[Bicycle Bob]

That old saw about the body losing most of its heat from the head is a warning about uneven insulation in the cold - if you neglect your head, it can be your main point of loss but still not feel cold due to circulatory priorities.
On a bicycle, wearing sweat-through clothing, you will lose most of your heat through your body. In a velomobile, your body may be sheltered while your head gets the full air blast, making it the much more significant in cooling, but that is just due to air flow patterns. However, in that case, you can have your body absolutely drenched in sweat, but as long as your head is cool, you feel little discomfort.