restriction is made more moot by travelling at higher speeds.
The issue I find a problem with helmets is with sweat, which people might be interpreting as evidence of being "hotter". Of course, this is more of a issue when going up hill, where one is going slower with more exertion. In such a situation, a helmet might be hotter.
^ plus sweat doesn't evaporate through thick styrofoam. this is like how wool can be both cool and warm, because wool is porous and lets air conduct, but does also traps heat.
The main issue with helmets being "hotter" is that sweat will not evaporate quickly on contact points with thick styrofoam. There are vents but its not effective as having thousands (or maybe hundreds of thousands) of micro holes in a fabric. Conversely helmets and decreased sweat evaporation may even make your head colder in frigid temps (just like when you have a non breathable slicker on during winter and your sweat can't evaporate and you end up soaking yourself and getting colder).
I think thats the next tech in helmets... maybe a totally breathable helmet? Like something thats dense and has impact protection, but also is not totally rigid but still breathable.