Well, I've looked into this a bit more and found some interesting stuff. Bear with the biology lesson, it's worth it.
Under normal circumstances the blood flow to the brain doesn't change because the demand for oxygen is constant. This being the case then the head accounts for about 7% of total heat loss. So far so good and pretty much what I said in the previous post.
Blood flow to the brain however does vary according to cardiac output. This means the faster your heart beats, then more blood flows to your head. This increase in blood flow causes an increase in heat loss. So when you start to exercise, more blood flows to your brain. This increases the heat loss from the head to about 50%! But, as you continue to exercise, muscles demand more oxygen and more blood. In order for your body to maintain its core temperature it now needs to shunt blood to other parts of the body eg blood vessels in the skin vasodilate to help cool the body.
This means that blood flow to the brain decreases causing the total heat loss from the head to decrease back down to about 10%. Once sweating starts this will drop back down to about 7%. Cool eh!
There is a big implication here for victims of hypothermia. Someone who is hypothermic but not shivering will be losing about 7% of total body heat through there head (see above). However, as soon as they start shivering, they are, in effect, exercising and so heat loss can rise to 50% or more through their head. So it is important to protect a hypothermia victim's head from the cold especially if they are shivering.
heat loss, hypothermia