Something that has long puzzled me is the use of full mail hauberks and the like in the Crusades. How did Crusaders and their various enemies avoid cooking inside their armor? I know many of them did die of exposure and the like but they did still fight and take territory. I'd assume the Byzantines and other local powers who used Cataphracts and similarly "covered" troops, ran into similar problems. Was it just a man it up and take it kind of a thing or did they fight at less hot times of the day? Change the campaign season?
I don't have much historical evidence to bring to this one, but I've worn heavy SCA armour on hot days (hot by British Isles standards) and discussed the problem with people who have done so in hotter climates (Texas and Israel, most notably).
So, first and foremost, they probably would not have worn the armour unless they were expecting to go into a fight. If they were travelling from one place to another, you might possibly wear the helm and a breastplate, but not much more. They'd only armour up fully before a battle. The concepts of today's guerilla warfare didn't really apply; many of the Crusader battles were sieges, one way or another, so they were pretty predictable.
Second, chain mail is heavy, but it's not airtight. Some breeze gets through, and with a cotton or even light woolen gambeson below that can be soaked, it's not that uncomfortable. It's certainly tiring, though. I don't imagine that many of the Crusaders would have worn full plate, which would indeed have the 'cooking' effect - full plate was really only coming in as the Crusades were coming to an end.
In any case, only a few of the people on the Crusades would have actually worn much armour - the nobles and knights. Their men-at-arms, infantrymen, and just plain foot soldiers would have been lucky to have a helm over a padded jack, rather than anything heavier.
After that - they'd have fought in cooler weather whenever possible, kept hydrated as much as possible (not that they had the term, but they certainly had the concept), and most importantly, not actually got into that many battles. A successful (that is, surviving) Crusader might have been in only a couple of real battles, and maybe a few scuffles on the side, many without armour.
I'm not sure but in the Encyclopaedia World History book it said many of them fought without armour, while others soaked cloth in water and put it underneath their armour. I would guess that under such heat exposure, as you said before, many died. Some of the battles, like you said, took place at night or less hot parts of the day. The armour they wore might have been heavy but may have still provided shade from the sun in some cases. Many coated themselves in oil; this may be because the oil could potentially repel the heat, but I'm not so sure about that one.
Mail is actually a heatsink: it draws the heat out of you. The same principle is employed in the construction of computers. In hot weather mail makes you cooler. Indeed, the first people to use mail extensively were the Romans, who fought mostly in and around the Mediterranean and other such hot climates.
The only problem is that when directly exposed to the sun, mail becomes quite hot to the touch, even burning. It's best to wear even a thin layer of cloth over the mail. The crusaders did this, painting the cloth with bright colors and large Christian symbols.
As far as the potential weight, when worn properly the weight is not felt at all. The weight is distributed across all of one's body. A thick belt helps: The mail hangs off the shoulders, and the belt takes half the weight off that.
Source: A lifetime of reenactments and various reading.
It is logical to assume that every crusader wore clothing made of various types of cloth, or leather, or skins, or wood (for shoes?), or reeds, etc., etc. but only some crusaders had any type of armor.
Thus it is logical to assume that every crusader wearing armor wore it over some type of clothing to prevent chaffing and pinching his skin, even if he was a really tough guy. And maybe that underclothing kept the armor from transferring too much heat to the Crusader's body.
And it seems logical to assume that many crusaders also wore some clothing over their armor to shade it and keep it from adsorbing heat from the sun. In fact books on heraldry often state that the surcoat was adopted during the crusades for that reason, and that the mantling over the helmet was also introduced during the Crusades to keep the helmet cooler.
Wikipedia says about the knight's surcoat:
And "The Six Parts of Coat of Arms" describes the mantle as: