Excellent and interesting post Jim
,
due to a number of experimental archaeologists many of the the 'Victorian made' views of armour have been radically changed.
Although heat 'build up' was a problem.......dehydration was the true enemy. To cool down, the body sweats...the evaporation of this cools the skin surface reducing body temperature. However, within the 'confines' of the armour the moisture 'transferred' to the air space within ...creating humidity. Sweating actually increases in an effort to counteract these conditions and the loss of water increases significantly.
A number of researchers have discovered that knights fighting in the melee, would take turns at the front....returning after a rest and the intake of much needed water.
Knights also improved their physical health with various routines often in full armour. This conditioning helped to increase their 'heat stroke' tolerance.
Also their physical weight made a difference, excessive body fat actually increases the 'heat up' factor.
Knights also had to learn to breath deeply and freely to counter the restrictive 'breathing holes' in their visors .....slightly less oxygen would have a massive affect on the body's performance.
Regards David